Category: Tech Tips

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,

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

Most of the content in this article is based on Alex Meaney’s contribution to the HVAC School podcast in the October 2nd, 2020 episode: “How to Get The Most From Online Education.”


Attending a class from your bedroom or home office sounds very convenient, right? That was probably what many Americans would have thought several months ago. Now, many of us shudder at the thought of learning remotely. With distractions and technical difficulties galore, many students and trainees have struggled with online education. Maybe you are one of those people, but fear not! Rock Star HVAC design educator Alex Meaney addresses the challenges of online education and offers advice for struggling students and educators alike. He has provided some crucial tips for overcoming the obstacles of distance learning and getting the most from your online education.


Take advantage of early reading


It’s a good idea for educators to provide educational materials before an online training session or class. If students or trainees have access to readings and videos ahead of time, they may enter the class or training session with a basic understanding of the concepts. Educators, it’s on you to set your students up for success.


Students, I’m afraid you aren’t out of the woods yet. It helps to go into a class feeling prepared. You may want to familiarize yourself with heat gain/loss equations before entering a psychrometrics class. That way, you will be ahead of the curve and won’t have to waste valuable time memorizing formulas.


Meaney also stresses the importance of learning vocabulary before attending a class. It helps to feel like you “speak the language” of the topic before attempting to grasp challenging new material. For example, you don’t want to look like you’ve seen a ghost when the educator says a word like enthalpy in a psychrometrics course. 


Look for opportunities to tutor difficult subjects


Meaney also gives a seemingly counterintuitive piece of advice for students: seek opportunities to tutor confusing subjects. Tutoring increases a student’s investment in the subject by making them accountable for your classmates’ successes. 


Some of you may enjoy the added pressure, and some may not. Regardless, you will obtain a more in-depth understanding of the material if you teach it to others. It also feels great to help a friend grasp a tricky topic. Who doesn’t want to be a good person and benefit from the experience?


Take a class with a friend


Taking a class with a friend is also a good strategy for getting the most from remote education. Studying with a buddy is useful for filling the gaps in learning. One person may better understand a topic than the other and vice versa. 


Manhattan Institute lists a few other benefits of friendship in education. Even in the virtual classroom, friends provide comfort and security. You may feel more comfortable asking questions or requesting clarification if you aren’t afraid of being judged by your classmates.


Friendship is also a source of support in the learning environment, even in nontraditional, remote settings. Students reinforce their learning when they tell their friends what they found exciting or challenging about a class.


Educators: show your faces


Educators can create a more engaging environment for their students by adding a video of themselves explaining the content in videos or PowerPoint slides. Although these learning formats do not compare to lectures in real-time, it’s nice to see a human face among diagrams and charts.


Work in a clean, distraction-free zone


This is a big one. Maintaining an organized workspace is a must for students and educators. For most people, this means limiting distractions and maintaining the cleanliness of the workspace. 


Putting away cell phones is a good start. Putting the cell phone away minimizes the temptation to look at text messages or social media during class. The last several times I checked, #HVAC wasn’t trending on Twitter anyway.


It also helps to remove irrelevant documents and materials from the workspace. Paying bills is essential, but nobody wants to think about that while they’re trying to learn a new skill. Even if you just need to shove them on the floor for a bit, it’s better than keeping them in front of you.


Closing background programs and internet tabs on the computer eliminates clutter on the screen. We know you enjoy listening to HVAC podcasts and checking out family photos on Facebook, but class time is not the time for that. The webpages will still exist when the lesson is over. 


Meaney also recommends taking notes with a writing utensil, not on a phone or tablet. Involving the hands in the learning process is an excellent way to engage the body while taking in new information, especially for those fidgety hands-on learners. (If writing is not required, try squeezing a stress ball while you learn!)


Ensure a stable internet connection


Anyone who shares an internet connection with other people may consider minding their bandwidth usage. This means ensuring that other people in the home or office don’t stream videos or play video games online.


Many people love watching The Mandalorian and playing Grand Theft Auto V online, but heavy bandwidth usage strains internet connection. A spotty internet connection can result in long download times, buffering, and disconnection from online lectures. 


