Tag: gas

Jesse Grandbois is one of the techs who reads the tech tips and he wrote a few tips that he wanted to share on some gas furnace control basics. This tip is about the basic terminal designations on typical 24v gas valve. Thanks Jesse!

Have you ever noticed the TH / TR terminal on a gas valve?. When I was in school, I’ll admit I didn’t know what it was. I’ve worked with people that are experienced and still think you need to use it. 
Nobody ever explained to me what it was. Being a newbie at the time it never clicked when I looked at the wiring diagrams. All my teachers would give the same response, “it’s a common.” 
That’s where the confusion set in. It’s a common. We throw this word around like it’s going out of style it seems in the HVAC industry. Everything is a “common” and technically a common is just a “common” point of connection but it really doesn’t clear up it’s intended purpose, or what part of the circuit it is intended for.
Following the diagram below while reading the rest of the article should clear things up.
Lets look at what each of these terminals mean:
TH – The 24v hot leg from the thermostat on a call for heat (R+W closing) to the gas valve (TH terminal) to open the solenoid to allow gas to flow. This is assuming the transformer is good and the high limit is closed.
TR – The 24v common/return side of the transformer.
TH/TR – This is not internally wired to the gas valve. Not using this makes no difference to the circuit. You could run your 24v hot from the transformer directly to your NC high limit in the above example and be just fine. This is nothing more than a convenience terminal. Joining these wires with a marette (Wire nut or terminal multiplier) does the same thing as this terminal.
Hope this helps clear up any confusion.
— Jesse

First, a thermocouple is not a flame rectifier like a modern flame sensor. A thermocouple actually generates a milivolt potential difference when it is heated by a flame.. Just to get that out of the way for any of you newer techs who are used to modern flame sensors.

With higher efficiency gas fired equipment being the norm for replacement systems, thermocouples and standing pilots are becoming a thing of the past. Newer appliances do not typically utilize a standing pilot, opting instead for hot surface or spark to pilot ignition. These types of ignition systems have benefits over standing pilot, from increased reliability and longevity to higher efficiency numbers. But there are many appliances in the field that still use a standing pilot, and a good service technician should be able to diagnose a thermocouple issue.


Many of you will say-

“Why even check the thermocouple? It’s a 5 dollar part, just throw a new one in!”

“Why are you so lazy? Do you even HVAC in real life or just on the internet?”

Yes, I know thermocouples are cheap and I am all for replacing them when they need to be replaced, or while replacing a gas valve or pilot assembly. But over the years I have seen a lot of guys ( me included) go on calls for pilot issues, find a pilot blown out, relight the pilot, and then because it’s the easiest, quickest fix, replace the thermocouple, only to have the same customer call in a day or two later with the pilot being out AGAIN. And when the tech goes back and relights the pilot, then what? Is that brand new thermocouple bad after a few days? Probably not. There is probably some other issue, but checking the thermocouple millivolt production is the first step for a proper diagnosis.


So how does a thermocouple work? Well, I’m no scientist ( I’m barely a writer), but I’ll tell you what I know. When different metals are joined, and there is a temperature difference between them, a magnetic field occurs between the joints where the different metals meet. The heat of the pilot flame is the source of the temperature difference in a normal pilot system. Through this process, a small amount of current is produced, generally around 30 millivolts. This voltage is sensed by the gas valve and is used to keep the pilot valve internal to the main gas open. If the pilot goes out, the heat that is generating the potential (voltage) is lost, thus current stops flowing to the gas valve, and the pilot valve is closed, closing off fuel to the pilot assembly. The thermocouple is a safety device. If the pilot flame goes and the pilot valve doesn’t close, the burner compartment and potentially the room the equipment is in can fill up with gas. That the consequences of that would require a different article.


When should you check a thermocouple? I am in the habit of checking thermocouples when I encounter them, whether it’s on a maintenance inspection or a service call. If you are in the habit of checking them, it usually doesn’t take more than a few minutes. If the millivolt measurement is less than 26-27, I typically recommend replacement.



To check a thermocouple, you need a multimeter that is able to measure millivolts. It is typically shown as mV or is just the third decimal over on the DC voltage reading. Remember, the meter should be set to DC voltage.

It’s also helpful to have a extra set of hands, but it is very possible to perform this check by yourself if you hold your tongue correctly (or just use alligator clips). First, disconnect the thermocouple from the gas valve. Then light the pilot. Most gas valves have a turn knob that has to be set from On/Off to Pilot. There usually is a push button that is pressed to manually open the pilot valve, sending gas to the pilot assembly in order to light the pilot. The trick is to light the pilot, and position the meter leads in the proper place to read the voltage. The push button must be depressed through the whole check. With the thermocouple being disconnected from the gas valve for checks, the pilot valve should not stay open and the flame should go out when the push button is let up.




Put on meter lead directly on the gas valve side of the thermocouple. Put the other lead on the copper line as shown by my right hand in the picture above. While holding the meter leads in this position, light the pilot. The thermocouple needs to heat up for 30 seconds to 1 minute in order to obtain a proper reading.


30 millivolts is the desired reading, with a swing of plus or minus 5 millivolts. If the readings are in that range, and you have been having pilot failure issues, more than likely there is some other cause. Dirty pilot assembly/ orifice is the most common other issue I encounter, but it could be down draft/flue or combustion air issues, fuel pressure problems, or a failing gas valve. But as stated above, the thermocouple should be eliminated as a potential issue before moving on with a proper diagnosis. Don’t throw parts at a problem and see what sticks. With thorough troubleshooting, you can save a lot of time, headaches, and maybe the customer a little bit of money and frustration.

