Tag: compressor

In HVAC/R we are in the business of moving BTUs of heat and we move these BTUs on the back of pounds of refrigerant. The more pounds we move the more BTUs we move.

In a single stage HVAC/R compressor, the compression chamber maintains the same volume no matter the compression ratio. What changes is the # of pounds of refrigerant being moved with every stroke(reciprocating), oscillation (scroll), or rotation (screw, rotary) of the compressor. If the compressor is functioning properly the higher the compression ratio the fewer pounds of refrigerant is being moved and the lower the compression ratio the more pounds are moved.

In A/C and refrigeration the compression ratio is simply the absolute discharge pressure leaving the compressor divided by the absolute suction pressure entering the compressor.

Absolute pressure is just gauge pressure + atmospheric pressure. In general, we would just add the atmospheric pressure at sea level (14.7 psi) to both the suction and discharge pressure and then divide the discharge pressure by the suction. For example, a common compression ratio on an R22 system might look like-

240 PSIG Discharge + 14.7 PSIA = 254.7
75 PSIG Suction + 14.7 = 89.7 PSIA
254.7 PSIA Discharge ÷ 89.7 PSIA Suction = 2.84:1 Compression Ratio

The compression ratio will change as the evaporator load and the condensing temperature change but in general, under near design conditions, you will see the following compression ratios on properly functioning equipment depending on the efficiency and conditions of the exact system.

In air conditioning applications compression ratios of 2.3:1 to 3.5:1 are common with ratios below 3:1 and above 2:1 as the standard for modern high-efficiency Air conditioning equipment.

In a 404a medium temp refrigeration (cooler) 3.0:1 – 5.5:1  is a common ratio range

In a typical 404a 0°F to -10°F freezer application 6.0:1 – 13.0:1 is a common ratio range

As equipment gets more and more efficient, manufacturers are designing systems to have lower and lower compression ratios by using larger coils and smaller compressors.

Why does the compression ratio number matter? 

When the compressor itself is functioning properly the lower the compression ratio the more efficient and cool the compressor will operate, so the goal of the manufacturer’s engineer, system designer, service technician and installer should be to maintain the lowest possible compression ratio while still moving the necessary pounds of refrigerant to accomplish the delivered BTU capacity required.

The compression ratio can also be used as a diagnostic tool to analyze whether or not the compressor is providing the proper compression. Very low compression ratios coupled with low amperage and low capacity are often an indication of mechanical compressor issues.

Compression ratio higher than designed = Compressor overheating, oil breakdown, high power consumption, low capacity 

Compression ratio lower than designed = Can be an indication of mechanical failure and poor compression

Understanding compression is critical to understanding the refrigeration process. Don’t be tempted to skip past this because it is a really important concept.

Look at the pressure enthalpy diagram above. Top to bottom (vertical) is the refrigerant pressure scale, high pressure is higher on the chart. Horizontal (left to right) is the heat content scale, the further right the more heat contained in the refrigerant (heat, not necessarily temperature).

Start at point #2 on the chart at the bottom right. This is where the suction gas enters the compressor. As it is compressed it goes to point #3 which is up because it is being compressed (increased in pressure) and toward the right because of the heat of compression (heat energy added in the compression process itself) as well as the heat added when the refrigerant cooled the compressor motor windings.

Once the refrigerant enters the discharge line at point #3 it travels into the condenser and is desuperheated (sensible heat removed). This discharge superheat is equal to the suction superheat + the heat of compression + the heat removed from the motor windings. Once all of the discharge superheat (sensible heat) is removed in the first part of the condenser coil it hits point #4 and begins to condense.

Point #4 is a critical part of the compression ratio equation because the compressor is forced to produce a pressure high enough that the condensing temperature will be above the temperature of the air the condenser is rejecting its heat to. In other words, in a typical straight cool, air cooled air conditioning system the condensing temperature must be higher than the outdoor temperature for the heat to move out of the refrigerant and into the air going over the condenser.

If the outdoor air temperature is high or if the condenser coils are dirty, blades are improperly set or the condenser coils are undersized point #2 (condensing temperature) will be higher on the chart and therefore will put more heat strain on the compressor and will result in lower compressor efficiency and capacity.