It would be best to let roommates and family members know when it’s class time. That way, they can do their part to respect and support your remote learning process. Netflix, Disney+, and the PlayStation Network can wait.


Set aside additional time to ask questions


Even with scheduled classes, students can benefit from managing their time. Students could consider reserving some time after a lesson to ask questions or do some independent learning. (For example, a student could set aside 2 ½ hours of their day for a 2-hour class.) If you don’t end up using the additional time, then it becomes free time. You can study some more, walk the dog, or take a nap. It’s your time. It is better to set aside more time than necessary than to plan poorly and need more time. 


Consider using the extra time to take advantage of the instructor’s availability. Ask questions! They love helping students understand their lectures. After all, they would not have chosen to teach if they did not enjoy helping others. There are plenty of jobs that pay better.


Don’t be tempted to skip material


It may be tempting to skip through sections of pre-recorded videos or speed up the playback, but it’s best to refrain from zipping through the first playthrough. Yes, the educator may talk slowly. Yes, you may already know what they’re talking about. That could change within seconds, and you don’t want to miss out on new information you’ll need later.


It’s okay to jump around and bypass some sections on subsequent playthroughs. You may choose to skip around the learning materials to focus on specific topics. Still, it would help to understand the terms and general subject before turning your attention to individual parts.


Take care of yourself


Not many people think about comfort in the learning environment, but it can significantly help the learning process. Exercising self-care is vital for getting the most from remote education. Hunger and thirst can ruin your mood and make it more difficult to concentrate. 


Bring a glass of water to class and have some snacks within reach. After all, eating or drinking during an online lecture is not as disruptive as eating in a classroom or auditorium.


Be aware of your microphone and webcam status


Students and educators can benefit everyone in the class by being mindful of their video and audio settings. 


Webcams are great because they can add a social component to distance learning. However, they can also disturb other students. Please wear appropriate clothing and monitor the activities of pets or other background distractions. Yes, everybody would love to see fido and tell him he’s a good boy. No, he is not helping anybody learn about the nuances of Manual J. . 


Students should ensure that they remain muted unless they have permission to speak. Even though we are strong proponents of staying comfortable in class, nobody wants to hear you crunch Doritos. Everyone’s online learning experience will improve if everyone makes an effort to be considerate and use technology appropriately.


Have faith that you will learn


The best thing students and educators can do to get the most from remote education is to remind themselves that we are all capable of learning. Distance learning is difficult for many people, and anyone who struggles with it is not alone. 


Getting the most from remote education primarily entails limiting distractions and temptations. Students can succeed with a healthy amount of educator guidance, discipline, and self-compassion.


The industry leaders are seeing positive trends and have faith in you, too. Dominick Guarino, chairman and CEO at National Comfort Institute, acknowledges that the HVAC industry is a little behind when it comes to online education. However, he and other industry leaders are optimistic about the progress contractors have made in distance learning over the past year. 


Educators can do the following things to maximize the effectiveness of their online classes:


  • Provide preliminary readings or vocabulary lists
  • Attach their face to the content to boost engagement
  • Make themselves available to answer questions
  • Be honest with what they do and do not know; follow up with students who ask difficult questions


Students can do the following things to get the most from their remote education:


  • Study materials and learn vocabulary ahead of time
  • Take a class with a friend
  • Seek opportunities to tutor difficult subjects
  • Set aside more time than the class itself
  • Ask questions!
  • Stay disciplined and committed to learning


Educators and students can benefit from:


  • Keeping a clean work environment
  • Minding bandwidth usage within their homes
  • Being aware of their microphone and camera usage


Remote education is efficient and cost-effective, but it presents a unique set of challenges for educators and students. 


To succeed in an online curriculum, students must hold themselves responsible for their learning. Taking responsibility includes limiting distractions within the workspace: putting away cell phones, closing unnecessary applications and tabs, and removing workspace clutter. 


Students should also challenge themselves and take advantage of opportunities to maximize their learning potential. These opportunities include tutoring other students, learning with a friend, asking questions, and setting aside additional time to learn the material. 


Students will learn to prepare themselves for class, manage their time, and take care of themselves in a challenging, unfamiliar educational environment. 