— Justin Skinner

Please Note: There have been some legitimate questions about a few of the points in this article and in the diagrams. While Justin Skinner is an experienced tech and totally qualified to write this article we are going to be specifically looking into the question of the best location of the circulator pump as well as addressing “point of no pressure difference”. This article is still full of very useful points so it will remain up until we can research and potentially make a few changes. It is also worth noting that Dan Holohan’s book “pumping away” is considered the authority on the subject in addition to his website HeatingHelp.com. Thanks!

This article is the third in a series by senior boiler tech Justin Skinner. Thanks Justin!

Boiler piping is a much-debated topic in the HVAC trade. In fact, many books have been written on the subject. Should the circulator pump be on the supply or return? Where should the expansion tank be located? The best way to bleed radiators? If you talk to 10 different technicians, it is very possible to get ten different answers. And the short answer is, they are all correct. Because there is no “one size fits all” approach to boiler piping and layout. What works on one boiler system may not on another, and when a new boiler is installed on an existing system, there are plenty of potential issues which could be unique to that specific set up. Water is weird, sometimes. Like air, water doesn’t always do what you engineer it to do. I couldn’t count the times that I’ve been involved with projects with issues that left engineers scratching their heads because how they designed the water to flow through a system vs. what the water is doing is completely different. A technician need to be able to identify and correct what causes flow and heat exchange issues when we find them, and to do that, we need to know how it’s supposed to work.


Hot water piping



Here is a basic drawing of a hot water boiler system. This is an optimal setup, in my opinion. I like to feed water into the supply side before the expansion tank. Typically this area is the lower pressure which allows feeding easier. A lot of the air should go to the expansion tank, and the rest will go out into the system, which is ok because if it is in the system, it can be bled as long as there are bleed valves. Also, it allows the cold feed water to heat up before it enters the boiler, avoiding shock. I prefer to put the circulator pump on the return side, as well. I’ve had better luck with flow and pump life when the pump is pulling from the system, rather than pushing into the system. Gravity and convection play a part as well. Hot water naturally wants to rise, and this natural circulation helps the return side pump move water much easier than if the pump were on the supply side of the same system. If a pump must be installed on the supply side, I prefer to install it after the expansion tank. Also, as indicated by all of the X’s, install shut off valves wherever you can. It will save you a ton of time and hassle later on when a repair is required.


This is all my opinion, and is based on my personal experiences. However, if someone calls and asks me a boiler piping question, my first suggestion is to do whatever the manufacturer recommends. Most boiler installation literature shows diagrams on piping set up, and that is the baseline for installing a new boiler, and possibly diagnosing a flow issue. Some manufacturers show the circulator on the supply, some on the return, and some don’t care either way. If you follow the manufactures specs, to the tee, 99% of the time you won’t have a ton of issues with flow and boiler operation.


Bleeding air from systems is necessary from time to time. Some boiler systems are much easier to bleed if they are piped to allow air to be removed by automatic vents or go to radiators to be bled. On system drain down and refill, I will typically bleed air after filling the system, while it is still cold and no pumps are on. After that initial bleed, I turn the boiler and the pumps on and allow the boiler to heat up to operating temperature. Once it is hot, I shut everything off, boiler and pumps. This allows the air that may be traveling with the water to go up, either to higher radiators or bleed points. I bleed air, turn everything on again, turn it off, and repeat until all the air is out.


A lot of older boilers guys I have worked with only bleed air with the pumps running. I could never get a satisfactory answer as to why, and I have had much better luck bleeding air from a system with the pumps off. If you do things differently, and it works for you, that’s great. Again, most of this is my opinion based on my experiences, and there is more than one way to skin a cat. Also, increasing boiler system pressure while bleeding helps speed things up. Most automatic water feed valves are factory set to keep the pressure at 12 psi, which is a pretty standard pressure. If your system is 12- 15 psi, bumping it up to 20-25 psi will help speed up the bleeding process. Always make sure you aren’t exceeding the pressure rating of the relief valve if you increase system pressure to bleed. And don’t forget to bleed excess pressure off after you have completed.


Steam Piping :

A basic steam system is much simpler than a hot water system. The natural rising of the steam allows it to move through the system, so there is no need for a circulator pump to move steam. Steam is a vapor, so there is also no need to bleed air, and no need for an expansion tank on a basic steam system. However, piping pitch is much more crucial to this system. The piping must have pitch or fall to help the steam rise, and more importantly, to allow the condensate to flow back to the boiler. Level piping holds water, so it must have fallen. Also, a Hartford Loop is required to connect the supply and return. This is basically an equalizer to balance the pressure between the two sides of the system. Also, as part of the loop, the condensate return line connects 2’’ below the water level of the boiler. The loop is used to prevent water from leaving the boiler through the return if the pressure is lower than the supply, or if there were to be a leak on the return. This piping configuration was mandated by code for a long time as a prevention for low water conditions causing the boiler to dry fire. With the invention of more advanced low water protection devices, it isn’t required by code everywhere but still is a good idea. It allows added protection if the low water safeties were to fail.


Steam traps are integral to steam systems as well. A steam trap is a check/float valve that allows condensate to pass through and return to boiler while preventing steam from passing. Steam traps are locating on the return (outlet) side of steam heat exchangers, radiators, etc. The goal is to keep the steam in the radiator as long as needed for it condenses to liquid water, thereby releasing as much heat to the radiator as possible. Without steam traps, the steam would blow right through the radiator, and would not stay there long enough to properly heat it up.

