Author: Bryan Orr

Two days prior to this article being published I sent one out about the popular fallacy that nitrogen “absorbs” moisture. That tech tip went out at 7 PM eastern time like usual, and I was sitting on the couch watching something on the Food network (like usual).

At 7:10 PM I get a call on my cell and I look down to see the name Jim Bergmann displayed boldly on my screen. Whenever this happens it means only one thing… Jim read my tech tip and he has something to say about it.

“What did I say wrong THIS TIME” I mumble sarcastically into my iPhone

It turns out it wasn’t what I said, but rather, what I had forgotten to say that cause Jim to speed read, then speed dial.

So this tech tip is really Jim’s, even though my hands are the ones typing the words. He had a really good point to make about sweeping nitrogen BEFORE pressurizing with nitrogen.

Air is mostly made up of nitrogen, oxygen, argon and water vapor. The nitrogen and argon are inert and while we don’t want much of them inside a refrigeration system they don’t react with the oil, refrigerant, and metals in the system like oxygen and water vapor can (and often do).

When we call nitrogen “dry nitrogen” we just mean that it is nitrogen vapor alone with no water vapor or oxygen mixed with it. When we flow nitrogen at 2-5 SCFH during brazing we are displacing the air or “atmosphere” with nitrogen that contains no oxygen or water vapor that cause the nasty flakes of carbon to build up.

Before we start flowing at low levels we should first Purge or “Sweep” the system with nitrogen so that all the air is displaced out, to begin with. This should be done at a reasonable low pressure of 3-5 PSIG to help ensure that we don’t condense the moisture in the system into liquid water.

Let’s take a quick pause there… You may ask

Why on earth would pressurizing the system with nitrogen lead to liquid water condensation?

This occurs for the same reason that water condenses inside an air compressor. When you squeeze together those water vapor molecules with pressure the dew point temperature increases… until finally, it condenses into liquid water inside the system.

By sweeping the system with low-pressure nitrogen for 30 seconds or so you help displace and carry out that air and it’s water vapor with it before it has a chance to condense. Then you flow nitrogen while brazing, finally you are ready to pressure test.

What if you did no brazing?

In cases where you opened a system and only made repairs to threaded fittings, or used a low-temperature solder that doesn’t require flowing or installed a ductless or VRF system that has no brazed connections or used Zoomlock…

In that case, you would still want to do the nitrogen sweep BEFORE you pressure test. This will help decrease your evacuation time and keep your pump oil cleaner, longer.

So there you have it… from Jim’s mind to my ears, to this article, to your brain. Pretty good stuff.

— Bryan

 


Full disclosure, as a technician I was guilty for many years of setting the fan to “on” at the thermostat. I never really thought of any of the negative impacts that could happen.
I wanted to circulate the air and to keep air moving through the high-efficiency air filter that most of our houses had. Later I learned that in many scenarios fan “on” is not a good idea.

For this discussion, I will be talking about the cooling season in a humid climate. Many adverse impacts may occur in the heating season, depending on the region.

Things to be aware of when running the fan “on”.

Condensate on the coil after a cooling call with the fan running will evaporate back into the living space. Some thermostats combat this by having a fan off period at the end of a cooling call to let the coil drain.

If the ducts are in unconditioned spaces, outside the thermal envelope of the house, the sensible heat will be added back to the space. If the ducts are warmer than the air traveling through them there will be a transfer of heat. If there is any duct leakage latent heat (moisture) will be added as well. Latent heat gains do not only apply to return duct leaks. Supply air leakage can also contribute to this.

It is common that the HVAC system can cause the house to go under negative pressure. When this happens sensible and latent heat will be added. A common cause of the pressure imbalance is when the duct system is in an attic or crawl space, and the return duct has fewer connections than the supply ducts.

Since the supply has more connections than the return there is more of a potential to leak air. If the supply air leaks into the attic or crawl space this can cause the living space to go under negative pressure. The leaked air is replaced with either attic, crawl space, or outside air. One CFM(M3/h) in = one CFM (M3/h) out.

Ducts in conditioned spaces with panned returns can add latent and sensible heat as well. This happens when the panned joists and studs are not sealed by the HVAC contractor on all six sides. Joists and studs are part of the building network that when not air sealed during construction, by the builder, they will communicate with the air outside the building envelope. With blower door testing, and air changes per hour requirements now code in many jurisdictions houses are being built much tighter. In an older home, with panned returns, expect to be bringing in some outside air.