Educators can improve their students’ learning process by providing videos of themselves explaining the content, providing preliminary reading materials, and following up with students who ask complicated questions.

The biggest thing is to prepare and be intentional and learn how to learn in this brave new world we find ourselves in.

Not if you are hungry anyways-

A true story. I have a good friend that owns an HVAC company. Not much of a PHD but he is known to be honest, doesn’t pull vacuums through manifolds and claims to almost always use American made capacitors. We’ll call him Captain Kirk.

He called me recently, quite worked up about one of his jobs. He had just replaced the AC system for a newly acquired client, a family of 4 that had just bought the house. As the summer was gaining momentum it started to get hot and muggy inside, so they figure they’d replace their AC system with a new one and all their comfort problems would go away.

The brand-new HVAC installed is a 4 ton, 16 SEER AC system with a constant torque air handler motor. Featuring a whopping 10 years warranty and a whole lot of disappointment for the client so far. “Why a 4 ton?” – I asked. “Because it’s what was there” – my friend the installing the contractor replied.

For the last month, however, Captain Kirk has had to keep going back. Double-checking everything, and despite doing little tweaks here and there, the system would just not keep up. “It only cools down at night when the thermostat shows it goes down to 72 & 88% relative humidity (WHAT?!) but then, even if we set it to 65, it runs all day and the temperature in the house will go up to 80 in the afternoon!” – The clients cried. “I want a 5 ton!” – LOL!! I actually heard them say that.
I agreed to try and help my friend out. So, I reached out to Mr. “I want a 5 ton” to set up an appointment.

The House & HVAC

The house is a 2 story, 2,260 square feet, 1989 built slab on grade with a vented attic. Ducts are running outside the envelope through the attic and in between floors. Typical of climate zone 1.
Seemed like enough capacity for the house to me. But to be sure I ran a blower door test and a load calculation. The blower door number came in quite high at 3,230 CFM50. That is 1.4 CFM of leakage per square foot of area at a forced pressure differential of 50 Pascals or a 1.4:1 LAIR. You can read up on the relevance of that number here.

That corresponded to a total of 186 cfm worth of infiltration (air from outside the space being forced in) under regular/average operating conditions in the load calculation estimate. The total cooling load calculation came in at 42,322 BTUH, with a .74 sensible heat ratio. That means that 31,169 of those BTUH must be of sensible cooling and 11,153 of latent.

Ok so, could the installed unit not be adequate then? I went and dug out the extended performance data from the manufacturer of this 4 ton. When compared to the cooling capacity requirements for the house this is what I’ve got:

Something wasn’t right. This unit should have plenty of capacity to handle the load in this house. Literally thinking outside the box, I took a house pressure measurement with reference to outside. For this, I slid a piece of tubing in between the door and the frame, this measurement was taken with all interior doors open and only the HVAC running. More about this procedure is explained in this video.

In this case, the pressure went down to 2 Pascals (0.008 inH2O) – a tiny pressure, but it can have a huge impact.  Now we’re getting somewhere!

If the pressure in a house drops when you run the HVAC equipment, that means air is leaking from the envelope to the outside, in this case, if the supply ductwork were leaking it would be to the attic, or the space in between floors outside the building envelope, since the blower door test revealed that it is so leaky. – As a side bar; in newer, well-sealed homes the space between two conditioned floors is considered conditioned space.  In a well-sealed home (draft stops and sealed rim joists), if this space is pressurized the air is more likely to go back into the house than to the outside.

With this in mind, my next step had to be running a delivered capacity check using MeasureQuick. For this, I averaged the supply air temperature at the vents of the two coldest rooms and two of the warmest ones (of course one of them was the master bedroom). The TESP was quite high at 1” WC. The fan tables for this air handler doesn’t go up that high. So, using a TrueFlow Grid I measured approximately 1,200 CFM. This is what I got back:

The Energy Conservatory makes a free software tool called See Stack that can be used to estimate how much leakage (or exhaust flow) it takes to change the house pressure by 2 Pa. The fact that the house is so darn leaky makes this small pressure very significant. These screen shots show the difference between no depressurizing leakage on the left, and enough depressurizing leakage to make the pressure in the house 2 Pa lower, shown on the right. It takes about 384 CFM of exhaust flow – or leakage – they have the same effect on the house. Remember this was with only the HVAC running. This is even worse if any exhaust fans, the dryer or the kitchen hood are running.