                   Steam Trap


There are an infinite amount of piping configurations that you will run into of the course of a career, some much better than others. And certain situations dictate changes and configurations that may allow one system to work well, and the same configurations could cause a different system to function poorly. In short, every system is different, and a lot of times I am required to think outside of the box to make a poorly designed system work. But as mentioned at the beginning, if you have a basic understanding of how things are supposed to work, it makes diagnosing why it isn’t working a lot easier.


–Justin Skinner


This article is written by Senior Boiler Tech Justin Skinner. Thanks, Justin.


Oil burner nozzles are present in most forced combustion air burners. They are used, with an oil pump, to atomize fuel oil and allow it to burn. Atomizing is raising the pressure of the fuel and forcing it through the nozzle. The fuel comes out of the nozzle essentially vaporized. It is then mixed with air and burned. Nozzles are also used to meter the amount of fuel being used, and to vaporize the fuel in an efficient pattern suited to the burner chamber of whatever equipment it is installed on. If you work on burners, you have probably seen and changed out your fair share, as periodic nozzle replacement is necessary for clean and reliable burner operation. But there is more to nozzles than what meets the eye. Let’s take a closer look.

     The numbers on the nozzle tell us the specific rating of the nozzle, the spray pattern angle, and the spray pattern type. The nozzle listing here has a .75 GPM rating. That means the nozzle will spray .75 gallons per hour of fuel oil at 100 psi. Nozzles are generally rated at 100 psi, and that is the pressure that most residential style oil burners run at, but not all.  It also has an 80-degree spray angle. That is the angle at which the spray comes out of the nozzle. The smaller the angle, the more narrow the spray pattern. Think of a garden hose with a spray nozzle. If you squeeze the handle only a little, the spray comes out at a wider angle. This would be similar to a higher degree spray angle. As you squeeze the handle more, the outer edges of the water get closer together. A closer spray pattern would be a smaller angle. Larger spray angles are generally used for wider, shorter burner chambers, and smaller spray angles are used for narrower, shorter chambers.


The letter on the nozzle indicates the spray pattern. Different manufacturers use different letters for the same patterns, so we will use Delavan as the example, as it is the most common nozzle manufacturer I use. Patterns are designated as solid (B), hollow (A), and semi-solid (W). A solid nozzle indicates the vaporized oil is distributed evenly throughout the entire spray pattern. A hollow nozzle distributes more of the oil to the outer ring of the pattern, and a semi-solid is neither. W nozzles are typically considered a replacement for both A and B nozzles, although that is not also the best option. I prefer to replace nozzles with the type and pattern specified by the manufacturer, but late nights and on-call situations do not always allow it. Typically the nozzle is designed to fit the equipment and not the other way around, so using the correct nozzle can save you a lot of headaches and a sooty mess.


The pictures above show an exploded nozzle. The back portion is a very small particle filter. It is composed of thousands of bronze pellets fused together. This filter is easily clogged by gunk, so an in-line filter should be used to catch most of the fuel sludge and trash before it gets to the burner. After the filter, a slotted distributor is present. The pressure of the fuel from the pump causes the distributor to spin, and the oil increases velocity inside of the nozzle. The oil is forced through the head of the nozzle, which contains a small hole/tube. The sudden decrease from high velocity/pressure to atmospheric pressure through the tube causes the fuel to vaporize. Once the vaporized fuel leaves the nozzle, it is mixed with air at the burner head and is ignited if the fuel/air ratio is correct and the ignition source is strong enough. Nozzles are rebuildable if you need to in a pinch. But they are finely machined to exacting specs, and fairly inexpensive. So I would only try to rebuild a nozzle in an emergency situation.


Nozzle flow is rated in GPH @ 100 psi pressure. One gallon of #2 fuel oil contains approximately 140,000 (give or take a 1000 or 2). So a 1 GPM nozzle @ 100 psi is a 140,000 BTU burner input. If the burner efficiency is 80%, that means 20% of the fuel energy goes up the flue as unused energy. So a  1 GPM nozzle on an 80 % efficient burner is equal to around 112,000 BTU’s available from the fuel. But what if you need a 1 GPM nozzle, but you only have a .75 GPM nozzle available? Well, increasing the pump pressure above 100 psi can allow for the same amount of fuel input with a smaller nozzle.


As the chart above shows, the same nozzle flow rate can be achieved with a variety of nozzle GPM sizes and pump pressures. It’s not advised to change nozzle size or pump pressure during an inspection unless there are issues. It’s more to get you by until you can get back with the correct nozzle. Also, if you change the nozzle size or pump pressure outside of what the manufacturer recommends, make sure you note it on the equipment for the next tech who may go behind you. It’s also an option to downsize the nozzle GPM and increase the pump pressure for hard/smoky light-offs and shut downs. The higher velocity from the increased pump pressure allows for the complete vaporization of the fuel. This allows for a cleaner light off, cycle, and shut down.

–Justin Skinner

This article was written by senior furnace tech Benoît (Ben) Mongeau. Ben hails from the frozen tundra of Ontario, Canada where high efficiency gas furnaces are commonplace.

While some codes and practices may be different from the US I find that most of it is common sense and translates pretty well. One glaring difference between Canada and the USA is the requirement in Canada for specifically certified PVC or CPVC vent pipe. Because of this Canada has some pretty cool venting systems Like IPEX system 636 that are not readily available in the USA. I’m leaving all this in because there is already talk about making the change in the US so I bet it’s coming.

Venting for High efficiency Gas Furnaces –  Assembly

Here are some good venting practices.  (This is mostly stuff I learned during a training session from IPEX, one of the major manufacturers of plastic piping, with a little of my personal experience and tips)

First of all, venting must be planned in order to be sized properly.  Depending on the BTU rating, length, number of elbows in the run, the size will vary, typically in residential from 1½ to 3-inch pipe.  Every manufacturer has its own vent sizing charts.  Read the manual, don’t guesstimate.

