Author: Bryan Orr

Bryan Orr is a lifelong learner, proud technician and advocate for the HVAC/R Trade

How many times have you looked at the bottom right hand side of an evaporator coil and seen all sorts of rust, even on a fairly new coil?

You may have noticed that many evaporator coils and even some condenser coils will start to corrode where the galvanized steel end plates touch the copper u-bends of the coil. This is a common example of “galvanic corrosion” and it occurs anytime two different (dissimilar) metals come into contact with one another in addition to the presence of an electrolyte such as salt water or condensate water when other particles are present in the water. The reason for this is when electrical contact is made between these metals, ions travel from one metal or “anode” to the receiving metal or “cathode”. When this occurs the anode metal corrodes and the cathode metal is protected from corrosion as the anode metal “gives itself up”.

In fact in the 1980’s, the statue of Liberty was found to have galvanic corrosion on the steel substructure where it connected to the copper skin of the statue, resulting in a major renovation.

The chart above shows that different metals have different galvanic properties and some act more as an anode, giving up to galvanic corrosion more easily and others resist galvanic corrosion and are protected by the other metals.

For example, you may be aware that galvanized steel is more resistant to corrosion than regular steel or cast iron. The “galvanized” part of galvanized steel is just a thin coating of zinc on top of the steel that gives itself up to corrosion, therefore protecting the steel below. This method is more effective than many other protective coatings because even if the coating were scratched or compromised the steel below is protected by the zinc and its sacrificial anode properties.

So let’s think about a common copper tube, aluminum fin, steel framed coil. Where all three of these come together. Where the aluminum touches galvanized steel the galvanized part will go first, then the aluminum, then the steel, then the copper. The galvanized (zinc), aluminum and steel that contact the copper tubing actually act to PROTECT the copper so long as they are in physical contact in the presence of an electrolyte. The only issue is that once that steel rots out the copper may not be held in place as firmly resulting in the occasional abrasion leak.

Now, because of recent studies, we know that most coil leaks are caused by formicary corrosion and copper is more prone to formicary corrosion than aluminum and this is why we are seeing so many units coming with aluminum evaporator coils. Just don’t be fooled into thinking that the rusty mess caused by galvanic corrosion is the cause of your evaporator coil leaks. That rusty steel may actually be protecting the copper more than harming it. There are even some companies that make sacrificial anodes that attach to the suction to help further protect the system from corrosion such as THIS. While many techs use a rusty coil as a system sales technique, you are better off actually performing a proper leak detection instead of assuming that rusty steel means corroded tubing.

— Bryan





I got in one of these tiny torches the other day to experiment with brazing aluminum in tight spots and one of the techs walked in and asked “what type of torch is that” to which I answered “It’s oxy/acetylene”, he picked it up and looked at it a bit then asked “Does that get hot enough to braze copper?”

It’s actually a tough question to answer simply

When we say does something get “hot enough” we often mean that the TEMPERATURE is high enough to melt solder or brazing rod, but heat is both an intensity (temperature) and a quantity (BTUs), so while this tiny torch is certainly high enough temperature to melt a rod, it may or may not be enough BTUs to heat up the base metal being joined.

The temperature of the flame is primarily dictated by the fuel or fuels being burned as well as the oxygen mixture. The BTUs depending on the pressure being used and the size/type of the tip.

Most torch manufacturers will list the size of the (copper) pipe that the tip is rated for as well as the proper oxygen and acetylene pressures for the task.

So, heat is both a quantity (BTU) and an intensity (Temperature) and when we say how hot or how cold it really depends on what you mean.  But to answer the question… Yes ,you can braze copper with the tiny torch, so long as it is small tubing.

— Bryan

As an A/C tech I can sometimes get the terms “defrost termination” and “defrost fail safe” mixed up because they sound pretty similar. Before we cover these terms lets set the basic defrost groundwork for refrigeration (coolers and freezers)

Defrost is accomplished in one of a few ways, these first two only apply to “coolers” where the box, air and product temperatures are above freezing but the coil temperature drops below freezing –

Off Cycle Defrost – In Medium temperature applications where the box air temperature is above freezing there is often no set defrost and instead the coil defrosts when the system naturally cycles off. This relies on appropriate over-sizing and can lead to issues when the heat or moisture load is high, especially when the door are opened a lot for loading and unloading.

