Month: October 2017

This article was written by boiler technician Justin Skinner. Big thanks to Justin for being one of the rare techs who cares enough to take the time to write something like this. Thank you Justin

Pressure switches are used in a variety of applications. Generally, they are a normally open switch that closes with either a rise or fall in the pressure it is monitoring, They are used mostly as a safety device, but are also used for operation control, such as fan cycle controls for low ambient cooling, starting and stopping for steam boilers, and the list goes on. Today I am focusing on the pressure switch used in gas furnaces that prove inducer fan operation.
As noted above, the pressure switches used for gas furnaces are almost always normally open.

The board first checks the switch to ensure it is open prior to inducer call. If it is closed prior to call it will go into fault because the switch is clearly jumpered, the circuit shorted or the switch failed closed.

During preignition, when the inducer motor is running before the flame is established, it draws the heat exchanger into a negative pressure (draft). The pressure switch is used to prove this draft by a connection, typically rubber or vinyl tubing, directly to the heat exchanger or to the inducer draft motor housing. In the normal sequence of operations, if the pressure switch does not close during this preignition period, the control board will not allow the furnace to light. This is to ensure a few things. First, that the inducer motor is operational and not failed, inducer wheel broken, etc. Second, it proves that the exhaust pathways are clear. If the chimney is caving in, or Mr squirrel makes a nest in the top of the flue stack, the inducer motor will not be able to establish the draft (negative Pressure) required to close the switch. The switch should stay closed during the entire run cycle also.

If the control board determines that the inducer motor has been running for an allotted amount of time and the pressure switch has not closed, it will lock out the furnace and go into a fault situation. It also locks out into fault if the switch opens enough times while the flame is lit. We will use Carrier as the example. When the switch does not close, or opens during operation, and the furnace locks out, the led light on the control board flash a fault code 31 which indicates the switch did not close. If the switch does not open after the furnace satisfies the heating demand tries to start again, the board will flash a code 23. Code 23 would be seen as 2 short flashes, followed by 3 long flashes.

So what’s the proper procedure for troubleshooting if you come across a furnace displaying these fault codes? Most of the time the issue is pretty apparent after you reset the board and run the furnace. The inducer motor is failed, or a family of possums used the chimney base as their summer home, or most uncommon of all, the pressure switch is bad. But what about the when it’s not so apparent? When you reset the board and the furnace lights and runs fine and customer runs down and tells you how great you are? More than likely if you leave it at that, that same customer who praised you will be calling in again, but probably won’t be as happy with your service skills as before. So let’s dig in and check a few more things.

In the thousands of no heat calls I have run, I can count the number of pressure switches that failed on their own with no other factors on one hand. They just don’t fail very often. I would need a lot more hands to count the callbacks I’ve run where another tech condemned and replaced a pressure switch, only to have the same issue not long after.

Callbacks cost money and they always hurt my pride when they were mine, so the goal is to save both. So the first thing to do in this situation is slow down. This time of year (December) crazy busy for everyone, but the most important call you will run today is the run you are on right now. When I am in a hurry, I’m much more likely to miss things, and when that happens I normally end up coming back to the call anyway.

Next, Grab your trusty volt/amp meter and manometer. I prefer the digital version for both. Check incoming voltage to the furnace and make sure it is within spec of the voltage rating on the data tag. Check the amperage rating on the inducer motor, and make sure the actual amperage is in range. Let the inducer motor run for a while. If the amperage starts to go up substantially, or the motor gets noisy or too hot to touch, there is potentially a motor issue. Check the tubing that connects the pressure switch to the draft point. Look for cracks, brittleness, and crud or water inside the tubes.

If you see anything, clean it out. Hook a manometer to the tubing and check the draft from the heat exchanger in inches W.C. with a manometer or Magnahelic. The pressure switch has a pressure at which it closes, which is typically negative. Ensure the draft is above the required pressure. By this point on an 80% efficiency furnace, if you haven’t found the culprit, you will have to put on your best thinking cap, because the issue could be any number of things, including a bad board, loose or bad wire connections, or a bad pressure switch.

If you are working on a 90+% (condensing) furnace, move to the condensate drain. These furnaces produce water as a result of lower exhaust temperatures, and that water is removed from the furnace in a few ways. Carrier has a condensate trap that is a white box that mounts in the blower compartment on an upflow application. If this trap is partially clogged, water can back up in the secondary heat exchanger. This can prevent the necessary air movement required to produce enough draft to close the pressure switch. And if it’s only partially clogged, it may have drained when the furnace was locked out and not running. So when you get there and reset it, everything runs fine until the condensate starts to back up again. Also, where the condensate water goes after it leaves the furnace is important to note. Does it go to a condensate pump? Does that pump the water outside? Is it freezing outside? It could have been last night, which caused water to back up and the furnace to lockout, but today its 40 degrees, everything has melted, and running smooth.

These are just a few things I’ve seen with consistency over the years, but the potential number of causes for the fault codes listed above are almost limitless. And every heating season I run into an issue I haven’t seen before. This is in no way a comprehensive checklist. The goal of this is to prevent a technician from replacing too many parts that don’t need to be replaced, which I did early on more times than Id like to admit. Have a safe and merry heating season, and like we all hear from time to time, don’t be a hack!

— Justin

(Approved and edited by Bryan)

Some techs and contractors swear that flex ducts are an evil invention and should never be used in ANY circumstance. I agree with what duct design expert Jack Rise said on the podcast when I asked him about flex ducts he said:

“There’s a lot of problems with flex duct, there really is and it’s a good product but we abuse it…. It’s a good product, it’s just poorly handled”

While the proper sealing of ductwork in unconditioned spaces is nearly universally recognized as important, it is rare that a flex system get’s installed properly in these other important areas.