Use the proper tools when installing plastic venting.  !!!Avoid using a sawzall or hacksaw to cut your lengths!!!: it creates a multitude of statically charged shavings that will stick to the inside wall of your pipe.  Once that condensate starts flowing, it will bring all those shavings to your drain and trap, blocking all those narrow passages and causing water backups, furnace failures, all kinds of things to piss off your service colleague who’s on call that night.  I highly recommend using a proper pipe cutter.  It is the best way to achieve a clean, straight cut.  The straighter and neater the cut, the more joining surface you have once you’re cementing it together.

vent pipe cutter and chamfer/deburring tool (REED venting solutions kit, which I highly recommend purchasing 

It is recommended to dry-fit the whole vent system before actually starting to cement joints together, just to be sure your lengths and angles are good.  Also, as mentioned in my condensate drainage tip, make sure the vent is sloped towards the furnace for the whole length.

Before applying cement, prepare the pipe ends by cleaning them up (wipe off any obvious dirt) and, most importantly, reaming them.  Use a proper reamer / chamfer tool (pictured with the cutter above).  This is a crucial step: if the pipe end is not reamed/deburred, the edge actually tends to slightly stick outwards, especially when cut with a proper cutter, ironically.  This will cause the pipe to push (I like to call it ‘’snow plowing’’) the cement at the bottom of the joint instead of letting it slip around the pipe, leaving large uncemented gaps in the structure of the joint and often causing leaks.  See comparative pictures of chamfer/un-chamfered pipe ends below.


Cut, not reamed /chamfered 636 PVC pipe


The same pipe end, reamed / chamfered and deburred  

Next, once all pipe ends are reamed and clean and ready for assembly, it’s time to start cementing.  Apply primer first if required, then apply the cement.  Don’t be shy, apply a generous coating around the whole joint surface of the pipe and fitting (yes, cement is applied on both the pipe and the fitting).  I recommend going around the pipe/fitting 3-4 times with the dabber/roller/brush to ensure a full coating.  Once both ends have cement applied, quickly (before it dries!) push them together, straight and all the way to the bottom of the joint, and as much as possible try to give the fitting a quarter-turn while assembling the joint to further evenly coat the entirety of the joining surface.  Very important: once you hit the bottom of the joint, hold the pressure for about 30 seconds (or longer) so the cement has time to set!  If you let go immediately, the still wet fitting and pipe will naturally pull back from each other and this can easily lead to leaks.  Wipe off any excess/runoff cement if necessary and proceed to the next joint.

Once assembled and when the cement has dried, as I mentioned before, the two pieces are basically welded together.  You cannot take them apart, so make sure your angles are correct, or you’ll have to cut it out and restart.

Support the pipe as necessary, per local codes/guidelines.  Support spacing usually varies depending on pipe size.  Avoid creating too much tension on the venting as it can lead to leaks/cracks.


Other tips:

  • Don’t leave your cement cans open longer than necessary.  The solvent part of the cement is quite volatile (evaporates easily) and as it evaporates, the viscosity of the cement will increase and it will get more difficult to use.  Once your cement has gelled (i.e. has a consistency very reminiscent of that of Jell-O) throw it out.  It is no good.  Keep an eye on your cement’s viscosity.  It should always be liquid, although with various degrees of thickness depending on the type, but NEVER jelly.
  • If a reducing fitting is used on your venting, always install it on a vertical portion, never horizontal, otherwise it will allow for condensate to pool in the vent.  Remember… slope for drainage!
  • Respect local guides and regulations and manufacturer’s specs regarding clearances when choosing where to terminate the venting outside.  Also, terminate the exhaust higher than the air intake (usually about 1ft minimum) if you are installing a sealed combustion system, to avoid recirculation of combustion products which can be quite disastrous.  Typically on a sidewall termination the air intake will be terminated with a downward-facing elbow and the exhaust will be snorkeled up, i.e. elbow up, 1ft pipe, then elbow out away from the wall.  There are also manufactured termination kits (concentric, for example) that are available and sometimes easier on the eye.  Make sure it’s certified, though!  Again, manuals will tell you if there are termination kits available and certified for use with the product.
  • Be careful to read install manuals for any specifics regarding the furnace you are installing.  There is often specific procedures for attaching the vent pipe to the cabinet’s internal exhaust fitting/flue collar etc. and it will vary from one manufacturer to the other.

— Ben

You can read the full IPEX 636 install instructions HERE

This article was written by senior furnace tech Benoît (Ben) Mongeau. Ben hails from the frozen tundra of Ontario, Canada where high efficiency gas furnaces are commonplace.

While some codes and practices may be different from the US I find that most of it is common sense and translates pretty well. One glaring difference between Canada and the USA is the requirement in Canada for specifically certified PVC or CPVC vent pipe. Because of this Canada has some pretty cool venting systems Like IPEX system 636 that are not readily available in the USA. I’m leaving all this in because there is already talk about making the change in the US so I bet it’s coming.

Venting for high-efficiency gas furnaces – Materials

Due to the condensing nature of a high-efficiency furnace, its venting must be made of a material that is resistant to corrosion. In a great majority of cases, plastic piping is used to vent high-efficiency equipment. It is classified as “Type BH” venting. The lower temperature of the exhaust gases also mean that the natural draft effect observed in conventional metal chimneys (heat rises) does not occur at a significant level. Which means those exhaust gases have to be forced outside. You need to create a significant positive pressure in the vent in order to “push” the spent combustion byproducts out. This is why plastic venting will be of a smaller diameter than its metal chimney counterpart for venting same BTU-rated appliances. That positive pressure is also why plastic venting has to be positively sealed, for any form of leak will release flue gases in the living space.