Timed Defrost – In medium temperature you can use a defrost timer to simply pump down or cycle off the compressor at particular times while keeping the evaporator fan running to force a defrost  few times per day.

Next we have the methods used for defrosting low temperature applications which are below freezing and generally 0ºF to -10ºF depending on what is being stored.

Electric Heat Defrost – On a set schedule (time) the compressor is pumped down or cycled, the evaporator fan is (generally) shut off and the electric heaters are turned on inside the evaporator coil.

Hot Gas Defrost – On a set schedule the evaporator fan is shut off and hot discharge gas is pumped through the evaporator coil.

In both of these situations the goal is to get the ice off the coils as quickly as possible but to stop the defrost cycle as soon as the coil is ice free but no sooner. We don’t want to terminate or stop the defrost too early and leave ice but we also don’t want to keep adding heat to the coil for no reason.

Defrost Termination

This is where defrost termination and fail safe comes in. The evaporator coil cannot go above 32ºF so long as there is still ice in that area, so it stands to reason that if heaters are running on the coil and the coil is still at 32ºF or lower then there is still ice. A defrost termination thermostat is mounted onto the coil to detect when the coil is free of ice and will often be set to “terminate” or stop the defrost heat when the coil reaches around 55ºF – 60ºF to ensure the entire coil is ice free.

So the defrost starts on a scheduled time of 2 – 6 times per 24 hour period and terminates once the coil defrost termination thermostat ends the defrost.

It is also common for defrost cycles to have a “drip” time once defrost end to allow water to drip off the coil after defrost and then a fan delay once the refrigeration begins again to prevent the fan from blowing water off of the coil into the box. This is often set to 30º or lower before the fan can come back on.


Fail Safe Time 

There needs a to be a time limit to how long a defrost can go before it goes back into refrigeration to prevent catastrophic product loss in the case of defrost termination failure. This is part of the defrost clock and is often called the fail safe or fail safe time.

The fail safe time can be a wide range of times depending on the application and frequency of defrost but 20 to 40 minutes is common. If your fail safe time is 30 minutes this means that once a defrost cycle begins the LONGEST it will remain in defrost in 30 minutes regardless of the defrost termination thermostat.

Demand Defrost

This strategy is of using time and temperature for defrost is still the most common found in the trade. There is a more advanced strategy called demand defrost that only initiates defrost when sensors predict that defrost  is required. This is often done via trend analysis between sensors to “learn” when ice is present and when it is fully defrosted and will require some manufacturer specific understanding of the particular controls scheme.


Regardless of the strategy the goal is the same

  1. Defrost when needed to keep heavy ice buildup off the coil
  2. Stop the defrost cycle as soon as the ice is gone
  3. Don’t blow water off the coil into the box / case by starting the fans too soon
  4. Use strategies that don’t cause catastrophic product loss if a sensor fails

From a technicians standpoint its important that you fully understand the defrost strategy being used and that you fully test the defrost cycle after you make any changes.

— Bryan



Low pressures are often measured in inches of water column or “WC. Like most units of measure, it has a very simple origin, in a water manometer 1″ of water column is literally the amount of force it takes to raise the column of water by 1”. While some water manometers (water tube) are still in use the vast majority are either dial or digital gauges that still use the same scale.

1 psi is equal to 27.71 inches of water column; this is why water column is most often used to measure pressures under 1 psi. These low pressures are most often read using a manometer or a magnahelic gauge.

When we measure water column with our tools it is calibrated at atmospheric pressure or the gauge scale instead of the absolute scale. This means that for a manometer or magnahelic to be properly used they  MUST be recalibrated before each (many auto calibrate to zero) to compensate for changes in elevation and barometric pressure. At altitudes over 2000′ above sea you will also need to follow manufacturer recommendations to adjust the gas valve and even change orifice sizes in some cases due to the effect the lower atmospheric pressure has on the gas.

Gas pressure is usually measured in “wc, most commonly we set single stage appliances to 3.5” wc on natural gas and 11″wc on propane. This varies based on manufacture specs, combustion analysis and meter clocking tests. Always read the manufacturer specs.

We also use “WC to check air static pressure on systems. Static pressure is pressure that is exerted in all directions in a contained space, it is not the directional force of the air.