Even if the ducts are sealed, and 100% in the conditioned space it still costs money to run the fan. Running a PSC motor 24/7 can be costly. (ECM motors on a properly-sized duct system do have considerably lower operating costs when compared to PSC motors.)

The duct leakages and pressure imbalances mentioned above will also occur during a cooling call. Most of the time these issues can go unnoticed because of the ability of the HVAC system to overcome or mask them.
The goal is to get maximum customer comfort with minimum power usage and maximum system longevity. In many cases, the fan being left in the ON position detracts from these goals.

Hopefully, this Tech Tip will make you think twice about running the fan “on”. Every situation is different. I encourage you to think outside the box, if you are not already.

— Neil Comparetto

Recommended Duct Velocities (FPM)

Duct Type Residential Commercial / Institutional Industrial
Main Ducts 700 – 900 1000 – 1300 1200 – 1800
Branch Ducts 500 – 700 600 – 900 800 – 1000

As a service technician, we are often expected to understand a bit about design to fully diagnose a problem. Duct velocity has many ramifications in a system including

  • High air velocity at supply registers and return grilles resulting in air noise
  • Low velocity in certain ducts resulting in unnecessary gains and losses
  • Low velocity at supply registers resulting in poor “throw” and therefore room temperature control
  • High air velocity inside fan coils and over cased coils resulting in higher bypass factor and lower latent heat removal
  • High TESP (Total External Static Pressure) due to high duct velocity

Duct FPM can be measured using a pitot tube and a sensitive manometer, induct vane anemometers like the Testo 416  or a hot wire anemometer like the Testo 425. Measuring grille/register face velocity is much easier and can be done with any quality vane anemometer, with my favorite being the Testo 417 large vane anemometer

First, you must realize that residential, commercial and industrial spaces tend to run very different design duct velocities. If you have ever sat in a theater, mall or auditorium and been hit in the face with an airstream from a vent 20 feet away you have experienced HIGH designed velocity. When spaces are large, high face velocities are required to throw across greater distances and circulate the air properly.

In residential applications, you will want to see 700 to 900 FPM velocity in duct trunks and 500 to 700 FPM in branch ducts to maintain a good balance of low static pressure and good flow, preventing unneeded duct gains and losses.

Return grilles themselves should be sized as large as possible to reduce face velocity to 500 FPM or lower. This helps greatly reduce total system static pressure as well as return grille noise.

Supply grilles and diffusers should be sized for the appropriate CFM and throw based on the manufacturer’s register specs like the ones from Hart & Cooley shown above. Keep in mind that the higher the FPM the further the air will throw and more mixing will occur via entrainment but also the noisier the register will be.

— Bryan

First, let’s state the obvious and clear the air a bit. The photo above is SUPER CHEESY! But this story is about three bad techs who don’t know it so three models clearly posing in clean clothes makes as good of a proxy for a bad tech as anything else.

First off, I’m not being negative about the trade or making fun of people, the point of this story is to identify some traits that many of us may exhibit or see in others techs and it can be hard to identify our own issues or issues within your organization. See if any of these techs sound a LITTLE TOO FAMILIAR and maybe we can learn something. Before you ask… No, these are not real people…. probably… maybe

Randy the Drama King

Randy, like most dramatic people who work in the trades, doesn’t see himself as being dramatic. He just thinks he is being constantly disrespected by management and co-workers and customers are crazy and the dispatcher is out to get him and it’s always about to rain and that ladder (and every ladder) looks unsafe.  These things aren’t DRAMA they are FACTS in Randy’s world and if you question this reality you get added to the long list of people who are disrespecting him.

Randy starts conversations with customers with phrases like “you aren’t going to want to hear this” or “Do you want the good news or the bad news”. He also tends to pass blame to his coworkers or his company because they are just clueless and he knows what’s REALLY going on.

Randy is actually a good tech, but he get’s in a lot of conflicts with coworkers, customers don’t always like his negativity (or as he calls it “being honest”) and he is inefficient and largely unpopular with other techs and management. Randy knows people think poorly of him because everyone is conspiring against him with that blankety-blank dispatcher Donna!

