Month: February 2018

Measuring airflow is easy… measuring airflow accurately is quite a bit more difficult. In many cases when we as technicians measure airflow we are trying to get to the almighty CFM (Cubic Feet per Minute) volume measurement. You can take CFM readings fairly easily with a hood like the Testo 420 shown above, but even a hood has some limitations when the goal is to measure total system CFM vs. register / grille CFM.

In this series of videos Bill Spohn from Trutech tools demonstrates all of the tools you can use to measure airflow from hot wire and rotating vane anemometers, to flow hoods, to smart grids and pitot tubes, all the way down to using a GARBAGE BAG.

I had the privilege of seeing this presentation in person (I am the one behind the camera) and I wanted to share it with you. It is well worth your time.

— Bryan

My technician (and brother in law) Bert made a good point the day (It’s hard for me to admit it, but it’s true). When he needs to open the refrigerant circuit to make a repair regardless of whether he is recovering or pumping down, he pulls out his nitrogen tank and his regulator (We like the VN500 shown above).

Once the refrigerant has been fully pumped down or recovered, instead of opening the system to the atmosphere and exposing it to air and moisture, he simply puts it on “BRZ” mode and introduces a very low flow of nitrogen. Now when he cuts into the system to replace a line drier, or a coil, or a compressor, or an accumulator (you get the idea) the system will stay dry and it will be less likely that anything undesirable enters the system. You simply connect the regulator to your center hose and direct the flow to the high side, low side or both depending on what part of the system you have open.

Once the system is all dry fit into place you are then ready to flow nitrogen while brazing, pressure test and even triple evacuate if nitrogen is needed for that.

The biggest hurdle to getting techs to flow nitrogen while brazing is getting the nitrogen tank off the truck. If you get in the habit of connecting nitrogen before you ever cut or open the lines it even further reduces the chance that you “forget” and increases the chances that your system is clean and dry.

Just a thought (from Bert)

— Bryan

This tech tip video comes from my friend Andrew Greaves of AK HVAC and HVAC Comedy on Youtube and the HVAC Vehicle Layouts group on Facebook. Many residential techs get confused when they see these multi-position valves in larger equipment and Andrew does a great job of demonstrating the basics in this video.


In the video Andrew describes the following positions

Back-Seated (all the way out, fully counter-clockwise)

This position provides full operational flow through the valve body but is closed to the access port. These types of valves have no schrader port so there will be no pressure on the port when the valve stem is back-seated.

Front-Seated (all the way in, fully clockwise) 

Front seated closes the valve and shuts off flow through the system at that point while remaining open to the port. Depending on the valve design the port may be open to the inside or the outside (inlet or outlet) of the valve, this is an important thing to be aware of when closing. Some compressors have suction and discharge valves and you must not front-seat (shut off) the discharge valve while the compressor is operating or extremely high pressures will build instantaneously.

Mid-Seated (valve in the center position, clockwise around 50%)

Mid seating will provide flow in all directions in, out and to the port. Ideal for vacuum and recovery with the system off

Cracked off the Back-Seat (turned clockwise just a little)

This is a form of mid-seating where you just turn the stem clockwise enough to get a reading on your gauges. This is used for testing and charging.

— Andrew & Bryan

P.S. – Many techs call these King valves but technically a King valve is specifically a liquid line valve on the receiver

I’ve always opposed the practice of grabbing a hub puller as the first method of removing a blower. This video by Brad Hicks with HVAC in SC on YouTube demonstrates the exact method I use to safely remove a blower wheel without damaging anything. Thanks Brad!