As the refrigerant is changed from a liquid vapor mix to fully liquid in the condenser it travels from right back left between points #4 and #5 as heat is removed from the refrigerant into the outside air (on an air cooled system). Once it gets to #5 is is fully liquid and at point #6 it is subcooled below saturation but ABOVE outdoor ambient air temperature. The metering device then creates a pressure drop that is displayed between points #6 and #7. The further the drop, the colder the evaporator coil will be. The design coil temperature is dictated by the requirements of the space being cooled as well as the load on the coil but the LOWER the pressure and temperature of the evaporator the less dense the vapor will be at point #2 when it re-enters the compressor and the higher the compression ratio will need to be to pump it back up to point #3 and #4,

This shows us that the greater the vertical distance between points #2 and #4 the higher the compression ratio, which means that both low suction pressure and/or high head pressure result in higher compression ratios, poor compressor cooling, lower efficiency and lower capacity.

In some cases, there isn’t much that can be done about high compression ratios. When a customer sets their A/C down to 69°F(20.55°C) on a 100°(37.77°C) day they will simply have high compression ratios. When a low temp freezer is functioning on on a very hot day it will run high compression ratios.

But in many cases, you can reduce compression ratios by –

  • Keeping set temperatures at or above design temperatures for the equipment. Don’t be tempted to set that -10°F freezer to -20°F or use that cooler as a freezer
  • Keep condenser coils clean and unrestricted
  • Maintain proper evaporator airflow
  • Install condensers in shaded and well-ventilated areas

Keep an eye on your compression ratios and you may be able to save a compressor from an untimely death.

— Bryan

Every contractor is different, I get that. we don’t all need to do everything the same way or include the same services with repairs but there are some “best practices” that can save you a lot of heartache before, during and after you make a big repair.

Catch it During Diagnosis

Let’s say you find a failed, shorted compressor on a 7 year old system that still has manufacturer parts coverage. If you simply quote the compressor and leave you may be missing a lot of other maintenance related issues that can affect operation once the compressor is replaced. A short list of items to check would be –

  • Look at the accumulator for signs of corrosion
  • Acid test to see if a burnout protocol should be used
  • Check the air filter
  • Inspect the condenser coil cleanliness
  •  Look at the underside of the evaporator coil
  • Perform a static pressure test on the system to check for duct issues
  • Check the crankcase heater (if it has one)
  • Inspect the contactor
  • Check condenser fan and blower motor amps
  • Test all capacitors
  • Visually inspect wires and cap tubes
  • Check high voltage electrical connections

And this is just for cooling side issues. If the system is a fuel-burning appliance you would inspect every part of the furnace operation as well.

Testing all of these things is commonplace AFTER a repair, but it makes so much more sense to do it beforehand so that you can either charge appropriately for any of these items that need to be addressed or let the customer know you are including them to differentiate you from the competition.

Things to Do Along With Major Repairs 

There are a few things you need to do as a matter of course during major air conditioning or refrigeration repairs that just make good sense to prevent callbacks. You can include them in the price or not or not but either way, it will save you more than it costs to do it.

  •  Clean the drain line and condensate pan (seriously…. do this)
  • Wash the condenser coil
  • Clean the blower wheel (if it is dirty)
  • Change the air filter
  • Test both modes of operation

Do these things along with all of the standards tests you perform to make sure that you have no issues and that whatever caused the fault in the system has been rectified and you will save a lot of problems. When the customer spends a lot of money getting a system fixed, they don’t want to turn around and have it fail for an “unrelated” reason.

While this list is clearly tailored to the residential and light commercial air conditioning market, every piece of equipment has its common maintenance items. So what do you do every time when you make a major repair?

— Bryan


This article is written by regular contributor, experienced rack refrigeration tech and RSES CM Jeremy Smith. Thanks Jeremy.

A technique that you can use to diagnose compressor problems and to help differentiate them from other possible issues is the use of compressor performance analysis.

Manufacturers do extensive testing of their compressors before they sell them, and a part of that testing is available to you as a troubleshooting tool. The compressor performance chart. I’ll primarily refer to Copeland compressors as they are what I service most, but I’ve been able to find charts and data from other manufacturers through their websites and tech support lines.

Let’s look at a real-world example. I went to do a follow-up check after a major leak and recharge on a set of freezers. On arrival, the cases, which had been running now for 14 or 15 hours since having been repaired, weren’t as cold as expected. Checking the unit, here is what I found:

Copeland compressor
27# suction
185# discharge
209v (average of all 3 legs)
13.9A draw.
Unit at 18-20°F

The suction line was cool to the touch and the sight glass had a thin ‘river’ of refrigerant in it. The high suction pressure really jumped out at me here as worthy of more consideration.

