Author: Bryan Orr

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

There are many examples of teaching using metaphor to help someone get a grasp of how something works without being EXACTLY correct.

Some examples are how we often use water flow to explain electrical flow or refrigerant circuit dynamics. It’s enough like the way it works to get our heads wrapped around it but there are many differences and the metaphors eventually break down.

This is definitely the case with air and nitrogen “absorbing” water

I’ve done podcasts and videos about how air can “hold” less moisture when it is cooler and more when it is hotter. You have likely heard old school techs talk about triple evacuation and sweeping with nitrogen to “absorb” the moisture from the system.

News Flash, Air and Nitrogen DO NOT absorb or hold moisture… They ignore one another at parties and they certainly don’t shake hands.

Water vapor in the air behaves much like all the other gasses contained in the air with the notable exception that water exists in both vapor and liquid states at atmospheric pressure and temperature.

When the temperature of water vapor is higher, a higher percentage of the air by volume can CONTAIN water vapor, but the air itself isn’t what is holding it. It does interact with it as the molecules move and bounce around and the percentage of water vapor in the air does impact the mass/weight of the air by volume (water vapor weighs less than dry air) so there are certainly impacts to the makeup of the air based on moisture content.

The percentage of the air around us that is moisture can vary from almost zero In cold arctic & Antarctic climates to nearly 4% in hot, tropical climates.

When teaching it we speak as though the air is a sponge and the hotter the air the bigger the sponge. This certainly helps us remember but it isn’t really how it works. In reality water in the air is all about the saturation temperature and pressure of the water and the air has little to do with it.

By Greg Benson

This is the same sort of thinking when a tech is having a hard time pulling a vacuum and they add dry nitrogen to the system to “absorb” the moisture. First off, you will want to sweep the nitrogen through the system, not just pressurize. Secondly, the nitrogen has no special properties that allow it to “grab” moisture. It can entrain the water vapor using Bernoulli’s principle, it will warm up the system a bit, it will certainly add in a bit of turbulence which can help move the oil around and potentially release some trapped moisture… but nothing more than that.

Don’t get me wrong, there is nothing wrong with sweeping with dry nitrogen, even better to use a heat gun and warm the compressor crankcase, receivers and accumulator and coils during a deep vacuum on a large system to help speed up the vaporization of moisture.

It doesn’t change the fact that air and nitrogen don’t “hold” moisture.

— Bryan

 

 

The most common and often most frustrating questions, that trainers and senior techs get goes something like this. “What should my ______ be?” or “My _____ is at ______ does that sound right?

Usually, when the conversation is over both the senior and junior techs walk away feeling frustrated because the junior tech just wanted a quick answer and the more experienced tech wants them to take all of the proper readings and actually understand the relationships between the different measurements.

In this series of articles we will explore the, “What should my _______ be?” questions one at time and hopefully learn some things along the way.


So what should the superheat be?

First, what is superheat anyway? It is simply the temperature increase on the refrigerant once it has become fully vapor. In other words, it is the temperature of a vapor above it’s boiling (saturation) temperature at a given pressure.

The air around us is all superheated! Head for the Hills!

How can you tell that the air around us is all superheated? Because the air all around us is made of vapor. If the air around us were a mixture of liquid air and vapor air, first off you would be dead and secondly, the air would be at SATURATION. So the air around us is well above its boiling temperature (-355° F) at atmospheric pressure which means it is fully vapor and SUPERHEATED. In fact, on a 75-degree day, the air around you is running a superheat of 430°

But why do we care?

We measure superheat (generally) on the suction line exiting the evaporator coil and it helps us understand a few things.

#1 – It helps ensure we are not flooding the compressor

First, if we have any reading above 0° of superheat we can be certain (depending on the accuracy and resolution of your measuring tools) that the suction line is full of fully vapor refrigerant and not a mix of vapor and liquid. This is important because it ensures that we are not running liquid refrigerant into the compressor crankcase. This is called FLOODING and results in compressor lubrication issues over time.

Image courtesy of Parker / Sporlan

#2 – It gives us an indication as to how well the evaporator coil is being fed

When the suction superheat is lower it tells us that saturated (boiling) liquid/vapor mixture is feeding FURTHER through the coil. In other words, lower superheat means saturated refrigerant is feeding a higher % of the coil. When the superheat is higher we know that the saturated refrigerant is not feeding as far through the coil. In other words higher superheat means a lower % of the coil is being fed with saturated (boiling) refrigerant.

The higher the % of the coil being fed the higher the capacity of the system and the higher the efficiency of the coil.

This is why on a fixed orifice system we often “set the charge” using superheat once all other parameters are properly set. Adding refrigerant (on a fixed orifice / piston / cap tube) will feed the coil with more refrigerant resulting in a lower superheat. Removing refrigerant will increase the superheat by feeding less of the coil with saturated (mixed liquid and vapor) refrigerant.