Randy always feels persecuted by the people around him and usually has something negative or conspiratorial to share about every topic. Politics, The weather, customers, co-workers, spouses… you name it.

Here is a test you can take to see if you might be a bit of a Randy

  • You have more than 5 people you are ticked off with or avoiding at any given time
  • You consistently see “danger” around you that nobody else sees
  • During work hours you have multiple conversations over 5 minutes with others about things that are “wrong”
  • You use a lot of negative and fear-inducing language with customers

If you find you are allowing negativity and drama get to you the best practice is to give yourself a break from negative speech. Like your grandma used to say, “If you don’t have anything nice to say”. Negativity is a hard habit to break but the best time to start is now and the best antidote for negativity is gratefulness.

Bob the Excuseful  

Yes, excuseful is a word… I made it up and I like it.

Bob is confident that he would be able to do his job if he was just given the proper training and tools and enough time to do the job and enough sleep and wasn’t forced to work these ridiculous hours. Bob often wonders if he should go back to school and get his degree in …. something and all the courses he would take if his cheapskate boss would just invest in him.

Sure he was given a book and sent to a seminar last month but that was all THEORY, he is a hands-on learner and he CANNOT learn from books or videos or seminars or anything unless he can get his hands on it.  Once he DOES get his hands on it he can’t be held responsible for any mistakes he makes because he has to be SHOWN what to do and how to do it and if he isn’t SHOWN how can he be held responsible? Now, when he is shown, he is a hands-on learner so he can’t learn things by being shown… he needs to do it himself.

His truck may be a mess but he would clean it if he ever had time with these ridiculous hours but in the slow season that is his one time of the year to relax, you can’t expect him to take his own time during the slow season to clean his van can you?

Here are some indications you may be struggling with a bit of Bobish excusefullness

  • You feel jealous when others succeed  and immediately give some reason why they have an advantage over you
  • You read fewer than 5 books last year but still feel like your lack of education is someone else’s fault
  • You find yourself using “hands-on learner” as a reason for failing to understand something
  • When you don’t understand something you call or text someone rather than looking up an answer yourself
  • You have a sense that your lack of progress is due to a lack of “opportunity”

The best way to stop making excuses is to begin living and working with what old-timers called “grit” or “gumption”. This means doing whatever it takes to solve problems, making excellence a goal and going after it no matter the barriers. Start by reading and learning on your own, don’t wait for someone to show you or tell you, go get it yourself.

Todd the Careless

Todd knows he is just forgetful, he TRIES to remember to tie down his ladder and put the caps back on and close his back doors on his van but he just forgets sometimes OK!

Sometimes Todd get’s defensive when other techs call him out for leaving the panel off or “forgetting” to clean the drain, but usually Todd just apologizes and says he will do better next time, but he knows he won’t because he didn’t do it on purpose, it just …… happened.

Some of the “Grouchy” old techs have told him that doesn’t seem to care about his job, but they are WRONG! (in Todd’s mind) he does care, he just has other things going on in his life and in his mind and sometimes accidents happen… like the time he stepped through the attic ceiling, or the time he slipped on the ladder, and that one time he rear-ended that car in the parking lot… oops

You may be a Todd if ….

  • You regularly make mistakes where you “just forgot”
  • You find yourself looking at your phone, texting and using social media during the workday
  • Your mind is preoccupied with personal matters during work and while driving

We have entered a new era of carelessness due to the advent of smartphones, social media, and texting. Many of us find our minds constantly distracted by things other than work during the work day and it leads to poor outcomes, mistakes and safety hazards. everything from climbing a ladder, to driving, to filling out a service call requires ATTENTION and distraction can lead to costly and dangerous mistakes. The best advice is to put the distractions way during the work day… unless it is reading this article. Just remember to put the panels back on and run test the equipment when you are done.

— Bryan

 

 

 

 

 

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 fireplaces. My questions for you are, (these are just ideas for your podcast though help is never turned down)

On a millivolt 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 system (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,
–Matt


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 the valve open (so, closed circuit) around 200-300mV. But this is very general, it may vary a lot 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, the board is bad. If you have 24V, the 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 to 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 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.

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


When I started in the trade in 1999 there were still a lot of oilable blower motors in service. As part of the maintenance, we would remove the housing, oil the motor plus vacuum / wipe it down.