We use a manometer or a magnahelic and measure the negative air pressure in the system return side before the blower (and after the filter whenever possible) and the positive pressure supply air side directly after the blower. By calculating the differential you come up with the total external static in water column. For example, if the return static is -0.3″wc  and the supply static is +0.2″wc the total static is 0.5” wc.

Many manometers and all magnahelic gauges (to my knowledge) have two ports so you can read the differential pressure all at once. This also comes in handy when reading/testing differential pressure on many furnace air pressure switches to ensure they make and break at the proper pressure.

— Bryan Orr

I’ve always liked old books.

Think about an old printing press somewhere in Chicago or Boston or Scranton, Pennsylvania.

Imagine workers with their hands covered in ink up to their elbows, setting type while giant machines of iron, steel, and brass stamped out a book page by page. Then those pages went on to be bound, crafted in a way that few things are nowadays.

At that time the pages were new and crisp, fresh ink and fresh paper giving off a distinctive odor.

But that’s not the part I like the most.

The part that piques my imagination is the people who wrote it and the world they lived in. In most of my imaginations, the past is all in black and white, full of dull people, living dull lives.

But that’s just wrong.

When I open one of these old books they talk about problems we still face today, with information that still applies

Pretty quickly you begin to see the genius of these writers. You start to understand that their lives and work were often very similar to our own with many of the conveniences stripped away.

These people had to be resilient and resourceful. They had to memorize more and read more because access to information was rare and precious. They were more reliant on experimentation and discovery because much of what they knew they had to find out for themselves and pass on person to person.

Many of these books for the trades are written between the industrial revolution and the Second World War. A time in the world when anything seemed possible both good and evil.

Great leaps in technology and progress on the positive side. Abuse of workers at home and a looming enemy abroad seeking to tear the fabric of civil society apart on the other.

In reading the Building Trades Handbook from 1899 I learned that there was a booming correspondence school in Scranton Pennsylvania that educated thousands by mail correspondence in the trades and engineering one page at a time.

The books start with very simple skills like working with fractions that can be so daunting for Tradesmen Even today.

I learned in the American Electricians Handbook from 1921 that we knew so much about electrical motors and electrical engineering at that time. So much of it is well explained in that text using explanations that would make sense to the average Workman.

On the other hand, electricians from that era were not nearly as concerned with preventing electrical shock. The practices used to diagnose electrical circuits are laughable and frightening by modern standards. It does show that the Tradesmen that came before us were tough… even to the point of being a bit crazy.

While all of this is very interesting I’ve noticed something else. Most of the really great educators in our trade have gone back to old books to find answers.

Jim Bergmann told me that he went to old books to find answers about carbon luminous flame in old furnaces and boilers.

Text From The Philosophical society of Glasgow 1881

Dan Holohan always speaks about going back to books by “dead men” to learn about steam heating.

Joe Lstiburek (Building Science) talks about going back to very old construction books to learn about capillary action and capillary breaks to prevent moisture intrusion.

Why is that? Why do old books contain information that some of the new ones don’t?

Remember when you played that game of telephone as a kid where you say a phrase to one person and it’s repeated around the circle. By the time it gets back it’s either nothing like the original or a good portion of the information is missing.

That’s what often happens in education.

Those who make significant discoveries, invent practical machines and applications and work out the math are the first educators in a particular field.

Not only do they write about it, but they also LIVED IT.

The generations after that tend to get split, with the educators focusing mostly on the teaching and the field workers focusing mostly on the doing. They both have a piece of the puzzle, but over time the message gets diluted and breaks down until nobody REALLY understands the whole anymore.

We see this in our field today all the time –

Engineers know lots of theory and math but not what commonly goes wrong or the practical elements of the field.

Manufacturers understand their products but not necessarily the application.

Installers know how to assemble systems but not why or how to properly design.

Techs know how to fix “most” problems but really understand the why of a design? Forget about it!

For those of us who really want to understand the work we do we are left with going back to those people long dead who made the discoveries themselves.

The ones who worked in unsafe buildings and grabbed hot wires, and worked in sweltering labs before A/C existed and also the ones who wrote old books.

I just got in a book like that…

Never stop learning, never stop reading old books. Take a look at Ebay and Amazon and let me know the treasures you find.

— Bryan



You are probably all familiar with radiant barriers. Sometimes it is thin foil draped under the roof deck, sometimes it’s used on the inside of stud walls or over furring strips before drywall goes up and there is even plywood with a radiant barrier attached to one side that is used for roof decking.