 

Transcript

This is going to be on removing blower wheels from blower motors I see a lot of guys that will just get a fancy puller they won’t do any prep work on the motor shaft they’ll just drop their puller on there and try to remove it. Sometimes it works sometimes it gets everything bound up to where you have to replace the blower wheel.
Basically, what I do is just loosen the set screw,  you can see how rusty this one is right here but loosen the setscrew and force the wheel on the hub down farther first.  You can see now we have some clean metal and then I just take a couple minutes to just with a piece of Emery cloth or sandpaper just clean this shaft up really really good. Take a couple extra minutes to prep that shaft once that’s nice and clean you just grab something some sort of lubricant rust breaker wd-40 in this case spray a little bit on there and nine times out of ten this wheel will come right off no problems at all without any puller. I do hope that helps thanks for watching.
— Brad

infrared_yuck

There are three reasons why I don’t like infrared thermometers for many HVAC tasks.

#1 – The Laser is Misleading
The laser dot is just a point of reference, not an exact point where it is reading. Often the thermometer will read lower, higher or over a MUCH wider area. Unless you are right up on what you are measuring you can’t be sure the result you are getting is correct.

#2 – They Only Read Surfaces
An infrared reads surface temp only, not air temp. This is necessarily a problem, but “shooting a vent” is not the same as measuring the air temperature coming out of it.

#3 – They Can be VERY Inaccurate
Basic infrared thermometers are only accurate on a surface that has high “emissivity” of near 1.0. These are usually darker, less reflective, generally non-metallic surfaces. Metals have a low emissivity (much less than 1 generally) which means that if you are reading a pipe an infrared could read much lower than the correct temperature.

Infrared thermometers can be useful to do comparisons where reading the correct temperature is less important than comparing one spot to another, such as looking for hot spots in a panel, or checking a zone to see if a damper is open.

So long as you use the right tool for the job you should be fine, but in general….

I don’t like techs using infrared thermometers for most tasks.

— Bryan

P.S. – While I don’t like infrared, I REALLY like thermal imaging. Check out these nice products from Trutech tools 

 

This article is written by Christopher Stephens of JVS Refrigeration in California with just a few additions by me (Bryan) in italics. Thanks, Chris!


Reach in refrigerators are an interesting side of our industry, often looked at as frustrating and troublesome. Often working in kitchens or convenience stores the refrigerators are never located in a convenient place to work on them, and that tends to lead to frustration on the technician’s part. Please understand my article pertains to medium temperature refrigerators. I also advise you to use manufacturers OEM parts when possible as the unit was designed to work with them. One of the more misunderstood and misdiagnosed parts is the temperature controller.

Keep in mind that some refrigeration temperature controls sense the evaporator coil temperature (not the desired box temperature) some use intake air sensors and some use supply air sensors. The medium being sensed (Coil, return air (intake) or supply air (discharge) will greatly impact how the controls function and what impacts them.

Personally, I break temperature controllers down into five different types, please understand that these are generic descriptions and you should always lean on the manufacturer if possible to understand their control strategies.