There exist many, but mainly three types of plastic are commonly used for high-efficiency appliance venting: ABS, PVC, CPVC.

(Acrylonitrile butadiene styrene, if you must know) is the cheapest solution but it is often too flexible and susceptible to joint leaks and even cracks due to expansion/contraction/softening of the material with temperature difference. Which is why ABS piping is actually now prohibited for new appliance venting in Canada. Never use primer on ABS.

(polyvinyl chloride) is what is most commonly used nowadays. There are different types/grades of PVC on the market and some of them may not be allowed for use as flue gas exhaust. Always check your local/state/province codes and regulations. For example, here in Canada Schedule 40 PVC DWV (drain PVC) may not be used. Only FGV (flue gas vent) PVC certified to a specific standard (ULC S636) may be used.

Note from Bryan: In the USA schedule 40 DWV pipe (the usual stuff) is still the standard, there is talk this may change soon so stay tuned.

(Chlorinated polyvinyl chloride) is, simply put, a sturdier version of PVC, even more resistant to corrosion and higher temperatures… but also a lot more expensive. It is more often seen on high-efficiency residential boilers, where, in some applications, even PVC is not sufficiently resistant. For easy recognition, vent/drain piping is usually color coded. Most often, ABS is black, PVC is white and CPVC is gray / tan. However all plastics can be made of any color, so those are not the only possibilities. Be extra careful about that especially when it comes to certain fittings supplied with the furnace. A prime example would be the vent flange on new Carrier/Bryant/Payne furnaces. It is black, but actually is made of CPVC. Which means you may not use ABS (or PVC) cement to attach it to your venting.

Note From Bryan: Read the manufacturer’s instructions

Those plastic piping systems are joined with a cement, which most people will incorrectly call glue (it’s okay, I usually say glue too). It is not glue. It is not an adhesive. Cement is basically the plastic you are working with, dissolved in a solvent. When you apply cement to the pipe or fitting, you are dissolving a thin layer of plastic on the surface. Once the joint is assembled, the solvent part of the cement evaporates, leaving only a continuous piece of plastic that is now basically part of the pipe and part of the fitting. The two pieces become as one (how poetic!). They are basically welded together. Always be careful to use the adequate cement. PVC cement will not bond properly to ABS or CPVC. An exception I know of would be the IPEX System 636 CPVC cement, which is certified for joining both PVC and CPVC pipes in any manner (PVC to PVC, PVC to CPVC, CPVC to CPVC) . Always use the correct cement, made by the same manufacturer as the pipe you are installing, since it uses the exact same plastic “recipe”, if you will. It is the only way to ensure a proper bonding (again, in Canada they utilize certified systems).

In addition to the cement, there is also primer, which is nearly pure solvent. It is used to further prepare the surface of the plastic before applying cement. Note: In practice it not necessary to always use primer on DWV pipe (UNLESS IF SPECIFIED BY YOUR LOCAL CODES). Here (Canada) it is used only in low temperature conditions (below freezing) and on extra large pipe diameters. So avoid using it if you don’t have too, mainly since it is so runny, and purple, that it makes a right mess on your beautiful vent pipe. Also, CPVC and ABS do not require a primer (according to Oatey)

As always, READ the manufactures instructions on the furnace / boiler being installed as well as the pipe / cement being used to ensure that you are using the correct
materials for the job. In part 2 we will cover more specific vent fitting tips.

— Ben

This article is written by Senior Boiler Tech Justin Skinner. Thanks Justin.

Steam plays a very important part in all of our lives, whether we know it or not. Virtually every article of clothing and accessory you are wearing right now relied on steam for either manufacturing or packaging. Hospitals use large steam boilers for dehumidification, sterilizing medical equipment, and plenty of regular old space heating and domestic hot water through heat exchangers. The power entering my house is generated by a high pressure steam boiler a few miles away. Here are some common components used to operated steam boilers safely and efficiently.

Operating/High Limit Controls:

The operating control shown here is made by Honeywell, and it is the most common that I run into. It is basically a normally open/ normally closed set of contacts with 1 common feeding both. There are 3 terminals : R, W, and B. R is common. The switch through R to B opens on a rise in pressure, and the switch between R and W closes on a rise in pressure.

Typical wiring is done through the R to B circuit, breaking the control voltage going to the burner that calls for it to start.  The pressure is set by the user. There are 2 scales to be set on this control. The first is the pressure at which R and B open, and the second is a subtractive differential in which the switch closes.

Lets say you have your first scale, the cut off pressure set at 10, and the second scale set at 3. That means the R to B switch will open at 10, shutting off the burner. It will close at 7 (10-3=7), indicating a pressure drop and enabling the burner to run. If you change the second subtractive scale to 5, it will close the switch at 5 (10-5=5). The high limit shown is a normally closed switch with a manual reset that opens on a pressure rise. This control will not reset itself should the pressure get high enough to open its switch. This is a safety device that should never be bypassed, and if one is tripping and needs to be reset, more investigation is required to determine the cause.

Modulation Control

The mod control is used to increase or decrease the firing rate of the burner. These controls aren’t typically seen on smaller single or 2 stage burners, but in larger burners that require different burner firing rates depending on steam load and requirements. They contain the same terminals as the operating control listed above ( R,W, and B), but act completely different.