“Pffft…all this trouble to come up with a duct is disconnected?! You could’ve just crawled in the attic and find it within minutes” – you might say. I did go up in the attic. It was quite spacious, actually (for an attic), so much so that all of the duct connections in the attic were accessible. There were no disconnected ducts up there. Whatever the issue was causing this depressurization, was not accessible unless some walls were cut open. Again, all exhaust ventilation devices disclosed were off.

Remember how in the load calculation the amount of infiltration that the HVAC had to handle was 186 CFM? Well, according to our new findings it looks like its closer to 380 CFM, that’s double the original number. If the load calculation is adjusted to reflect this then:


First, notice that the trued load with the HVAC driven infiltration is about 12,500 BTUH more than the Manual J load calculation tells us – more than 1 ton!  Now look at the sensible and latent numbers.  The sensible load didn’t go up much – only about 12% or about 4000 BTUH.  But the latent load is up by 80%!  And that’s a problem because the system doesn’t have that much latent capacity, hence the 88% relative humidity reading by the thermostat, especially when “it cools down to 72 at night”.

The manufacturer says that the equipment makes 47,000 BTUH total, which is 35,600 BTU/h sensible and 11,400 BTU/h latent.  Those numbers cover the sensible and latent loads of the original Manual J loads nicely.  But after we add in the HVAC driven infiltration, we’re covering just over half the latent load.

So, between the real load being higher and 26% of the HVAC’s normalized capacity being lost out somewhere we can’t see, now we’re only covering about 60% of the total load, and 63% of the latent load.  Things are starting to make sense.  The inability to maintain set point on any days near design and very high indoor humidity fit very well with what we’ve calculated so far.

The second bar of the graph below shows the capacity reduction, and the 3rd bar shows the load increase. They are now WAY too far apart.

So, do they need a 5 ton instead of 4?

As I’m mid-briefing the installing contractor on my findings so far, he asks: “So, do they need a 5 ton instead of a 4? I just want this to go away you know”

No, they don’t. Installing just about any HVAC in this house without improving the shell and ducts is like eating soup with a fork.  If you go at a bowl of soup with a fork, it will get you some of the big chunks in the soup, but there’s no chance of getting much broth. In fact, if you keep eating it that way, what’s left in the bowl will get more and more watery. Leaky duct(s) and envelope are similar, you’ll get some of the “big chunks” of cooling delivered where it should, but lots of it is slipping through the cracks, and the house is getting wetter and wetter.

But what if you just install a bigger unit, a 5 ton in this case? What we’ve got with the 4 is something like this:


Replacing it with a 5 ton it’s like getting another fork with a longer handle. It won’t solve the problem in any meaningful way. In fact, it might make it even worse.  Connecting a 5 ton to the existing duct system would increase the airflow, but also the pressure in the supply duct significantly. This will exponentially exacerbate the additional HVAC driven depressurization, creating even more leakage and even more net loss of delivered capacity. Even with a nominal increase in refrigeration capacity. A classic no-win scenario.

This author loves Manual J calculations. It provides us with a framework for quantifying estimates of heat loss and heat gain in a house. But, whenever you can turn an estimated number into a measured value, you are much better off, as the inaccuracy of one large thing can destroy the accuracy of everything. And to be accurate we need to test.

A caveat on measuring house pressure with reference to outside

We discussed measuring the house pressure, but that’s not enough by itself.  There are a few reasons why measuring only the house pressure might not tell you everything you need to know.

  • Sometimes the leakage on the supply side and return side can be large, but nearly equal. In this case, you might have a big increase in the load and a big decrease in your capacity, but the change in house pressure would be nearly zero – Luckily, this wasn’t the case in this instance.
  • If the house is very leaky, even large duct leaks might not create much pressure change.
  • Windy weather can make the house pressure jump around as much as 5 – 10 Pa, which makes it much more difficult to measure a few Pascals of pressure change when you turn the HVAC system on

So, to really understand what’s going on, these steps can be taken.