Now, a high suction pressure in this instance can be caused by high load (note the high unit temperature) or it can be caused by a compressor problem. Looking over the data here, I was concerned about the health of the compressor and its ability to pump properly. I did a quick “pump down test” and found it inconclusive. The compressor pulled to 24” Hg easily and held there. Still, I wasn’t happy with this, so I pulled out my smartphone and opened the Copeland Mobile app.

A quick note on pump down ‘tests’. They really aren’t effective on most modern compressors. I performed the test and included the results here to illustrate exactly that fact.

Entering the model of the compressor leads you to select the application (R502 low temp which is closest to R404a low temp). Selecting the “Diagnostics” tab brings you to a screen where you and input pertinent data and the app then outputs both the expected amperage at your conditions and the percent deviation
from the norm.

In this case, my expected amperage was significantly higher than my observed amperage, so the high suction was definitely caused by a compressor problem.
I recovered the refrigerant from the machine and removed the compressor head and valve plate for internal evaluation.

Finding a single broken suction reed, the rest of the internals were intact making this a good candidate for a new valve plate. I Installed new valve plates, evacuated and restarted the machine and re-evaluated operation.

Had this been a hermetically sealed compressor, I would have had no choice but to condemn and replace the compressor. This time, the amperage was within 5% of specifications (sorry, didn’t get a screenshot) and I continued to monitor unit operation until equipment reached 0° F, verified and completed proper
charging of the unit and called it a day.

Why not use RLA (Run Load Amps) (? Or use LRA÷6 (Or is it 8?) to diagnose?

The simple answer is that they just aren’t sufficiently accurate enough for me dealing with high stakes, high dollar equipment and they shouldn’t be accurate enough for you, either.

Let’s return to my real-world example…

The compressor has a listed RLA of 25.8 and a LRA (Locked Rotor Amps) of 161.0. Now look back at the original screenshot of the app. It calls for an amp draw of 17.09A at that set of conditions. If we compared that to the RLA, even the correct amperage looks low. If we use common LRA divisors 161 ÷ 6 gives us 26.83A and 161 ÷ 8 gives us 20.125A. Maybe a little better than the RLA method but still off by a significant amount. Enough to cause concern and possibly lead to an incorrect diagnosis.

Not a one of these methods gives us an accurate expected amperage for this machine. That inaccuracy can lead us to draw a bad conclusion and potentially wasting time and money pursuing a “bad” compressor that is in fact, working exactly as it should.

Like most things in HVAC/R using a fixed operational target without considering the specific conditions can lead to misdiagnosis and a lot of wasted time. You would be surprised what is available within manufacturer specs if you take the time look.

— Jeremy Smith, CM

This is a basic overview of the refrigeration circuit and how it works. It isn’t a COMPLETE description by any means, but it is designed to assist a new technician or HVAC/R apprentice in understanding the fundamentals.