  1. Standard Pressure Control – these work on the principle that at any given pressure saturated refrigerant is a constant temperature. This style of control is not used very much anymore as a means of temperature control because it is not very precise and to an untrained technician, it can be hard to set the temperature correctly.To use this control strategy, you need to understand what evaporator T.D. (Temperature differential) your reach-in was designed with, you will need a temperature pressure chart, you will need an accurate set of refrigeration gauges, and an accurate thermometer. With all these tools you can take your desired box temperature and find it on your pressure chart read across the pressure chart and find the corresponding pressure for your desired box temperature and that will be your cut-in pressure to set your control at. Now we need to find the cut-out setting typically we want the system to have about a 5-8 degree differential between the cut out and cut in to reduce system short cycling this will likely be about 8 degrees colder than the cut-in temperature, so take your desired box temperature subtract your differential of 5-8 then subtract your designed evaporator T.D. (specific to the equipment but likely 20 -30 degrees for reach ins) and find that number on the temperature pressure chart than read across the pressure chart and find the corresponding pressure and that will be your cut out setting.  Understand that pressure controls are never exact, so you will need to adjust accordingly in the field.
  2. Constant Cut in Control (electromechanical) – These are one of the most common temperature control’s that you will find in reach in refrigerators because they are the most economical for the manufacturers as they have an off cycle defrost built into them. They work by inserting the sensing bulb into the evaporator coil and they have a set temperature that they turn back on (cut in) at no matter how cold you turn the dial. They work very similar to the pressure control as they are designed with the evaporator T.D. (Temperature Differential) in mind, but instead of using pressure they sense the evaporator temp on the surface of the coil, they do have a knob to adjust the cut-out temperature, but you have no control over the cut in temperature that is why they are called constant cut in. By design they also have a built-in defrost as the cut in temperature is usually 37 to 41 degrees (for a cooler/refrigerator) depending on the manufacturer. They rely heavily on proper superheat and proper refrigerant charge. If the charge is incorrect or the superheat is not correct the coil could get too cold and the control could prematurely shut off. This could lead a technician to diagnose a bad control if they did not understand how they work.  If you come upon a reach in that is short cycling and shutting off too soon, make sure to check the charge and measure the evaporator superheat before you diagnose a bad control.
  1. Constant cut in (Digital) – These work the same as the electromechanical control, but they typically have two probes one to be located in the coil and one to be located in the return air stream. They tend to have more features that are available, such as an added defrost cycle based off time (every four hours, every six hours, etc.…..)  while still using box temperature as a fail-safe. For example, say the control has a defrost every four hours if the coil temperature comes above a pre-determined temp say 40 degrees the control will terminate the defrost. The controls can also shut off the evaporator fan motors during the off cycle to save energy and reduce warm is intrusion into the unit. These types of controls are on many HC (Hydrocarbon) units being built today.
  2. Universal electromechanical – these typically have one sensing bulb that you mount in the return air stream and they turn on and off via the temp setting.
  3. Universal digital – These are usually aftermarket controls and can have several different control strategies and can usually be customized to do anything, from heating to cooling to defrost depending on the manufacturer.

 

Something to understand is that reach in refrigerators are usually designed to perform in a certain environment and if something changes such as the ambient temp in that environment, or if doors are left open. The box will not perform correctly, I suggest you take a step back before you start throwing parts at a reach in and evaluate the environment you may find your problem there!

— Chris

As always I suggest “Commercial Refrigeration for Air Conditioning Technicians” by Dick Wirz as the bible for refrigeration training 

Microchannel is a coil type used in many evaporator and condenser coils and can easily be identified by its flat tubes and fins between them that appear as waves between the tubes. The technology was developed for use in the automotive industry and is used for radiators and automotive A/C condenser coils.

These coils are made of all aluminum and are used because of their superior heat transfer due to increased surface contact between the refrigerant and the metal as well as the lighter weight and smaller refrigerant charge.

These coils have come under a lot of criticism by technicians due to an undisputed high failure/leak rate of the condenser coils in some systems. Some have felt these failures occur to inherent issues with the design while others have stated that the leaks were due to specific manufacturing issues on a few coils and that these issues are largely in the past. No matter how you feel it’s likely that microchannel coils are here to stay due to the increased heat transfer efficiency and decreased weight. Here are a few things you need to know when installing and servicing microchannel coils.

Don’t Pump it Down 

MicroChannel condensing units are not sent with the full system charge and must have the charge added to manufacturers specs even with a short (say 15′) line set with more charge carefully added for line length.

You cannot, and must not attempt to pump down a system with a microchannel condenser or you risk causing a catastrophic failure of the coil.

Instead, you must recover the charge when making a refrigerant circuit repair and then carefully weigh the proper charge in after the repair is made.