The mod control pictured is not a typical open/closed switch, but rather a potentiometer that changes in resistance with a change in pressure. Basically, as the pressure rises, resistance is decreased between R and W, which causes a connected modulation motor to drive shut, and drive the burner to a lower firing rate. Similarly,  when the pressure falls, resistance is decreased between R and B, which drives the mod motor open, increasing the firing rate. There are mod controls that are normally open/closed switches, and they look identical to the one pictured above. If you look at the inside cover, you can usually find a diagram that shows operation, or a part number to be able to look it up. Below is a typical modulation motor. This motor is connected to air and fuel dampers and valves, adding or taking away both to the flame in unison based on the steam load and input from the modulation controller.

Condensate/Deaerator Tank

In a steam system, superheated water (steam) is sent out to the piping system. As the steam cools, it condenses in liquid water. This water is collected in a tank and reused to feed the boiler. There are a few reason for this. The most important reason is safety.

When a boiler gets up to temperature and is making steam, there is a optimal water level in the boiler where below is water and above is steam, almost like a accumulator on a heat pump system. At atmospheric pressure, water boils at 212 degrees F. In a pressurized steam system, that temperature is higher, as thermodynamic laws require. Increase pressure, increase boiling (vaporizing) temperature. So the liquid water in the lower portion of the boiler is above 212 degrees when steam is being produced. If cold water is introduced to this environment, it rapidly (and potentially violently) expands. Water expands 1,700 times when it is converted to steam. We collect the condensate, which hopefully is still hot, and feed it to the boiler because hot water does not flash to steam as easily as cold water. It is safer and easier on the boiler vessel to use pre heated water to make steam. Cold water going to a hot boiler can be loud and pretty scary. It bangs and pops and the boiler can move some. Optimally, the water being fed from the tank should be as close to the boiling temperature as possible. Also, water heated in the feed tank releases oxygen molecules from the feed water, which is important for preventing corrosion and scale build up in the boiler. Less oxygen in the boiler, the less rust. Its common to run steam to a feed tank to ensure the feed water is sufficiently heated. Water treatment chemicals are often added to the feed tank, and pumped to the boiler by way of the feed water to inhibit rust and corrosion. And of course, reusing water is a cost saver for the customer. It’s more expensive to treat and heat fresh water than it is to reuse pretreated and already hot water. Below is a typical condensate tank.

Water Level Controls

Water level is important in steam boilers, and they aren’t completely full. There a level at which the internal components are sufficiently covered in water, and there is room for the water to boil off above. There are many types of level controls, the most common for larger boilers being shown below. Made by mcdonnell-miller, this is a float type controller with a series of normally open and normally closed switches. The switches control the feed pump that feeds the boiler from the condensate tank, and can be configured to shut off the boiler if the boiler water level gets too low, too high, and can activate an alarm circuit. Smaller steam boilers require a similar device, and there are many varieties out on the market. These controls are crucial in the safe operation of a steam boiler. If the water level drops and the boiler dry fires, bad things happen. A quick google search of serious boiler explosions will indicate in the reports that bypassing or jumping out the switches on these devices are the most common cause. If you don’t know how its supposed to work and are not pretty familiar with the device, the last thing you should do is mess with it. Call a senior guy or tech support or someone. Boilers do explode, dont be the cause of it. There are probe type water level safeties as well. They use power to detect conductivity from the probe through the water. If the water level drops, no conductivity is detected and the boiler should shut down. Level controls and safeties should be inspected and tested, and if they fail they should be repaired or replaced immediately.

Float type level controller and sight glass

Level switches inside float head


Sight Glass

The sight glass is a real time view into the boiler water level, and a quick indicator that all the feed water controls are in working order. I am in the habit of looking at the sight glass before anything else when i come into a boiler room, and constantly checking it while I am working in there. As stated above, boilers in low water conditions can be catastrophic, so it’s crucial to pay attention to. If you come across a boiler with no water in the sight glass and you don’t know what to check or why, the best thing is to turn the burner switch off, get the heck out of there, and call for back up. The last thing you should do is start turning on pumps and opening valves to try to fill a hot boiler that is under pressure. Your personal safety is much more important than getting the boiler up and running.

Steam boilers can be dangerous if not properly serviced and maintained. I’ve run into situations that were pretty intense before, and had to call for help many times. It’s tough for a lot of techs to admit they dont know something and they need help (me included), and it’s especially tough when the boss is telling you to just get it done. But safety and getting home to your family is the most important thing we will do today. And if you are in a uncomfortable situation with a larger steam boiler, asking for help may mean the difference between going home or not. There are plenty of other components involved with steam boilers, but this is a basic overview. By reader request, the next article will cover basic boiler piping systems. If there is something that you would like clarification on, or a topic i did not cover, feel free to email or message me or comment below.

–Justin Skinner

This article is the second in a series on boiler basics by senior boiler tech Justin Skinner. Thanks Justin.

There are many types of boilers that do a lot of different things, but most all of them have some of the same basic components. Some because they are required by regulatory agencies, some because they are necessary for proper operation and safety of the boilers. But no matter how large or small the boiler is, you can probably find most, if not all of these components.Unless noted otherwise, these are typical for hot water boilers.  



Every boiler needs some source of heat, obviously to heat the water. The type and sizes of burners vary so much that a few complete articles would be required to really get into detail as to how they operate and the specific operations of each burner. All burners serve the same function, which to safely and efficiently burn fuel and create heat. Typically, the flame safeguards are integrated in the burner control sequence, in that certain conditions need to be met in order for the flame to light, similar to gas furnace pre-ignition sequence of operations. A blower with air dampers for adjustment is often part of the burner, and air/fuel ratios are adjusted at the burner during combustion analysis. Ignition transformers/control boards, ignitors, draft pressure switches, flame sensing devices (flame scanner/ flame rod), and a primary controller to sequence all of it together are all apart of most burners. Honeywell, Fireye, Siemens, and a few others are common  burner controls that are all different, but essentially do the same thing. Some of the most common burner manufacturers ( at least in my world) are Powerflame, Webster, and Beckett, but there are a lot of burners out there. It is  common to see a dual fuel set up on larger burners, meaning they are capable of burning 2 types of fuel, typically gas and oil. Steam and hot water boilers both use burners.