First, let’s address some areas of possible confusion 

  1. The Word “Condenser” Can Mean two Different Things Many in the industry will refer to the outside unit on a split air conditioner, heat pump or refrigeration unit as a “condenser” even though it will often contain the condenser, compressor, and other parts. It’s better to call the outside component the “condensing unit” or simply the “outside unit” to reduce confusion.
  2. Cold and Hot are Relative terms Cold and Hot are both an experience, a description, a comparison or an emotion. Cold is a way to describe the absence of heat in the same way that dark describes the absence of light. We will often use the words cold and hot to compare two things “Today is colder than yesterday” or to communicate comfort “It feels hot in here”. These are useful communication tools, but they are comparisons not measurements.
  3. Heat and Absolute Zero Can be Measured We can measure heat in BTUs and light in lumens, we cannot measure cold or dark. Absolute cold is the absence of all heat.  -460°F(-273.3°C) (cold) is known as absolute zero, -460°F(-273.3°C) is the temperature at which all molecular movement stops. Any temperature above that has a measurable level of heat. While this is a known point at which all molecular movement stops, it has not (and likely cannot) be achieved.
  4. Boiling Isn’t Always Hot When we say it’s “boiling outside” we mean it’s hot outside. This is because when we think of boiling we immediately think of water boiling in a pot at 212°F (100°C) at atmospheric pressure, which is 14.7 PSI (Pounds Per Square Inch)(1.01 bar) at sea level. Boiling is actually just a change of state from liquid to vapor, and the temperature that occurs varies greatly based on the substance being boiled and the pressure around the substance. In an air conditioner or a refrigeration system, refrigerant is designed to boil at a low temperature that corresponds to the design of the system. On an average air conditioning system running under normal conditions with a 75°(23.88°C) indoor temperature, the evaporator coil will contain refrigerant boiling at around 40°F(4.44°C). In air conditioning and refrigeration when we refer to “boiling”, “flashing” or “evaporating of refrigerant” we are talking about the process of absorbing heat, otherwise known as cooling.
  5. Cooling and Heating Cannot be “Created” We are not in the business of making heat or creating cool; it cannot be done. We simply move heat from one place to another or change it from one form to another. When we “cool” a room with an air conditioner, we are simply absorbing heat from the air into an evaporator and then moving that heat outside to the condenser where it is “rejected” or moved to the outdoors.
  6. Heat and Temperature Aren’t the Same  Imagine a shot glass of water boiling away at 212°F(100°C). Now imagine an entire lake sitting at 50°F(10°C). Which has a higher (hotter) temperature? That answer is obvious-I just told you the shot glass had 212°F(100°C water in it so it is CLEARLY hotter. But, which contains more heat?  The answer is the lake. You see, heat is simply energy and energy at its basic form is movement. When we measure heat we are measuring molecular movement; the movement of molecules–atoms stuck together to make water or oxygen or nitrogen. When molecules move FASTER they have a HIGHER temperature and when they move SLOWER they have a LOWER temperature. Temperature is the average speed (velocity) of molecules in a substance, while heat is the total amount of molecular movement in a substance. The lake has more heat because the lake has more water (molecules).
  7. Compressing Something Makes it Get Hotter (Rise in Temperature) When you take something and put pressure on it, it will begin to get hotter. As you pack those molecules that make up whatever you are compressing, they get closer together and they start moving faster. If you drop the pressure the molecules will have more space and will move slower causing the temperature to go down.
  8. Changing the State of Matter Moves Heat Without Changing Temperature  When you boil pure water at atmospheric pressure it will always boil at  212°F(100°C). You can add more heat by turning up the burner, but as long as it is changing state (boiling), it will stay at 212°F(100°C). The energy is changing the water from liquid (water) to vapor (steam) and the temperature remains the same. This pressure and temperature at which a substance changes state instead of changing temperature is called its “boiling point”, “condensing temperature” or more generally “saturation” point.
  9. Superheat, Subcool, Boiling, and Saturation Aren’t Complicated  If water is boiling at sea level it will be 212°F(100°C). If water is 211°F(99.44°C) at sea level we know it is fully liquid and it is 1°F(-17.22°C) subcooled. If water is 213°F(100.55°C) at sea level we know it is vapor and superheated. If something is fully liquid it will be subcooled, if it is fully vapor it will superheated, and if it is in the process of change (boiling or condensing) it is at saturation.

Where to start 

Take a look at the diagram at the top of this piece and start at the bottom left. Are you looking at the part at the bottom left? OK, now read this next line OUT LOUD:

Compressor > Discharge line > Condenser > Liquid Line > Metering Device > Expansion Line > Evaporator > Suction line and then back to the Compressor

When I first started in HVAC/R trade school this was the first thing my instructor forced me to LITERALLY memorize forward and backwards before he would allow me to proceed.

While I am not always a huge fan of rote memorization as a learning technique, in this case, I agree with committing this to memory in the proper order.

These four refrigerant components and four lines listed above make up the basic circuit that every compression refrigeration system follows. Many more parts and controls may be added, but these basics are the cornerstone on which everything else you will learn is based. Once you have these memorized we can move on to describing each.


The compressor is the heart of the refrigerant circuit. It is the only mechanical component in a basic refrigeration system. The compressor is like the heart that pumps the blood in the body or like the sun that provides the earth its energy. Without the compressor to move the refrigerant through compression, no work would be done and no heat would be moved.

The compressor creates a pressure differential, resulting in high pressure on the high side (discharge line, condenser & liquid line) and low pressure on the low side (suction line, evaporator and expansion line).