The point of this article is to remind you that you eliminate the benefit of a radiant barrier when you sandwich it between materials in other words when there is no “air gap”, but I also want to help you understand why this is.

How Radiant Heat Transfers 

Heat energy is the “force” that makes the atoms move and molecules jiggle and it’s in everything over absolute zero (-460°F). Heat is transferred or moved in one of three  ways but heat itself isn’t these things, these are methods by which heat is moved like walking, flying in a plane or riding a surfboard.

  • Conduction – Heat moving when one molecule bumps into another and imparts some it’s force. It’s like standing in a line and shoving someone, they move because you impart force directly on them.
  • Convection – Heat moving when the molecules in a fluid are free to move around. It’s like flying on a plane, you are moving freely through the air and bringing your energy with you.
  • Radiation – Heat moving through the air or a vacuum via electromagnetic waves. It’s like surfing because your energy is riding a wave DUDE!…. and that stupid metaphor was the whole reason for the other two lame ones…

So from a practical standpoint in a building we control conductive heat transfer with insulation, convective heat transfer by air sealing to the unconditioned spaces and radiation with low emissivity barrier with the shiny side facing an air gap, this is if you need a radiant barrier at all.

Radiant heat can only transfer when you have two surfaces pointed at one another that have a different temperatures. The rate at which heat will transfer between them is a function of the temperature difference, the distance between them and the emissivity of each surface. A suface with an emissivity of 1 is a so called “perfect black body” and is a theoretical perfect emitter and absorber of radiant heat.

A surface with an emissivity of of 0 is perfect reflector of radiant heat energy and neither absorbs or emits radiant heat. In practice we do not see 1 or zero but a fraction of 1 with a black dull surface being close to one and a shiny, reflective radiant barrier generally being around 0.10 meaning only 10% of the radiant energy is absorbed or emitted.

So why can’t we sandwich a radiant barrier? 

Imagine getting a pan on a stove nice and hot and then hovering your hand over it, you would feel the radiant heat emitting from the pan. Now place a sheet of aluminum foil over the pan and hover your hand again, very little radiant will be absorbed and emitted by the foil and your hand will be much cooler.


Push your hand down on the foil and squeeze it into the pan…


Spoiler alert, it will burn you.

While aluminum foil has a low emissivity it is very thermally conductive and heat travels through it easily via conduction (molecule to molecule). This means that the only way it helps you block heat is when one shiny, low emissivity side faces an air gap (or vacuum or other fluid that allows the electromagnetic waves to pass easily through). This is why you see white radiant roofs on shopping centers that face the sky, or plywood for roof decking with a radiant layer that faces down into the attic.

If you press anything solid up against both sides of a radiant barrier you make it a conductive layer and it does NO GOOD.

Some of you may (incorrectly) assume that a radiant barrier must be pointed at a light source (like the sun) to do any good. Remember, you don’t need visible light to have radiant heat transfer just a temperature difference. So a radiant layer on the underside of roof decking will help block radiant heat from leaving that roof decking and entering the ceiling and trusses and whatever else is in that attic even if it is pitch black up there because the radiant barrier is bad at absorbing AND emitting radiant heat so even though the radiant barrier on the underside of the roof deck would be hot to “touch” (conductive) it does much less emitting then wood so more of the heat stays put.

— Bryan


We have been discussing a lot of methods for checking a refrigerant charge without connecting gauges over the last few months. This got me thinking about the “approach” method of charging that many Lennox systems require.

Approach is simply how many degrees warmer the liquid line leaving the condenser is than the air entering the condenser. The approach method does not require gauges connected to the system but it does require a good temperature reading on the liquid line and suction line (Shown using the Testo 115i clamp and 605i thermo-hygrometer smart probes).

When taking an approach reading make sure to take the air temperature in the shade entering the coil and ensure you have good contact between your other sensor and the liquid line.

The difference in temperature between the liquid line and the outdoor temperature can help illustrate the amount of refrigerant in a system as well as the efficiency of the condenser coil. A coil that rejects more heat will have a leaving temperature that is lower and therefore closer to the outdoor temperature. The liquid line exiting condenser should never be colder than the outdoor air, nor can it be without a refrigerant restriction before the measurement point.

Here is an approach method chart for an older 11 SEER Lennox system showing the designed approach levels.