  1. Measure the house pressure with the HVAC system and all other exhaust fans in the house off, and then again with it (HVAC) on using a precision micromanometer. It should change by less than 1Pa.
  2. Do a blower door test. If the house is very tight, a 3Pa house depressurization might not indicate very much air is leaking from the duct work. But if it’s a leaky house, 3Pa might mean a LOT of air leaking in.
  3. Use a tool like SeeStack to estimate how many CFM are coming in or blowing out to create the pressure you measured and your blower door results.
  4. If the house is very leaky, it might be hiding the duct leakage problem. In this case, a DuctBlaster® test is the best way to be certain how much duct leakage you have.

Back to Captain Kirk and Mr., I want a 5 ton’s conundrum

Armed with the mighty power of fairly complex to comprehend data, I sit down at the kitchen table with the homeowner and the installing contractor. We’re not 6 feet apart but we are all wearing our masks.

I explain the interpretation of the data to the best of my abilities, hopefully, better than I did here. The homeowner seems to get a good grasp on the basics of what the real problem is, can’t tell about Captain Kirk tho. He seems mildly relieved but not too confident. I conclude advising something along the lines of: “Your system is not too small, but we need to tighten up the house some and it would be great if we can test for duct leakage specifically”.

“It’s like eating soup with a fork!” – says my friend the installer in an outburst of newfound confidence – “You wouldn’t eat soup with a fork, would you? Not if you are hungry anyways”.

Genry Garcia – Comfort Dynamics, Inc.

Steven Rogers – The Energy Conservatory.

Russ King – Coded Energy, Inc.

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

Nomenclature on HVAC/R equipment is a sequence of numbers and letters a manufacturer uses to speak directly to the technician. Lots of initial upfront information is handed to the technician by the manufacturer the moment the technician reads the nomenclature in the model and serial numbers.

So how do we make sense of these seemingly random sequences of alphabetical/numerical characters? Every manufacturer has developed its own “language” of sorts when it comes to their equipment. Let’s take a look at several different examples.





Now let’s take a look at all the different information these model numbers can give us. Across the manufacturers listed as examples, we can find information about Product Series, Type of Equipment, SEER, Cooling/Heating Capacity, Grille Variations, Voltage, Configuration, Cabinet Dimensions, Type of Motor, Airflow Capacity, Refrigerant Type, Efficiency, etc.

Understanding the importance of the information in model numbers is essential, and can save a lot of time in diagnosis and installation. Serial numbers, too contain valuable information. Most manufacturers don’t publish the nomenclature for serial numbers, because it’s mostly used for warehouse and warranty tracking, but a few helpful tips per manufacturer are:


Carrier: The first 2 numbers in the serial number indicate the week of the year the equipment was manufactured. The 3rd and 4th numbers indicate the year of manufacturing. 

Goodman: The first 2 numbers in the serial number indicate the year of manufacturing. The 3rd and 4th numbers indicate the week of the year it was manufactured.

Trane: Typically has the year of manufacturing printed elsewhere on the data tag

Mitsubishi: The first number in the serial number indicates the year of manufacturing. The decade must be assumed. (an “8” could mean the equipment was manufactured in 2008, or 2018)


Bottom Line 


Always take a moment to familiarize yourself with the manufacturer model/serial number nomenclature. The information you find will help you understand the system more fully and can help speed up the diagnosis/installation process.



My goal in this tech tip is to help those who struggle to understand heat pumps to get their head around it as quickly as possible as well as understand some of the things a tech needs to know about them.

The basic idea of a heat pump is to use the compression refrigeration cycle to move heat in the opposite direction from what would be considered usual by making the coil that would usually be an evaporator into a condenser and the coil that would usually be a condenser into an evaporator.

This is generally done with a valve called a reversing valve or 4-way valve that connects to the suction and discharge line near the compressor and can redirect the flow from one coil to another as shown in the images above.

Pretty simple really…. but there are some things that can trip up techs who are used to cooling only systems.

Let’s look at some of the unique aspects of a heat pump one at a time.

Low Voltage O & B Terminals 

On a heat pump, the Y terminal isn’t really a “cooling” signal, it is now a circuit that energizes the compressor contactor in both heating and cooling. The shift from cool to heat is done by the reversing valve solenoid with the most common being a 24V call on the O terminal to designate cooling. Some systems use a 24V B call for heat instead of cool but this is far less common.