There are many different types of compressors, but you will most likely see Scroll and Reciprocating type compressors most often. A reciprocating type compressor uses pistons, valves, and a crankshaft. Reciprocating compressors operate much like car engines; pulling in suction vapor on the down-stroke and compressing that vapor on the up-stroke. A scroll compressor does not have any up-down motion like a reciprocating compressor. A scroll compressor uses an oscillating motion to compress the low-pressure vapor into high-pressure vapor.

The compressor pressurizes low-pressure vapor into high-pressure vapor, but it also causes the temperature of the gas to increase. As stated in the gas laws, an increase in pressure causes an increase in temperature and a decrease in volume. In the case of refrigerant cooled compressors, heat is also added to the refrigerant off of the kinetic (bearings, valves, pistons) and electrical (motor windings) mechanisms of the compressor. Compressors require lubrication; this is accomplished through oil that is in the compressor crankcase, as well as oil that is carried with the refrigerant. Liquid entering the compressor through the suction line is a very serious problem. It can cause liquid slugging, which is liquid refrigerant entering the compression portion of the compressor. Liquid slugging will most likely cause damage to the compressor instantly. Another problem is bearing washout or “flooding”. This occurs when liquid refrigerant dilutes the oil in the compressor crankcase and creates foaming, and it will greatly reduce the life of the compressor because it will not receive proper lubrication and too much oil will be carried out of the compressor and into other parts of the system. The compressor also (generally) relies on the cool suction gas from the evaporator to cool the compressor properly, so it’s a delicate balance to keep a compressor from being flooded and also keep it cool.



Condensers come in all different types, shapes, and sizes. Regardless, they all perform the same function: rejecting heat from the refrigerant. The refrigerant entering the condenser was just compressed by the compressor, and this process increased the temperature by packing the molecules together which added heat to the vapor refrigerant due to the motor and mechanical workings of the compressor. This process in the compressor also greatly increased the pressure from a low-pressure in the suction line entering the compressor, to a high-pressure vapor leaving the compressor.

The condenser has three jobs:

  1. Desuperheat the refrigerant (Drop the temperature down to the condensing temperature)
  2. Condense (saturate) the refrigerant (Reject heat until all the refrigerant turns to liquid)
  3. Subcool the refrigerant (Drop the temperature of the refrigerant below the condensing / saturation temperature)

The condenser’s job is to reject heat (drop the temperature) of the refrigerant to its condensing (saturation) temperature, then to further reject heat until the refrigerant fully turns to liquid. The reason it must fully turn to liquid is that, in order for the refrigerant to boil in the evaporator, it must first have liquid to boil.

The way in which the condenser removes the heat from the refrigerant varies. Most modern condensers flow air over the tubing where the refrigerant is flowing. The heat transfers out of the refrigerant and into the air. The cooling medium can also be water. In the case of a water source system, water is circulated across the refrigerant in a heat exchanger.

In either case, the condenser relies on the removal of heat to another substance (air, water, glycol etc..). For instance, if you turned off the condenser fan so that no air was flowing over the condenser coil, the condenser would get hotter and hotter. This would cause the pressures to get higher and higher. If it kept going that way it would trip the internal overload on the compressor or cause other damage.

The hot vapor from the compressor enters the condenser and the superheat  (temperature above condensing temperature) is then removed. The refrigerant then begins to change state from vapor to liquid (Condense). The refrigerant maintains a constant temperature until every molecule of vapor is condensed. The temperature of the liquid again starts to fall. This is known as subcooling. When we measure subcooling we are measuring degrees of temperature rejected once the refrigerant has turned completely to liquid.

Temperature above the saturation temperature is called superheat. Temperature below the saturation temperature is called subcool or subcooling. So when something is fully vapor (like the air around us) it will be superheated, and when it is fully liquid (like the water in a lake) it is subcooled. 

Metering Device

The metering device is a pressure differential device that creates a pressure drop to facilitate refrigerant boiling in the evaporator coil.