While most manufacturers don’t publish an approach value, you can estimate the approach by finding the CTOA (Condensing Temperature Over Ambient) for the system you are servicing and subtracting the design subcooling.

6 – 10 SEER Equipment (Older than 1991) = 30°F CTOA

10 -12 SEER Equipment (1992 – 2005) = 25°F CTOA

13 – 15 SEER Equipment (2006 – Present) = 20°F CTOA

16 SEER+ Equipment (2006 – Present) = 15°F CTOA

I did this test on a Carrier 14 SEER system at my office so the CTOA would be approximately 20°

Then Find the design subcooling. in this case, it is 13°F

Subtract 13°F from 20°F and my estimated approach is 7°F +/- 3°F. I used the Testo 115i to take the liquid line temperature and the 605i to take the outdoor temperature using the Testo Smart Probes app and I got an approach of 4.1°F as shown below.

More than anything else, the approach method can be used in conjunction with other readings to show the effectiveness of the condenser at rejecting heat.

If the system superheat and subcooling are in range but the approach is high (liquid line temperature high in relation to the outdoor air), it is an indication that the condenser should be looked at for condition, cleanliness, condenser fan size and operation and fan blade positioning. If the approach is low it can be an indication of refrigerant restriction when combined with low suction, high superheat and normal to high subcooling.

If the approach value is low with normal to low superheat and normal to high suction pressure and high subcooling it is an indication of overcharge.

The approach method is only highly useful by itself (without gauges) on a system that has been previously benchmarked or commissioned and the CTOA and subcooling or the approach previously marked, or on systems (like Lennox) that provide a target approach specific to the model.

— Bryan

In order to wrap my head around diagnostic issues it really helps me to engage in thought experiments where I think of more extreme examples of an issue or situation or consider the ideal in order to find the “edges” of a concept. Once I find the edges of the extreme then I can begin to sort down to a more exact conclusion.

So let’s consider compressors and mass flow

First, don’t get overwhelmed by the phrase “mass flow” I’m not going to start in with confusing words and fancy math. As techs we rarely need to do advanced calculations anyway, it’s more about understanding relationships between factors or IFTTT (If this then that). If this thing occurs or I change this what happens to that.

Mass flow just means how much fluid is moving over a a given amount of time. The “stuff” in this case is refrigerant and the mass measurement is generally lbs in the USA and the rate could be minutes, seconds or hours.

Our goal with a compressor is to move as many lbs of refrigerant, as quickly as possibly with the minimal amount of watts in energy used in order to move the greatest # of btus/hr we can … pretty straight forward so far.

The typical single speed, single stage compressor with no unloading capability runs ESSENTIALLY the same speed and with the same volume in the compression chamber (cylinder, scroll etc…) this means that a traditional compressor has a fairly constant volume in the compression chamber and rate of compression. I say “fairly” constant because as the compressor moves greater mass or works against greater pressure the motor will tend to slip more resulting in a slower rotational speed of the rotor.

So let’s imagine an old single speed, single stage reciprocating compressor with no unloading. It’s compressing refrigerant with a constant volume in the cylinder that goes from its largest cylinder capacity at the bottom of the down stroke (suction stroke) to its minimum capacity at the top of the up stroke (discharge stroke). This variation in volume in the cylinder as the pistons actively move and down is what creates the pressure differential between the high side and the low side and this pressure difference is what allows the refrigerant to move through the circuit.

So you may think to yourself (as I have in the past)

“If pressure differential is what causes the refrigerant to move then don’t we want a big pressure difference between the compressor discharge and suction so that more refrigerant will move?”

The answer is absolutely NO

We actually want the minimum pressure differential we can get away with while still accomplishing the task of maintaining an evaporator (or evaporators in the case of multi-circuit systems) at the desired temperature and (nearly) full of boiling refrigerant.

The reason we want lower pressure differential has to do with mass flow rate, if the compressor has a fixed volume in the cylinders and the pistons are pumping away at the same speed then that part of the equation is fairly fixed. The only way to increase the amount of refrigerant being moved by the compressor is to

#1 – Increase the density of the refrigerant

#2 – Reduce the amount or re-expansion waste in the cylinder

#3 – Reduce the pressure to overcome in the discharge

The first part of that equation is simple, when suction gas is higher pressure it is also higher density, when the suction pressure entering the compressor drops the density also drops. When then density of the refrigerant drops entering the compressor the compressor moves less refrigerant because there is just less there for it move.