Use Proper Brazing / Evac / and Refrigerant Practices

It’s right in the name “micro” channel. The flat tubes have tiny refrigerant channels in them and they are susceptible to blockage by any solid contaminants in the system. Make sure to flow nitrogen while brazing, install a new liquid line drier after making a refrigerant circuit repair and pull a proper vacuum (as always). You also need to take extra care to keep shavings out of the system when cutting and reaming and keep tubing ends and hoses away from dirt and debris. For example, if you replace a compressor, anything allowed to get in your pipework will hit the condenser coil before it ever reaches the liquid filter/drier and has the opportunity to clog part of the microchannel coil.

Wash Carefully

Most manufacturers advise against using any cleaner on microchannel coils to avoid damage. Either use a garden hose, low pressure “fan” pattern pressure washer less than 100psi or a cleaner that is approved for use with microchannel and work carefully. The refrigerant channels go all the way to the edge of the coil and can be easily damaged if impacted.

The Charge is CRITICAL

When charging microchannel you will want to follow manufacturers specs and weigh the charge in whenever possible. If you see low suction doesn’t just start dumping in charge until you are certain it is a charge issue and not an airflow issue or a restriction. Subcooling on microchannel systems tends to be more erratic due to the lower volume of the condenser coil.

Install Thoughtfully

Many manufacturers will swear that microchannel is just as resilient as tube and fin coils, based on my personal experience I would suggest taking greater care to protect microchannel. It may make sense to keep microchannel away from areas of the lawn that will have equipment going near the unit and possibly shooting debris into the surface.

When a microchannel condenser leaks it is often fairly evident by the oil stain that appears on the surface. Because of the channels, these leaks can be quite small, so if you see the telltale oil spot it is best to investigate.

Repair

I confess I have never attempted a microchannel repair myself, but there are many who claim to do it regularly. Here is a video showing it being done.

So take extra care when installing and servicing microchannel systems when cleaning, charging and repairing.

— Bryan


Breakers are designed to trip anytime the circuit draws a current above the rating for a period of time. The time the breaker takes to trip is a function of how high the circuit amperage in comparison to the breaker rating.

The higher the amperage above the rating the faster the breaker will trip

Breakers can accomplish this either thermally, by tripping on increased heat or inductively, by tripping on increased magnetic field when amperage increases.

The majority of residential circuit breakers are thermal which means they are more prone to trip during high ambient temperature than during low ambient temperature. This is one factor in why you will receive more nuisance or intermittent breaker tripping calls on a hot Summer day.

Many times breakers get replaced just for doing their job and tripping when they should.

There are five common causes of breaker tripping. Improper circuit design, Overload, ground fault, leg to leg short and breaker issues

Improper Circuit Design

Improper circuit design can result in an overload condition when the circuit ampacity (amperage capacity) or the circuit breaker size is not properly matched to the load to begin with and / or someone added additional load to the circuit later on.

For HVAC equipment this means that the circuit size should be matched to to MCA (Minimum Circuit Ampacity) and the circuit breaker or fuse should be matched to the MOCP (Maximum Overcurrent Protection)

If the conductor is smaller than the MCA rating or the breaker is smaller than the MOCP rating It can result in a tripping breaker.

You will also see cases where more than one system will be connected to one circuit breaker which is incorrect unless the systems have additional, independent overcurrent protection.

These issues usually causes an intermittent trip as it takes time under load to show up depending on the severity of the issue.

Overload

An overload condition occurs when the loads draw more current / do more more work than they are designed for. Common overload conditions would be a compressors locking up, motor bearings binding, blower belts too tight or sheaves adjusted improperly. And overload generally occurs with inductive (magnetic) loads like motors in cases where the motor is either being placed under a greater torque load than it’s designed for or the motor itself is beginning to fail mechanically.

Overload conditions often don’t trip a breaker because the motor itself will usually have an overload that specifically protects the motor. This is why when a compressor is locked it is much more likely to shut off on thermal overload than it is to trip a breaker even though it will draw far higher amps than the breaker rating on startup. In these cases the thermal overload is designed to respond quicker than the breaker.