This method of “setting the charge” by superheat does not work on TXV / TEV / EEV systems because the valve itself controls the superheat. This does not negate the benefit of checking superheat, it just isn’t used to “set the charge”.

#3 – We can ensure our compressor stays cool by measuring superheat

Most air conditioning compressors are refrigerant cooled. This means that when the suction gas (vapor) travels down the line and enters the compressor crankcase it also cools the motor and internal components of the compressor. In order for the compressor to stay cool, the refrigerant must be of sufficient volume (mass flow) and low temperature. Measuring superheat along with suction pressure gives us the confidence that the compressor will be properly cooled. This is one reason why a properly sized metering device, evaporator coil, and load to system match must be established to result in an appropriate superheat at the compressor.

#4 – Superheat helps us diagnose the operation of an active metering device (TXV / TEV/ EEV)

Most “active” metering devices are designed to output a set superheat (or tight range) at the outlet of the evaporator coil if the valve is provided with a full liquid line of a high enough pressure liquid (often at least 100 PSIG higher than the valve outlet / evaporator pressure). Once we establish that the valve is being fed with a full line of liquid at the appropriate pressure we check the superheat at the outlet of the evaporator to ensure that the valve itself is functioning properly and /or adjusted properly. If the superheat is too low on a TEV system we would say the valve is too far open. If it is too high the valve is too far closed.

#5 – Superheat is an indication of load on the evaporator 

On both TEV / EEV systems and fixed orifice systems (piston / cap tube) you will notice that when the air (or fluid) going over the evaporator coil has less heat, or when there is less air flow (or fluid flow) over the evaporator coil the suction pressure will drop. However, on a TEV / EEV system as the heat load on the coil drops the valve will respond and shut further, keeping the superheat fairly constant. On a fixed orifice system as the load drops so will the superheat. It can drop so much on a fixed orifice system that when the system is run outside of design conditions the superheat can easily be zero resulting in compressor flooding.

When the load on the evaporator coil goes up a TEV / EEV will respond by opening further in an attempt to keep the superheat constant. A fixed metering device cannot adjust, so as the heat load on the coil goes up, so does the superheat.

When charging a fixed orifice A/C system you can use the chart below to figure out the proper superheat to set once all other parameters have been accounted for or you can use our special superheat and delta t calculator HERE

Using this chart requires that you measure indoor (return) wet bulb temperature so that the heat associated with the moisture in the air is also being accounted for as well. This is one of MANY target superheat calculators out there, you can use apps, sliderules etc… Here is ANOTHER ONE

Remember, this chart ONLY applies to fixed orifice systems.

So what should your superheat be in systems with a TEV / EEV? The best answer is… like usual… Whatever the manufacturer says it should be.If you really NEED a general answer you can generally expect

High temp / A/C systems to run 6 – 14 degrees of superheat

Medium Temp  – 5-10 

Low Temp – 4-10

Some ice machines and other specialty refrigeration may be as low as 3 degrees of superheat

When setting superheat on a refrigeration system with any type of metering you often must get the case / space down close to target temperature before you will be able to make fine superheat adjustments due to the huge swing in evaporator load. Once again, refer to manufacturer’s design specs.

— Bryan

 

 

Image Courtesy of Eaton SNAP ‘N SHIELD

Piping support is covered in Section 305 of the IMC (International Mechanical Code) which once again, isn’t binding but is the code that most local codes are based on.

Piping / Tubing MaterialMaximum Horizontal Distance Between SupportMaximum Vertical Distance Between Support
Copper Tubing 1 1/4″ & Smaller 6′ 10′
Copper Tubing 1 1/2″ & Larger 10′ 10′
PVC Pipe 4′ 10′
CPVC 1″ & Smaller 3′ 10′
CPVC 1 1/4″ & Larger 4′ 10′
Pex Tubing 32″ 10′

You will notice pretty quick that 10′ is the vertical support distance in all of these common cases. When supporting horizontally It’s also important to use supports that won’t compress insulation on insulated suction lines and drain lines.

There are really nice saddles made nowadays for insulated lines like the B-line Snap ‘n Shield supports shown above. We recently re-insulated a large grocery store with overhead copper and replaced the existing supports with these to eliminate condensation.

— Bryan

Bryan examines Symptoms of Low Refrigerant Charge in this in-depth video.

www.hvacrschool.com/quick-sheet
www.hvacrschool.com/5-pillars
www.hvacrschool.com/terms

To learn more about the evaporator and superheat – https://youtu.be/ZboChiHDITY
To learn more about the condenser and subcool – https://youtu.be/TkpF0e7jyPs

–Bryan

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