Think of this like an old PSC blower motor on undersized duct work. When the static pressure on the return increases the amount of air being moved decreases because the density of the air is decreased. The blower is still spinning the same speed (on a PSC), heck, it may even be spinning faster due to the motor experiencing less resistance, but the airflow decreases. This happens not becasue the motor is doing anything different, it moves less air mass becasue the air is less dense entering the blower and therefore you are moving less air.

When you drop the suction pressure entering a typical compressor you drop the mass flow rate becasue the mass entering the compressor is reduced, lower mass flow rate means moving fewer lbs of refrigerant which (by itself) means lower capacity.

Now let’s move to the second part which is re-expansion and this one applies more to reciprocating compressors where there is a clear compression and expansion stroke vs. a scroll , rotary or screw where the compression is essentially a continuous cycle.

Imagine a compressor sitting in a room with no tubing connected just pumping air. The compressor would be pulling from 14.7 PSIA and discharging into 14.7 PSIA (atmospheric air pressure at sea level). When the piston draws down it would pull in air and fill up and then as the piston pushes up it would start to discharge air out of the cylinder really quickly in the up stroke because the only thing pushing against the discharge is 14.7 PSIA and therefore the highest pressure that would build up inside the compressor is slightly more than 14.7 for it to overcome the pressure of the discharge valve and push out into the air.

If that same compressor were pumped into a chamber where the pressure built up to 200 PSIA what would change?

The compressor would move less air even if  the suction was still left open to atmosphere (and therefore the same air density) because now the discharge valves wouldn’t open until the pressure in the cylinder went above 200 psi meaning that the effective stroke would be reduced due to the pressure being pushed against (#3 on the list above). It would also need to pull down further to re-expand the gas left over in the cylinder to below 14.7 PSIA for more air to enter the cylinder again.

In a scroll, rotary or screw there isn’t valves and cylinders in the same way but the amount of refrigerant being moved is still impacted by changes in suction density (suction pressure) and the pressure exiting the compressor… in other words the COMPRESSION RATIO.

Have you ever noticed that the BTU ratings on compressors have dropped over the last 10 years as units become more efficient? Where a 3-ton unit may have previously had a compressor with a 36 in nomenclature for a nominal three tons you may now find it has closer to 30 or even less.

You may also notice that high efficiency systems often have larger condensing coils and larger evaporators which bring the head pressure and therefore the condensing temperature closer to the outdoor temp and the evaporators are also running a higher temperature bringing up the suction pressure. Manufacturers are increasing how much refrigerant the compressor can move (mass flow rate) by bringing the design head pressure down and the design suction suction pressure up. They can then afford to downsize the compressor achieving the same capacity with less input watts also known as greater energy efficiency.

Let’s give some real world examples of altering mass flow rate by impacting these factors in the field –

  1. Dirty Condenser Coil – Decreases mass flow rate and system capacity because the head pressure and compression ratio go up
  2. Low Indoor Airflow – Decreases mass flow rate because refrigerant density goes down entering the compressor and compression ratio goes up (to a degree). Keep in mind that when there is low air flow or low load head pressure will also tend to drop as the mass flow rate drops. It is held up by the outdoor temperature as a limitation on how low the condensing temperature will drop however.
  3. Overcharge – The impact of overcharge on mass flow rate will vary depending on the metering device and how overcharged the system is. On a TXV / EEV system it will always result in lower mass flow becasue the head pressure will increase. On a fixed orifice it may result in a slight increase in mass  flow initially as suction pressure increases.
  4. High Indoor (Evaporator) Load – Increases mass flow unless there is some control preventing it from doing so like a CPR (compressor pressure regulator). Increased heat entering the evaporator will increase the pressure and density of the refrigerant returning to the compressor, this will increase the mass flow rate, system capacity and head pressure if all else remains the same.

What happens if we change compressor capacity on the fly?

For years in residential and light commercial we’ve been used to fairly fixed compressor volume flow rates but nowadays we see many different types of multi-stage and variable capacity technologies from a simple dual capacity unloading scroll to a digital scroll all the way to variable frequency, variable speed scroll compressors. These compressors have their “rated” capacity which is the state at which they are tested for bench-marking against other units. They can then reduce their capacity below their rating and some can every produce a higher capacity then their rating.