The metering device is located between the liquid line and the evaporator. The liquid line is full of high-pressure liquid refrigerant. When the high-pressure liquid hits the small restrictor in the metering device, the pressure is immediately reduced. This drops the pressure of the refrigerant to such a degree, that the saturation temperature is lower than the temperature of the air surrounding the tubing that the refrigerant is in. This causes the refrigerant to start changing from liquid to vapor. This is called “boiling” or “flashing”. This “flashing” brings the refrigerant down from the liquid line temperature to the boiling (saturation) temperature in the evaporator, and in this process a percentage of the refrigerant is immediately changed from liquid to vapor. The percentage of the refrigerant that changes during flashing depends on how great the difference is. A larger difference between the liquid line temperature and the evaporator boiling temperature results in more liquid lost to flashing and reduces the efficiency of operation.


There are a few different types of metering devices. The most common ones being the Thermostatic Expansion Valve (TXV/ TEV) and the Fixed Orifice (often called a piston)– as well as electronic expansion valves, capillary tubes, and others.



The evaporator is also known as the cooling coil, because the purpose of the evaporator is to absorb heat. It accomplishes this through the refrigerant changing from liquid to vapor (boiling). This boiling process begins as soon as the refrigerant leaves the metering device, and it continues until the refrigerant has absorbed enough heat to completely finish the change from liquid to vapor. As long as the refrigerant is boiling it will remain at a constant temperature; this temperature is referred to as saturation temperature or evaporator temperature. As soon as the refrigerant is done boiling, the temperature starts to rise. This temperature increase is known as superheat.

When the indoor air temperature or the air flow going over the coil is higher, the evaporator pressure and temperature will also be higher because more heat is being absorbed into the coil. When the air temperature or airflow over the coil is lower, it will have lower pressure and temperature in the coil due to less heat being absorbed in the coil.

The refrigerant leaves the evaporator, travels down the suction line and heads back to the compressor where the cycle starts all over again.

Refrigerant Lines 

Suction Line = Line Between the Evaporator and the Compressor

The suction line should contain low-pressure superheated suction vapor. Cool to the touch on an air conditioning system, and cold to the touch in refrigeration.

Discharge Line = Line Between the Compressor and the Condenser 

The discharge line should contain high temperature, high pressure superheated vapor

Liquid Line = Line between the Condenser and the Metering Device

The liquid line should be high pressure, slightly above outdoor temperature subcooled liquid

Expansion Line (When applicable) = Line Between the Metering Device and the Evaporator

On most systems, the metering device will be mounted directly to the evaporator making the expansion line a non-factor. Some ductless mini-split units will mount the metering device in the outside unit making the second, smaller line and expansion line. The expansion line is full of mixed vapor/liquid flash gas.

Yes, this was long, but more than anything else just keep repeating over and over: compressor>discharge line>condenser>liquid line>metering device>expansion line> evaporator>suction line and on and on and on…

Jim Bergmann did a great whiteboard video on the MeasureQuick YouTube page that explains the basic refrigerant circuit





First off I want to thank Ulises Palacios for taking these photos. He is in the habit of cutting open the compressors he replaces to see why they failed (when possible). I think that’s pretty boss.

So why would the compressor have copper plating on the inside? They certainly aren’t manufactured that way.

The short answer is the acid inside the system eats away at the copper and brass components in the system. The copper is then deposited in the high pressure/temperature environment of the compressor.

Why does this happen?

The presence of any acids in the system can cause this to occur but the most likely causes are the combination of air and moisture reacting with the refrigerant oil (most prevalently POE) to create an environment in which the copper is dissolved internally and redeposited on the steel in the compressor.

The result inside the compressor is reduced clearances and ultimately locking, overheating and even short circuits if the mechanical failure results in winding damage as is fairly common.

So for a technician, what we can do in ensure we are properly evacuating the system and installing appropriate filter driers to reduce or eliminate the presence of air and moisture.

— Bryan

P.S. – For an in-depth analysis of a study on copper plating in compressors you can read here

I’ve got a confession to make.
I’m ‘that guy’ call it OCD, call it being anal retentive, but I’m always making an effort to be as technically correct as possible, and one aspect of that effort has been the use of torque indicating or torque limiting tools when tightening fasteners.


It started after I put new valve plates and gaskets on a Carlyle 06E compressor. As I was always taught, all the torque you needed to apply to a fastener was the torque you could apply with a normal sized combination wrench, so that is exactly what I did. The compressor gaskets failed and were bypassing the very next day because I didn’t get those bolts tight enough. The tech who took that call asked me if I had and used a torque wrench. At first, I didn’t understand.. sure, I tighten the bolts. No, he asked, did you TORQUE them to specifications? 90-100 ft/lbs was the spec and, while you can get that kind of torque out of a standard length ¾”
wrench, you’re working at it.