As oilable motors have become extinct I see fewer and fewer techs pulling the blower housing. Here are some reasons you may want to consider doing it more often.

  • Cleaning the motor itself can help it run cooler and last longer. A hot motor not only is more susceptible to winding breakdown but also to bearing/lubricant failure. Grab a vacuum, soft bristle brush, and a rag and get the dust buildup off the motor. If you have any dust that gets stuck inside, use some low-pressure nitrogen or compressed air to get it clean.
  • Get in there and look carefully at the wheel. A wheel that is even slightly dirty can have a significant effect on air output. If it’s dirty,  recommend cleaning.
  • Check the blower bearings, it’s easier to do when it’s out
  • On high-efficiency furnaces pulling the blower is a good way to check the secondary heat exchanger. On 80% furnaces, you can check parts of the primary exchanger and even the evaporator coil with a mirror or inspection scope.
  • Pulling the blower gives you the ability to wipe down the inside of the furnace or Fan coil.
  • You can check blower mounting bolts and set screws as well as blower alignment and balance more easily.

Obviously, when and why you pull the housing will vary from contractor to contractor but I advocate it being done more often than it is now.

What say you?

— Bryan

I remember it like it was yesterday… It was my first day of work as a trainee at my first technician job, just a wet behind the ears kid fresh out of trade school.

It was a Monday morning and technicians and I were standing in the dusty warehouse surrounded by stacks and stacks of brand new condensing units drinking the nasty warehouse coffee…

and I was LOVING IT

Finally, I had made it, one of the guys, listening to the war stories and well-natured ribbing and getting a caffeine fix for the day.

One of the senior techs was telling a story of low suction pressure and he said “So I figured it has to be the wrong sized piston” and he stopped and looked over at me and said “you know what a piston is….. RIGHT”

It seemed like an eternity passed as the whole group stared at me, I mumbled “a piston sure” and gave a weak nod hoping that “LIAR” wasn’t emblazoned on my forehead for All to see.

The tech turned and finished his story and my mind raced….

Of course, I knew what a piston was in an ENGINE or even a reciprocating compressor but I had no clue that the little hunk of brass with a hole in it that we called a “fixed orifice” in school was called a piston.

Later I learned all there was to know about sizing and replacing pistons. The installers I worked with often forgot to put in the correct size.

In case you are like I was, a piston is a fixed orifice metering device used in systems for many years. They are especially in residential heat pumps and straight cool systems. Even now that TXVs and EEVs are becoming more popular you will still see pistons in many new Carrier models being used outside as the heat mode metering device.

Piston Facts

There are three common piston designs I see regularly and while different manufacturers may use them I will group them by the manufacturers I know them by.

Lennox / Rheem Type

The piston shown above is the Lennox / Rheem style. It is directional, meaning it can only be installed one way with the cone (tapered side) pointed at the evaporator and the other side pointed at the liquid line. This type uses seals toward the end of the cone to help prevent refrigerant bypass and it also uses an o-ring to seal the “chatleff” style housing.

Carrier Type

Carrier used to call their pistons “accurators” and maybe still do although I haven’t heard that term for years. These pistons can be installed in either direction but still use the same “chatleff” style housing as Lennox

Trane Type

The Trane style has a much smaller size and is directional. The Trane housings do not use o-rings.

Piston Size

The physical exterior dimensions of the piston must be the same as all the others for that brand/series otherwise it will not fit properly. It is only the internal bore diameter that changes.

Pistons are sized in decimals of an inch like a gas orifice, usually from the 40’s up to the low 100’s. When a piston is described as being a “65 piston” that means it is 0.065 of an inch and a “104” would be 0.104 of an inch.

Check Flow Operation

In a heat pump system, every metering device needs some method of bypassing the metering device when the refrigerant flows in the opposite direction. This is done in TXVs by means of an internal or external check valve but with a piston, the piston itself is allowed to slide in the housing allowing restricted flow in one direction and unrestricted flow in the other.

This is actually where a piston gets its name, because like a piston in an engine it is a cylinder within a cylinder that can slide back and forth.

Any carbon, wax or other solid material that gets into the piston housing can cause one of three undesired conditions

Piston Restriction in the Desired Mode

If something gets into or covers the orifice bored into the piston it can cause a restriction resulting in low evaporator pressure, low suction, high superheat and normal to high subcool. When a piston is restricted and the system is a heat pump with a liquid line filter/drier properly installed, we will often alternate the system into cool and heat and see if that will break free the contaminants and catch it in the line drier. Otherwise, the piston should be removed, inspected and cleaned or replaced and a new line drier installed.