In all of these cases the compressor is altering the amount of refrigerant it is moving by making a change within the compressor itself resulting in lower mass flow when the compressor stages or ramps down and higher mass flow when it ramps up.

Lets imagine a theoretical 4-ton rated unit with a compressor that can ramp down to 2-tons or it ramp up to 5-tons.

What that means in practice is that the compressor is capable of moving an amount refrigerant consistent with two tons of capacity up to a mass flow that can produce 5-tons of capacity at the same rated conditions.

So here is what you would see change when that compressor changes mass flow in comparison to rated capacity if everything else remained the same –

Low Stage (2-ton)

High Suction Pressure

Low Head Pressure

Low Subcool

High Superheat (potentially)

Low Evaporator Delta T

Poor Dehumidification due to high coil temperature

Low compressor amps

Low Compression Ratio

Low Discharge Temperature

Low Approach (liquid line temperature above outdoor temperature)

High Efficiency (EER / SEER)


High Stage (5-ton)

Low Suction Pressure

High Head Pressure

High Subcool

Low Superheat (potentially)

High Evaporator Delta T

Strong Dehumidification due to lower coil temperature

High compressor amps

High Compression Ratio

High Discharge Temperature

High Approach (liquid line temperature above outdoor temperature)

Low Efficiency (EER/SEER)


Now think about how a system responds when the compressor isn’t pumping properly. It is almost exactly the same as the low stage / low mass flow example listed above with the exception of the efficiency.  When we have lower mass flow than rated these are symptoms we will see whether it is by design or due to a failure.

In practice these variable capacity systems will often be matched with a variable speed blower and a wide range TXV or EEV so that the coil temperature and feeding can adjust with the change in mass flow to help mitigate some of the negative effects of staging down.

There are come interesting things that can be done with modern controls and variable mass flow compressors. One example is Bosch branded condensing units that vary the compressor mass flow to set a fixed evaporator temperature, effectively adjusting the capacity to match the load on the evaporator coil. Another is Carrier Greenspeed heat pumps that ramp the compressors up during heat mode to drive up the pressure on both coils to increase the heat produced inside and reduce defrost requirements.

— Bryan


There has been much written and many jokes made about the misdiagnosis of TXV (Thermostatic expansion valves) and rightly so. This article will cut straight to the point to help those of you who may still need a bit of clarification and hopefully, we will save the lives of a few TXVs and the pocketbooks of some customers.

Q: What is a TXV?

A: A TXV (TEV) is a type of metering device. The metering device’s job is to create a pressure drop from the liquid line into the evaporator which will result in refrigerant boiling (changing from liquid to vapor) through the majority of the evaporator coil. This low temperature “boiling” absorbs heat from the space or product being cooled.

Q: How does a TXV Function?

A: A TXV “measures” the temperature and (usually) the pressure at the end of the evaporator coil with a bulb and a tube called an external equalizer. The bulb measures temperature and provides an opening force, the equalizer measures pressure and provides a closing force. There is also a spring that may have an adjustable tension that provides additional closing force. When working properly these forces achieve a balance and maintain the evaporator superheat to the designed of set levels at the end of the evaporator.  The TXV’s job is to maintain superheat within certain operational ranges and conditions. 

Q: How do they fail?

A: A TXV may fail either too far open or too far closed. Too far open is also called “overfeeding” and it means that boiling refrigerant is being fed too far through the evaporator coil, this would show up in low superheat. If the TXV fails closed it can be said to be “underfeeding” which means not enough boiling refrigerant is fed through the evaporator coil and superheat will be too high at the evaporator outlet. 

These failures can and do occur, but they are usually caused by contaminants or moisture in the system that have worked their way to the valve and caused it to stick or become restricted. Another cause of valve failure is a rub out on bulb tube and an external equalizer without a core depressor installed on a port that has a Schrader core in place.  

When a valve is overfeeding the first thing to check is bulb insulation, placement and strapping. If the numbing isn’t properly sensing the suction line it can lead to the valve remaining too far open.

Q: Why are they misdiagnosed so often? 

A: TXV’s are often incorrectly condemned in cases of low evaporator airflow or load. This happens because techs will find a system with low suction pressure and assume that means it is low on refrigerant. They will then start to add refrigerant and the TXV will respond by closing further the more refrigerant is added. The tech will see that the suction isn’t increasing and they will conclude that the TXV is failed. 