Fully Extend The Flex 

Some guidelines suggest pulling a 25′ piece of flex fully extended for 1 full minute before attempting to install it. This reduces the compression and the depth the of the corrugation (the accordion spiral inside the duct). The more compressed the duct is when it’s installed the greater the air resistance of the duct will be. The air duct council states that 30% of compression can result in 4 TIMES the air resistance. This means that fully extending the flex is a big deal and may be one of the most overlooked aspects of flex system installations. Cutting off that 2′ – 6′ of extra flex on the end instead of just “using the whole bag” can mean the difference between a good and a poor duct system in many cases.

Strap and Support the Flex 

Jack Rise spoke about how he tested a duct and measured a .2″ wc change in static when he altered a duct from sagging to properly strapped. In retrofit applications, many companies focus on “sealing” connections but they often don’t truly address sagging ducts with proper strapping. the allowable amount of sag is only 1/2″ per 4′ of length which isn’t much. Don’t ONLY rely on the code required strapping in your jurisdiction, just because a system passes inspection doesn’t mean it’s installed correctly.

Keep the Curves to a Minimum 

When designing a duct system you must calculate TEL (Total Effective Length) not just length. In a flex system each curve has a HUGE impact on the TEL and when a field install doesn’t match the design it can throw the whole system out of whack both from an air balance standpoint as well as a system performance by increasing the TESP (Total External Static Pressure). Every bend and angle matters so keep it extended, properly routed and well supported and all will be well so long as the design is correct.

For more info go to the ADC (American Duct Council) website at or download their excellent guide HERE

— Bryan




Good friend and contributor to HVAC School Neil Comparetto made this video showing the way in which he creates access ports for static pressure and gas combustion analysis. As techs we find ourselves in the tough position of needing to drill access holes to take measurements but the drilling and sealing of the holes can sometimes create real and perceived issues with the equipment. Many techs use high temp RTV Silicone, Rectorseal Duct Seal compound or even tape. In some cases these sealants may be appropriate but Neil shows how he uses plugs to make a good permanent access point. Always make sure to leave any work is a well sealed and workmanlike condition.

You can find many of these items at Trutech tools HERE 

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


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

I’ve heard a lot made of clocking gas meters over the years and honestly, in Florida there isn’t too much call for for heat and even fewer furnaces.

I was pleasantly surprised when I found out how easy it actually is. Here is how you do it, step by step.

#1 – Make sure all gas appliances are off other that the one you are clocking. Even shut off pilot lights or it can mess with your reading.
#2 – Make sure the appliance you are checking is running at high fire (max output)
#3 – Get a stopwatch (your phone has one)
#4 – Watch the smallest unit dial on the gas meter, it will often be 1/2 cubic ft
#5 – Time how long that dial takes to make one full revolution with the stopwatch
#6 – Multiply the dial size by 3600 (3600 is the # of seconds in an hr) so if it’s a 1/2 cu/ft dial it would be 1,800
#7 – divide that # by the # of seconds it took. So lets say it took 22 seconds that would be 1,800 / 22 = 81.82
#8 – Multiply that # by the BTU heat content of 1 Cu/Ft of gas provided by the utility. If it is 1,000 (which is common for NG) the total BTU per hr would be 81,820

The complete formula is Cubic Feet per Hour (CFH) = (3600 x Dial Size) / Time (seconds)

Then to get the ACTUAL device output in BTU’s you would multiply for the AFUE efficiency. In this case if it was an 80% furnace the input is 81,820 btu/hr and the output would be 65,456

Pretty cool huh?


In a Series circuit (loads connected in a row end to end) it’s easy to calculate total circuit resistance because you simply add up all the resistances and you have the total.

In a Parallel circuit the voltage is the same across all the loads, the amperage is simply added up but the resistance is a bit more tricky.

It gets tricky to imagine because the total circuit resistance of parallel loads goes down the more loads you add. 

For example, if you have one light bulb connected to a power source, the total resistance of the circuit is just the resistance of the bulb. 

Add in another bulb in PARALLEL and the resistance of the circuit goes DOWN

When you are calculating the total resistance of a parallel circuit you take each individual resistance and divide it into (not by) one. You then add up all the resistances that were divided into one and divide that sum into one. The formula looks like this for the diagram at the top of the article.

1÷Rt (total resistance)= 1÷R1 + 1÷R2 + 1÷R3

For this particular application as shown above it would be.

1÷Rt(total resistance)=1÷120 + 1÷45 + 1÷360

So 1 ÷ 120 = .0083 + 1 ÷ 45 = .022 + 1 ÷ 360 = .0028

Then we add them all up

.0083 + .022 + .0028 = .0331 

Then to find the total you divide one by the total

1 ÷ .0331 = 30.21 Ohms total 

As you will notice, 30.21 Ohms is less than the lowest resistance in the circuit. This makes sense when you think about ohms law.

The lower the resistance the higher the amps. Adding in additional parallel loads INCREASES the amperage in a circuit, and we see this ever day when we notice that compressor amps and condenser fan amps added together equals total condenser amps.

So it stands to reason if lower resistance equals higher amps and adding in more parallel loads increases the amps, then adding in more parallel loads reduces the resistance.

Another myth this busts is the idea that electricity ONLY takes the path of least resistance. Electricity actually takes all paths between positive and negative charges and every additional path (parallel circuit) just decreases the resistance between the two points of potential difference. This increases the total circuit amperage, which is why when you try to run two hair dryers on one 15a circuit the breaker trips. Two hair dryers in parallel = lower  total circuit resistance = higher amps. 

Not that I would use two hair dryers….. maybe that’s why I’m almost bald.

— Bryan

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