Keep in mind that some systems have a screen built into the piston housing inlet that can also block up. Look for this once the piston housing is disassembled.

Piston Bypassing (Overfeeding)

If the piston fails to seat properly it can overfeed the evaporator in the same way it would if the system had a larger bore size than it should. This will result in high suction pressure, low superheat and low subcooling. In these cases, the piston should be removed and inspected for proper bore size and signs of contamination around the outside or near the seal surfaces of the piston and the housing.

Opposite Mode Piston Restriction 

In some cases, a heat pump piston may fail to fully unseat in the opposite mode. This will result in a pressure drop and an undesired restriction similar to a clogged liquid line filter drier.  In this case there will be a clear temperature drop across that piston when there should be little to none.

For example, if you are running a system in cooling and you notice frost starting to form on the liquid line side of the outdoor, heat mode piston housing, you can be sure it is restricting in the opposite direction. Sometimes this can be resolved by switching back and forth from heat to cool a few times but often it will require disassembly and inspection.

This condition is similar to what happens when an external TXV check valve fails.

In Closing

A piston is a simple little hunk of brass, it drives me nuts when a tech incompletes a call so that someone can “replace a failed piston”. A piston doesn’t just fail, if one does have an issue it’s either the wrong size or something got into it and got stuck in it or caused it to stop seating properly. Many of these issues lead back to improper vacuum, failing to flow nitrogen, getting copper shavings or sand in the system etc…

Every good residential tech should have a little plastic container with various brands and sizes of piston in it in case you find one that is the wrong size or worn down from improper seating. I may be a little late to the game here since pistons are a dying breed but they are simple enough that a return trip for a “failed piston” seems like a huge waste.

— Bryan

I had an old-timer tell me that you can never connect two transformers together because they will “fight one another”.

If you are anything like me (and heaven help you if you are), whenever someone says something like that, a cartoon in your head starts playing.

In this case, I imagine two transformers with boxing gloves on duking it out to see which one “wins”.

The truth is you can connect two transformers together so long as you are careful, but you need to know why you’re doing it and then do it properly.

Transformers have a VA (Volt-amps) rating that dictates how many volt-amps (volts x amps, which is watts simplistically but there is a more complicated reason it is called VA in transformers that we won’t get into here) the transformer can handle on the secondary.

Above we show two 75VA transformers with 24V secondary windings.

75VA÷24V=3.125A

So with a 75VA transformer, you can run a maximum of 3.125 Amps, if you needed more power you would need to either go get a larger, more expensive transformer or…. you could connect another identical one in parallel. If you connected two 75VA transformers in parallel you would then have 150VA of secondary capacity which can be necessary in some cases with multistage commercial units or some large accessories.

In this case, parallel simply means connecting the two primary and secondary windings together in the exact same way as we show above… Pretty easy

It is SUPER important to get the polarity exactly the same and use two transformers with identical winding turns in the primary and secondary and identical secondary coil impedance (resistance).

In fact, it is so important that I advise that you only do this if you have two identical model transformers.

To be even safer, connect the primary windings first and check the secondary’s against one another with a voltmeter before actually connecting them to the system. For a typical 24v secondary you can connect the two common wires to ground to act as a stable reference first then check the two R or Hot side leads to one another and then to common. They should read 0v to one another and 24v to common. If you get anything other than 0v from hot to hot then you want to recheck your primary wiring and ensure that they are exactly the same.

— Bryan


In Florida, there are not many gas furnaces, At least not as many as up North. Sometimes we can look like real dummies compared to techs who work on them every day.

One thing to know about 80% gas furnaces with cased evaporator coils is that you can often check the evaporator coil by removing the high limit and running an inspection camera up through the opening.

You may also be able to use a mirror and flashlight but you usually won’t see much due to the heat exchanger being in the way. Otherwise, you are stuck removing the entire blower assembly… and that’s no fun at all.

Another practice is bench marking the static pressure drop across a new coil when it is dry and wet when installed or during the first service call. You can then easily watch coil loading over time without the need to look at the coil visually.

— Bryan

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