Reversing Valve Solenoid 

The reversing valve solenoid is an electromagnetic coil that mounts onto the reversing valve and is generally 24V on residential heat pumps. The solenoid does not actually shift the main valve, it only shifts a much smaller pilot valve that then uses system pressure to shift the valve. The solenoid should never be energized unless it is properly mounted on the valve or it can overheat and fail.

Two Metering Devices

Most heat pump systems will have two separate metering devices with one being outside for heat mode and one inside for cooling mode. This is due to the fact that the evaporator is inside in cool mode and outside in heat mode. In some cases, you may even find that the system has a TXV metering device inside and a piston outside. This can cause confusion for some techs because they may see the piston housing outside and just assume it is also a piston inside which can lead to charging issues.

Keep in mind that each of these metering devices must have a method of refrigerant bypass in the opposite mode by either an internal or external check valve. The goal is to have properly restricted flow in one direction and unrestricted flow in the other direction.

Bi-Flow Liquid Line Filter-Drier

In a heat pump, the liquid line is always the liquid line but the flow direction goes from outside / in during cooling mode and inside / out during heat mode. For this reason, we must use a bi-flow filter/drier on the liquid line that can filter the refrigerant in both directions.


High Head Pressure in Heat Mode

Because the indoor coil becomes the condenser in heat mode, low indoor airflow can cause really high head pressure, compressor overheating, and high-pressure switch trips. When you find abnormally high head pressure on a heat pump always look at indoor airflow (dirty filters, coils, blowers, duct issues etc…)



When outdoor temperatures get low enough the outdoor coil may become icebound and require a defrost. Different manufacturers use different control strategies but the common sequence is the system will switch into cooling mode, turn off the condensing fan and turn on aux. heat where applicable until the defrost is complete. Because of the sound of the valve switching and the steam leaving the coil this can cause nuisance service calls if the customer happens to observe a defrost.

You Need Compression For The Valve to Shift

The reversing valve solenoid relies on system pressure to force the valve back and forth. If the compressor isn’t running or has poor compression the valve can fail to shift or can fail to shift completely resulting in a possible misdiagnosis of the valve as the issue.

Suction Pressure Drops As Outdoor Temperature Drops

Because the evaporator is outside in heat mode the suction pressure and suction saturation will decrease as the outdoor ambient temperatures decrease. This will also increase the compression ratio the colder it gets, which reduces system capacity unless other strategies are employed to increase the capacity.

I have seen many techs overcharge a heat pump when ambient temperatures are low in an ill-advised attempt to increase the suction pressure which will only result in other issues.

Connect the Suction Gauge to the “Common” Suction Port

The large line we would normally refer to as the “suction” line becomes a vapor line, this is because it is high-pressure discharge rather than suction in the heating mode. In order to check suction pressure, you need to connect to the specially designed common suction port that connects to suction between the compressor and the reversing valve.

Obviously, this is just an introduction but don’t be afraid… Heat pumps are getting better and better and more technician friendly all the time. Start with reading the product info on the particular unit you are working on and go from there.

— Bryan

I’ve seen a lot of guys recently who reach for the motor puller tool first thing when attempting to remove a blower motor from a wheel/fan blade. Motor puller tools are an awesome backup tool when needed, but that shouldn’t be the go-to method of removing a motor.


The main issue with using a motor puller for every single motor is its tendency to bulge out the shaft. Motor pullers work by clamping down on a hub and then twisting a small shaft against the motor shaft in order to push/pull the motor/wheel away from each other. Sometimes, when technicians don’t sand down a shaft and spray the area with WD-40 or other water displacement lubricants, the shaft will get stuck and a tremendous amount of force is required to crank the motor puller shaft against the shaft of the motor. These opposing forces can significantly bulge the motor shaft. If the technician is successful in removing the motor that way, they often find it more difficult to get the motor shaft back inside the bore of the wheel. 

My hope is every technician reading this understands that the cardinal rule of removing a motor is to never use any of the following methods:

  • Use a hammer/wrench/blunt object to beat the shaft out of the assembly
  • Use channel locks of set screws
  • Use channel locks on the motor shaft
  • Over tighten the set screw

Any of the above-mentioned sins can result in expensive problems.