That question started a dive down something of a ‘rabbit hole’ for me and I’m going to share
some of what I learned.


I started with one torque wrench, just to tighten the bolts on compressor heads. From there, I’ve expanded to have three torque wrenches (¼ ⅜ and ½ drive) and a torque limiting screwdriver mostly for electrical connections.


What is torque? Simply put, torque is a measurement of the amount of force required to turn a fastener. For bolts, torque is normally measured in ft/lbs or in/lbs. To help you understand a foot-pounds (ft/lb) or inch-pounds (in/lb). A foot pound is one pound of force applied on a lever one foot long measured from the center of the fastener.


As we continue to increase the torque applied to a bolt, the male threads on the bolt move deeper into the female threaded hole while the part being fastened, for example, a compressor head, prevents the head of the fastener from following them. This stress results in a stretching of the bolt. That stretch applies what is called ‘clamping force’ to the assembly. This stretching permanently deforms and weakens the bolts, and sometimes proper assembly requires
the use of new bolts. Lubricant applied to the fastener makes it easier to turn which decreases the torque required to achieve the same clamping force. Be careful to always follow manufacturer guidelines.

One place where torque wrenches are making inroads in our industry are with ductless mini splits. While I haven’t broken down and added one of these to my kit yet, I do have an easy way to torque flare fasteners using a regular torque wrench. Crowsfoot wrenches.


These little guys allow you to turn any ratchet or breaker bar into a handy wrench. To use them with a torque wrench, however, requires a little extra step.
Remember how we measure torque? It’s based on the distance between the center of the fastener to the point where force is applied. Well, a torque wrench is calibrated to have force applied on the knurled part of the wrench handle and centers that force on the centerline of the drive spindle. Adding a crowsfoot wrench to the end of the wrench changes the center of the applied force. What we need to do is account for the extra length of the crowsfoot and the extra leverage that


For this, use your required torque force as TA to solve the math. That said, I’ve found very little actual difference when using a crowsfoot wrench, and since often torque values are given in a range, it isn’t really necessary to calculate the difference, just set your wrench to the low end of
the torque range and use the crowsfoot. That will generally keep you within the specified torque range.


Another trick, where possible, is to install the crowsfoot at a 90° angle to the drive. Doing this makes the actual and effective length of the tool the same and allows direct use of the tool without calculations. Keep in mind that a standard socket drive extension won’t affect your torque wrench settings because it doesn’t affect the length of the tool in the direction that matters.


I hear a lot of guys argue against using a torque wrench because they can tighten things up just fine without one. Probably so. I did a lot of jobs prior to the one I mentioned earlier without using one and I was “just fine” or was I? Did I tighten those flanges evenly or did I warp the flange by over-torquing one side? Did I over-torque that flare and set myself up for a leak later? Did I tighten all the bolts evenly ensuring even clamping force on all those gaskets? A torque tool simplifies things for us. Tighten to specified torque, and you’re done. You don’t have to think about that variable anymore. It’s as tight as it’s supposed to be and no tighter.
It’s one less thing to worry about.

— Jeremy Smith


This tip will be like an episode of Columbo, we will start with the what and who and then get to the why.

  1. Don’t pump down a scroll into a vacuum
  2. Don’t run a scroll in a vacuum
  3. Don’t run a high voltage megohmmeter or Hi-pot test on a scroll (As a general rule don’t go over double the rated running volts)
  4. Don’t do a any megohmeter test with a scroll under vacuum

These points have been confirmed with Copeland (Emerson) as being on the naughty list this Christmas.

Resistance / Megohm Testing

A scroll is like any other compressor in that it has a motor and a compression chamber “hermetically” sealed inside the shell. There are many differences between a scroll and a reciprocating compressor but let’s focus on the few that are pertinent to this conversation (or at least the pertinent ones I can think of).

  • In a scroll, the motor is located on the bottom, this means that the motor is immersed in refrigerant and oil. When the compressor has been off and is cold, there can even be some liquid refrigerant in the compressor.
  • A scroll is more compact and balanced design as there is no need for “suspension” like a reciprocating compressor. This results in closer tolerances / distances between the electrical components and the other metal parts.