If a breaker is tripping because of an overload condition it will usually be after several seconds, minutes or even hours of operation. It will not be “instantaneous” unless someone installed the wrong breaker or fuse and used an “instantaneous trip” instead of a typical “slow blow” or slow acting type. This would be quite rare.


Ground Fault

A ground fault is a short circuit (no load path) between an energized circuit and equipment ground.

A ground fault is the most common cause of instantaneous breaker tripping

In most ground fault situations there will be very amperage, very quickly resulting in a breaker that trips right away.

Common cases would a shorted motor, such as a shorted compressor or a rubbed out wire.

A combination of visual inspection, isolation and ohm measurement to ground and megaohm / hi-pot tests or hot verification as needed is the best way to diagnose a short to ground (ground fault).

Leg to Leg Short (Bucking Phases)

When you have two legs of power that have different sine wave patterns such a 240V single phase or 3 phase power you must prevent the legs from coming into contact except through a load.

If they do come in contact there will be an enormous transfer of energy and a significant arc.

This can happen when two wires rub out, when switch gear becomes compromised or within a motor.


Many times techs will look for short circuits from “leg to leg” or “winding to winding” in a compressor or a motor without first measuring to ground.

This is not a good idea
Even when a motor does short “winding to winding” it is rare that it just stays shorted. Usually it will ALSO be shorted to ground or it will be open after the arc flash that resulted from the short.


Think of a circuit board. Circuit boards short out all the time and the result is a big black spot on the board and nothing works anymore (open). It rarely results in a continued short circuit because the arc from the short blew the connection apart.

The reason I encourage caution is because I have seen many junior techs condemn good compressors due to a “leg to leg” short just because the ohm reading between Run and Common appeared low to them.

The only way to know if a single phase compressor is shorted “leg to leg”  with an ohmmeter is to know what the windings should read in the first place.

On a three phase motor all three legs should read the same ohms leg to leg which makes it considerably easier.

When you do encounter leg to leg (only) short circuits it is more often on fan motors than on compressors.

Breaker Issues

Because most breakers trip due to heat, anything that causes the breaker to get hotter than normal can result in tripping.

This can be due to a poor connection inside the breaker itself, but often it is due to a poor wire connection on the breaker or a poor connection between the breaker and the bus bar.

Usually these types of breaker issues are caused by installation problems such as loose connection, wrong breaker type, failure to use anti-oxidation paste on alum to copper or excessive tripping / using the breaker as a switch.

Here are some tips for diagnosing a tripping breaker

Tripping instantly

  • Perform a visual inspection of all wires and connections. Look for signs of rubout, damage and arcing
  • Isolate components and ohm to ground
  • If you are unable to locate with an ohmmeter use a megohmmeter to ground (with caution especially on scroll compressors)
  • Finally, once you believe you have identified the cause, fully disconnnect the shorted component and power the unit back up and make sure everything else functions.

Tripping intermittently or after more than 3 seconds

  • Visually inspect all electrical connections and ensure they are clean and tight.
  • Inspect the breaker and bus bar connections
  • Check breaker and wiring size
  • Measure running voltage and ensure it is within +/- 10% rating
  • Measure for voltage drop during startup (less than 15%) as well as between the power source and right at the unit (less than 5% overall)
  • Measure component amperages while starting and running and compare to manufacturer specs
  • Measure motor and compressor temperatures and watch for temperature increase over time. Infrared and thermal imaging can assist with this
  • Watch for anything that can cause overload such as failing bearings, belts too tight, or sheaves adjusted for too much RPM
  • Measure current right at the breaker, if it remains below the breaker rating and the breaker STILL TRIPS, only then replace the breaker.

I also learned recently that AFCI (Arc Fault) breakers generate heat internally which mean that you will see a hot spot on them with a thermal imaging camera or IR thermometer.

Don’t replace a breaker unless you know it’s failed and don’t condemn a part as being shorted unless you can isolate it out of the circuit and every other component still functions (as possible)

— Bryan

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

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