Please note the two things that must be completed before using a motor puller: sanding the shaft and lubrication. Guess what…


That’s all you need to do to remove a shaft!

  1. Sand the motor shaft until shiny and smooth.
  2. Spray with water displacement lubricant
  3. Loosen the set screw (but don’t remove it. They are easy to lose)
  4. (Optional) Take an adjustable wrench and gently turn the shaft independently of the wheel
  5. (Optional) Slightly push the wheel down the shaft to sand the portion of the shaft that was previously unreachable, which may have a lip that needs to be sanded down.
  6. Gravity is your friend. Let the motor fall out of the assembly. A shake or two may be required.


Voila! Those are steps a technician needs to do before using a motor puller, yet 90% of the time, those steps are all that’s needed to do the job. 


One extra tip…Blow off the sandpaper/rust debris before applying the lubricant, and don’t apply lubricant before you sand the shaft. The debris can get stuck and make things even more difficult, and sandpaper that is saturated in WD-40 doesn’t do much good.


For a video on this method, we shared a post by Brad Hicks earlier this year of him demonstrating how it’s done!

The Surefire Way to Get a Blower Wheel Off


– Kaleb

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 hear the following phrase a lot

It’s the amperage that kills you not the voltage

While there is truth to the statement it is sort of like saying “it’s the size of the vehicle not the speed that kills you when it hits you”…

OK so that’s a pretty bad example, but hopefully, it gets the point across. BOTH of them are needed to cause injury or death and in the case of voltage and amperage the higher the voltage the higher the amperage.

This statement about amperage being the real danger as led to many people inaccurately believing it is the size of a panel or the gauge of wire that makes something more or less dangerous… which is 100% incorrect.

Let’s take a quick look at OHM’s law –

Amps = Volts ÷ Ohms 

The resistance (ohms) of the human body depends on a lot of factors including things like the moisture content of the skin, what other objects the current path is traveling through, what path the current is taking through the body etc…

While the resistances vary based on these factors Ohms law still holds true that when you increase the voltage you ALSO increase the amperage.

Take a look at this chart from the CDC

Effects of Electrical Current* on the Body [3]
Current Reaction
1 milliamp Just a faint tingle.
5 milliamps Slight shock felt. Disturbing, but not painful. Most people can “let go.” However, strong involuntary movements can cause injuries.
6-25 milliamps (women)†
9-30 milliamps (men)
Painful shock. Muscular control is lost. This is the range where “freezing currents” start. It may not be possible to “let go.”
50-150 milliamps Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions. Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Death is possible.
1,000-4,300 milliamps (1-4.3 amps) Ventricular fibrillation (heart pumping action not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely.
10,000 milliamps (10 amps) Cardiac arrest and severe burns occur. Death is probable.

*Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns.
†Differences in muscle and fat content affect the severity of shock.

Let’s say that a particular shock is traveling through a 20 KOhm (20,000 ohm) path in your body

At 120V this would produce a 6mA shock

At 240V it would be 12mA

At 480V it would be 24mA

It becomes clear pretty quick that higher voltage does lead to more dangerous shocks as does the resistance of the path.

High Resistance and Low Voltage = Safer

Low Resistance and High Voltage = Danger

This is why working around live electrical should only be done with insulated tools, proper PPE and in dry conditions. These all serve to keep the resistance up to reduce the likelihood of a fatal shock. The higher the voltage the more diligent you need to be.

Some people may bring up high voltage shocks from a taser or static electricity as proof that “voltage doesn’t kill”.

In these cases, the power supply is either limited, intermittent or instantaneous. This means that while the voltage is high it is only high for a very short period. Unfortunately in our profession, those sorts of quick high voltage discharges aren’t the big danger we face, most of the electrical work we do is on systems that will happily fry us to a crisp before the power supply cuts out.

A circuit breaker or fuse will never protect us because we draw in the milliamp range when we are being shocked as almost all fuses or breakers don’t trip or blow until much higher levels are reached.

Be safe around high voltage and keep your resistance high.

— Bryan




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