This occurs because the tech is paying too much attention to suction pressure without considering the other readings.

Q: What is the correct way to diagnose a TXV? 

A: First take all of your refrigerant readings as well as your liquid line and suction temperature at both ends (on a split system). This means superheat, subcooling, suction saturation (evaporator coil temp) and liquid saturation (condensing temp). For a TXV to do what it is supposed to you need a full line of liquid before the TXV, this means you need at least 1° of subcooling in theory but in reality, you will want to make sure that you have the factory specified subcooling which is usually around 10°. In refrigeration, we do this same thing by looking for a clear sight glass. On a split system checking the subcool outside and then confirming there is no big temperature difference inside to out is a great way to ensure that kinked lines or plugged line driers aren’t an issue. 

The next thing that a TXV needs is enough liquid pressure to have the required pressure differential. This amount of required pressure differential will vary a bit based on the valve but usually, we want to see a 100 PSI minimum difference between the liquid line pressure and the desired evaporator pressure. If the head pressure drops too low due to low ambient conditions this can come into play and impact the ability of the valve to do its job. 

Once this is all confirmed then it is simply a matter of checking the superheat at the end of the evaporator. Most A/C systems will be maintaining 6-14° of superheat at the evaporator outlet. If it is in that range then the valve isn’t bad, it’s doing its job. 

If it is lower than 6° of superheat at the evap outlet then it could be overfeeding (double check your thermometer and gauges) and if the superheat is well above 14° at the evaporator outlet, with the proper subcool and liquid pressure entering… then you have a failed closed (underfeeding valve). Keep in mind that some valves will have a screen right before the valve and this can be the cause of the restriction rather than the valve. You can intentionally freeze the coil and try to see the freezing point or use thermal imaging to help spot if it’s the valve or the screen. When you find the point of temperature you find the point of pressure drop, just remember that the TXV is DESIGNED to provide pressure to maintain a fairly fixed superheat. 

Q: Do TXVs Ever Fail

A: They can fail internally but most often they fail because of a blocked inlet screen (if they have one), contaminants entering the valve, loss of charge from the power head, bulb location and positioning issues and overheating of the valve. In commercial and refrigeration applications you can often replace or clean the screen and replace the power head rather than replacing the entire valve. 

As I have said many times before diagnosis make sure your tools are well calibrated and working and that you are ACTUALLY reading the pressure correctly. I’ve seen many misdiagnoses just because a Schrader wasn’t pushing in or a multi-position valve cracked properly.

I walked into my first real job interview in the HVAC trade. The manager was a guy named Ernie and he walked me out to the warehouse.

Quick warning.. guys named Ernie are tough. Don’t mess with a dude named Ernie.

He walked up to a box, snatched a pen out of his shirt pocket and scribbled a circle, 3 dots, and three numbers on it while grunting “which is common, start and run”

I was in luck…

While I may have had almost zero practical knowledge of air conditioning, this was one thing I HAD actually learned in school.

I marked the terminals and I got the job.

Now, of course, this only applies to single-phase compressors and this leg to leg reading is helpful for identifying terminals but tells you very little about the condition of the windings unless you know the resistance in the first place or have historical readings or another identical compressor to compare to.

Before you say that this information is useless let me stop you.

It isn’t useless. It may not be something you use every day, but I have needed to ohm out a motor or compressor a handful of times and it got me out of a pinch.

So here it goes –

The lowest ohm reading is between Common and Run

The middle ohm reading is between Common and Start

The highest ohm reading is between Start and Run

Common is just a point between Start and Run and therefore the Common to Start and Run to Start readings will add up to the run to start reading. Many will tout this as a diagnostic reading you should check. it’s more a mathematical fact than something useful to check. If you did see a higher reading Start to Common + Run to Common vs. Run to Start it could really only indicate an increased resistance through the motor thermal overload that breaks common.

Here is how I remember which winding resistance is which (let the mockery begin)

Starting is hard… so it has the highest resistance

Running is hard also… but not as hard as starting, so it has a resistance less than Start.

Common is easy… being common requires the lowest resistance

So common to run is the least and start to run is the most.


The orientation when read like a book (top left to bottom right) is usually… if not always Common, Start then Run. Many techs remember that with the phrase “Can She Run”.

Understanding common, run and start is uncommon… so it requires a lot of resistance… so start… knowing it

OK, I’m done.

— Bryan

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