Oil Fired Boiler

Gas Fired Boiler


Water feeder

Most water boiler systems are sealed, meaning that they are filled with water, the air is bled, and the same water is circulated throughout the system. In a perfect world, no additional water would be required, but most systems lose water and pressure through a variety of ways. A automatic water feeding valve is used to keep the system at a set pressure. There are many types with many different pressure ranges. The proper term would be pressure reducing valve, but i’ve always heard them called water feeders, so that is what I call them here. Back flow preventers are often used with water feeders. Once the water enters the boiler system, it should not be allowed to go back and re-enter the domestic cold water system. Boiler water can be pretty gross, and often contains chemicals for water treatment so be mindful and safe when opening the water side of any boiler system.

Typical Water Feeders (PRV’S)


Pressure/Temperature Relief Valves 

Relief valves are used to protect the boiler pressure from rising above the safe maximum that the boiler is rated for. The pressure rating on the relief should never be above the pressure that the boiler is rated for. Also, relief valves come with a BTU rating and are sized to match the fire ratings of the burner. This is crucial to keep in mind when replacing a relief valve. A valve that is too small may open prematurely, and a valve that is too large may not open at the pressure it is supposed to. There are calculations and recommendations that are used to size relief valves that I’m not gonna give here, but if you are replacing one or having issues with one and you are unsure, ask a senior tech or contact the manufacturer for recommendations. But keep in mind that the relief valve may be the last line of defense in preventing a boiler explosion, so treat it as such. NEVER plug a leaking relief valve, it is kind of illegal. Found on both steam and water boilers.

If you do this, you are fired!


Operating/Modulation/High Limit Controls 

These are controls to maintain a set range for water temperature, the burner modulating from high to low fire (for modulating burners), and as a safety to prevent temperature rise above set point. These are used in both water and steam boilers, and i will go over them in more detail in the next article.


Circulator Pumps 

Are used to move water through the boiler and the system. Some pumps are controlled on and off by the boiler, some are controlled by building automation, some are just on   and run constantly. Flow sensors are often used to insure proper water flow is present in the boiler, and will disable burner operations if the flow is decreased below what is recommended.

Pumps come in all shapes, sizes, and voltages.


Low Water Protection 

When a boiler gets low on water, it can be a very dangerous situation. Low water safeties are used to disable the burner when low water conditions are present. Steam and water boilers both require protection, but low water controls for steam are generally much more crucial than a typical water boiler, as the risk with steam and boiler low on water can be severe.


This is by no means a comprehensive list, just a general overview, and I’m sure i missed something that you all will let me know about. With the huge variety of boilers out there, it would be tough to list every single thing that you might run into. These are all very common and the things that i seem to replace or have issues with the most.   I will expand on steam specific controls and components in the next article.

— Justin


I received an email from a podcast listener with some furnace related questions. Based on the nature of the questions I figured it would be better to ask an experienced furnace tech. Benoît Mongeau agreed to help by answering the questions. 

My name is Matt and I am a newer tech (fully licensed this September, have been doing the work for 2ish years) who lives in Northern Ontario, Canada. I really enjoy the HVACR school podcast. I don’t do any A/C stuff but I still enjoy listening and wrapping my brain around it. I have always struggled with the theory behind getting cold from hot. The bulk of my work is residential gas heating, mainly high efficiency furnaces and gas fire places. My questions for you are, (these are just ideas for your podcast though help is never turned down)

On a milivolt system (runs off of a thermopile)
– How to easily test for gas valve failure, what are the expected resistances across the solenoid in the gas valve.
– What expected readings should we consistently get from a properly working systems (voltage of thermopile alone, with gas valve open, with thermostat closed etc)

On high efficiency
– What is the relationship between the pressures in the collector box of the secondary exchanger and the pressure switch.
– How does a clogged condensate trap lead to the pressure switch not closing.

Another Question
– Is it possible to check readings from the circuit board when the wires are in a harness. For example, I troubleshot a gas valve failure. It was either the board or the valve. The wires coming to the gas valve from the board are in a harness. How do I know which to check and what am I checking for. (Given that everything else was working I leaned toward a faulty gas valve and was right, just so you know!)

Thanks for your time and for doing the podcast.

All the best,

For the collector box/pressure switch:
During normal operation, the collector box is under a vacuum (negative pressure) when the inducer is running. That vacuum is what the pressure switch checks for. If the vacuum is sufficient the contacts will close and signal the board everything is good. If your condensate trap is blocked, the collector box will still be under a vacuum. That doesn’t change. However, the pressure switch port (where the tube is attached on the collector box) should be at the bottom of the box, usually near the drain port. The backed up condensate will simply end up blocking that port and the switch will no longer be able to ”feel” the vacuum, the contacts won’t make and you will get an error (pressure switch not closing or stuck open). What may also happen, but not always, is that the port will block during a cycle and the vacuum will remain stuck in the pressure tube. As your inducer comes off and normal pressure returns, the air can’t go in the pressure tubing because it’s blocked with condensate, and you’re basically trapping that vacuum inside. So the contacts will stay closed, until the next call for heat. When that call starts, the contacts will already be closed before the inducer starts, and that will also give you an error (pressure switch stuck closed).
Now if your exhaust is blocked, this will create back pressure and your collector box won’t be under the appropriate vacuum, and once again won’t close.