The motor being located at the bottom is the biggest thing. Copeland states in bulletin AE4-1294 that megohm readings as low as 0.5 megohms to ground are acceptable. Besides that fact that this makes a scroll difficult to successfully meg (essentially impossible with a tool like the Supco M500 because it only reads down to 20 Mohms) it is a clear indication that a scroll compressor is running tighter resistance tolerances and a higher risk of internal arcing due to many factors. Another thing to consider is the scroll will read lower ohms to ground when it is cold than when it is running due to higher refrigerant / oil density at lower temperature and of course you are generally doing a meg test when a scroll has been off…. so that makes it tricky.

Some of the factors that can decrease resistance further and lead to problems are:

  • Moisture contamination
  • Free metallic particles due to copper leaching (acids), small metal pieces left from copper fabrication or metal from compressor breakdown due to other issues like overheating, flooding and improper lubrication.
  • Other contaminants

All of this to point out that tolerances are tight in a scroll to begin with.. add in some extra nastiness and you are at risk.

Pump Down 

First, many scroll compressors won’t even allow you to pump them down into a vacuum. Either they are equipped with a low pressure cut out or some sort of low pressure / low compression bypass like shown in this USPTO drawing


For example, in Copeland AE4-1303 it states “Copeland Scroll compressors incorporate internal low vacuum protection and will stop pumping (unload) when the pressure ratio exceeds approximately 10:1. There is an audible increase in sound when the scrolls start unloading.’ This is to prevent the compressor from pulling down into a vacuum.

In addition to that there are lot’s of threats and warnings about running a scroll while it is in a vacuum, as in, if you had just evacuated the system and then accidentally turned the system on. Which is a bad idea on any compressor, but worse on a scroll.


The totally obvious reason is that the compressor itself isn’t designed to run in a vacuum and it will overheat as well as fail to lubricate properly, but that isn’t the only reason or even the primary reason.All of the literature mentions arcing and I spoke to more than one tech rep who mentioned the “fusite” plug arcing or being damaged.

First, Fusite is a brand name and one of the companies in the Emerson family. So when we say “fusite” we are using a ubiquitous term for a sealed glass to metal compressor terminal feed through. There are many different types and  designs of Fusite terminal just as there are many different types and designs of compressor. There are scroll compressors that use them, there are reciprocating compressors that use them, the ice cream truck that plays that obnoxious music driving through your neighborhood probably has one…. on the refrigeration compressor. Do certain fusite terminals short out more easily than others? I’m sure some are more susceptible than others. Is that what is going in here… maybe.. but if so it’s only part of the story.

What we do know about a scroll is the electrical tolerances are tighter… and when electrical tolerances are tighter there is a greater likelihood of arcing.

It’s about to get really nerdy here so if you don’t care just stop reading and go back to the very beginning, memorize the 4 points and move on with your life.

I can’t do that… because I’m broken.

Why is vacuum an issue? Isn’t vacuum the absence of matter and isn’t matter required for electrons to arc from one surface (cathode) to the other surface (anode)?

The answer is not really simple AT ALL but the summary is that under certain circumstances vacuum increases the likelihood of arcing and scroll compressor terminals inside the compressor happen to be one of those circumstances.

First thing to remember is that while electrons do travel through matter, electromagnetic fields do not require matter to exist and in either case.. we are incapable of achieving a perfect vacuum so no matter how deep we pull a vacuum, some molecules are still present.

I’ve some some techs attribute this to the corona discharge effect which can occur due to the ionization of particles around a high voltage conductor. I really don’t see this as being the answer both because the voltages applied are not THAT high and corona discharge is not a arc or a short in the traditional sense, just a “loss” to the environment around the conductor and a pretty cool looking light (as well a decent Mexican beer).

My opinion (and this is an opinion not a fact) is that the arcing is due to something called field electron emissions which can result in insulator breakdown in vacuum conditions (NASA has to deal with it all the time in space because space is a vacuum ).

The conclusion is that while this phenomenon can happen in ANY compressor, it is made more likely in a scroll due to tighter tolerences and “motor down” configuration. This means that doing a high voltage meg test, or any running / meg testing under vacuum is a bad idea.

If you want to read more about Fusite, Copeland scroll compressors and a great overall guide that includes evacuation procedures just click the links.

Nerd rant over.

— Bryan



I walked in to my first real job interview in the A/C business. 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 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.

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.

Here is how I remember this (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.

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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