For millivolt systems:
Unfortunately I can’t say what typical resistance values would be for a mV gas valve, because I don’t know. I would say however that in three and a half years I haven’t had to replace a fireplace gas valve. They rarely go bad. In most cases the pilot tube/orifice is dirty, the thermopile is too weak, or, if it works with a wall switch, very very common: the switch is bad. Standard wall switches are meant for AC voltage. Running millivolt DC thru them will work, but as soon as you have a bit of resistance in the switch contacts, that voltage will not get through. If it runs on a thermostat, usually it works better but you can still get the same problem.
For typical readings, I’d say between 450-650mV from the thermopile alone, open circuit. With valve open (so, closed circuit) around 200-300mV. But this is very general, it may vary alot between systems.
If your thermopile alone doesn’t produce enough mV’s, check your pilot flame. Make sure it hits the thermopile well. You might be able to adjust it (on some valves) to make it bigger. As I mentioned, the orifice or tubing may be blocked. That is relatively common especially if the pilot was kept off for a long time.
If your thermopile gives enough voltage but the valve won’t open, check your switch/tstat and even the wire itself for any significant resistance or short. Isolate section by section and ohm it out. If everything is good and sufficient mV’s come back to the valve and it still won’t open, then yes, that valve might be bad. But I’d probably even replace a switch/tstat before I condemn the valve regardless, just to be sure, just because changing those valves in most cases is a total pain in the butt.

For the gas valve/board dilemma:
If your wires are all in a harness with a big fat connector on the board, there’s a good chance you won’t be able to pull it off and diagnose on the board pins, because by removing the connector you remove most or all of the safety circuits. If you want to look at the gas valve, you need to hook your meter on the wires at the valve itself. If it’s just a standard 24v valve with 2 or 3 terminals (Common + hot or common + low and high solenoids) just pull the wires off (or connector) at the valve and you have to check for 24V on the wire across common and hot. Even with the valve disconnected if your board is OK it will still send 24V in that wire at the proper time in the sequence of operation (i.e. wait until the ignition sequence completes!!). If you don’t have 24 volts, board is bad. If you have 24V, gas valve is bad.
If it’s one of those Honeywell SmartValves, then that’s another story entirely. A good portion of the controls are actually inside that gas valve and it will have multiple wires going to it. They are a bit more difficult to diagnose. My best advice is follow your electrical diagram.
If there’s no way for you to disconnect wires at either end (which should never happen as far as I know…) you could always cut the wire and check your voltage in the wire itself. But try to avoid doing that.

— Ben

This series of articles is written by senior boiler tech (and all around swell guy) Justin Skinner. Thanks Justin.


There are quite a few different types of boilers out in the world. They come in all shapes, sizes, pressures, and types of fuel burned. I’m going to go over some of the more common ones, their common components, and why it all matters.


First, lets establish what a boiler is. A Boiler is defined as a fuel-burning apparatus or container for heating water, in particular. The in particular part is thrown in there because a lot of boiler systems heat a fluid other than water. Glycol, oil, and process chemicals to name a few. But to keep things fairly simple, we will stick with water. So your basic boiler burns a fuel source to heat water. A water heater, basically. But water heaters are used to produce domestic hot water for showers,sinks, and other household hot water uses. Boilers are used to produce hot water for space heating purposes, dehumidification, and other processes (or even potable water indirectly through and exchanger).


For residential and commercial/industrial purposes, there are 2 types of boilers. The most common is the fire tube design. This would include the common sectional type boiler seen in most residential applications. The fuel is burned, and the hot gases pass through a series of flue passages or tubes that are generally steel. The steel composing the flue passages or tubes is heated by the gases passing through. This area does not contain any water, only heat and combustion gases. Water surrounds the flue gas area, but does not actually enter the area. Heat transfer to the water occurs by conduction, primarily. The steel comprising the flue passages becomes heated, and transfers the heat to water.  Hopefully these illustrations help.​


 Commercial Style Fire tube


 ​Residential style sectional, note flue passes

As you can see, the tubes or sections that contain the hot flue gases are surrounded by water, which is how this boilers transfer the heat to water. The passages that contain the hot gases also act as the heat exchanger in the boiler. The FIRE (flue gases) is contained in the TUBE (tubes or passages).


A water tube boiler is typically used in commercial and industrial applications, and not seen often in residential. As the name implies, the tubes contain the water being heated, and are surrounded by the hot combustion gases. This type often looks like a rectangular box with a burner mounted to it. Its virtually a large fire box with water tubes. ​


 Water Tube Boiler Design


Whats the difference in application and why use one type over the other?


Water tubes are generally considered safer. They contain much less water than fire tubes, so if a disturbance occurs (tube breaks, boiler melt down) there isn’t as much water/steam to have the potential to escape the boiler.


The main determining factor of water or fire tube is application. Water tubes are able to handle much higher pressure ( 1000s of psi), and fire tubes generally aren’t designed to be used over 350 psi. Water tubes are available in much larger capacities than fire tubes, and are able to recover a lot faster from a large increase in load demand from a pressure stand point. Meaning if the pressure drops on a fire tube boiler, it takes longer to come back up than on a similarly rated water tube boiler. Fire tube boilers typically have lower operating and maintenance costs, have easier access to the fire and water sides for inspections, and its much easier to replace tubes on a fire tube than a water tube. Generally speaking, if you have fluctuating demand and large swings in steam requirements, a water tube is probably a better fit. If you have a pretty constant load requirement without a lot of swing in steam demands, a fire tube would work fine. My next article will cover boiler components and safeties.

— Justin


Water Tube Boilers


Fire Tube Boiler

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