Special thanks to Ty Branaman and Bert Testerman for their work and feedback on this tech tip about compressor failure. A PDF checklist that covers all of the procedures in this tech tip can be found at http://www.hvacrschool.com/compressor-replacement-checklist.

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Important Note Before You Begin

Industry data indicates that up to 30% of compressors returned under warranty are found to have no fault. Technicians frequently misdiagnose the compressor as the problem when the actual root cause lies with another component. This checklist is designed to help you definitively confirm compressor failure, thereby saving time and money and avoiding unnecessary replacements.

A quick preliminary inspection can help you identify some potential issues. Look for obvious signs like oil on the ground, a lack of refrigerant in the system, or a pungent smell when attaching gauges. You should also check the electrical panel for a blown capacitor, hard start kits, pests (like bugs, lizards, or rodents), or damaged wires. However, it's crucial not to jump to conclusions or assume a single issue is the only problem.

This guide provides systematic steps to ensure a thorough and prepared diagnostic process. While it doesn't cover every possible scenario, it addresses the main points to help you accurately identify compressor failures.

General Safety First

Before beginning any diagnostic work, always prioritize safety. Failure to follow proper safety procedures can result in serious injury or death.

Lockout/Tagout (LOTO): Always de-energize and lock out all electrical power sources to the equipment before servicing when you don’t have full control of the point of disconnect. Verify with your multimeter that the circuit is dead before proceeding.

Personal protective equipment (PPE): Always wear appropriate PPE, including safety glasses and gloves. When handling refrigerants, ensure you are using refrigerant-rated gloves.

Refrigerant handling: Be aware of the dangers of refrigerant, including frostbite and asphyxiation. Always work in a well-ventilated area and follow all regulations for the safe handling, recovery, and storage of refrigerants.

Confirming Electrical Failure

This section will guide you through a systematic electrical diagnosis to verify the health of the compressor and its related components.

1. Performing an Initial Assessment at the Panel

Your first check starts where the power does. This assessment helps you understand the initial state of the system.

Has a breaker tripped, or has a fuse blown?

YES: This can indicate a short or a component drawing excessively high current. In some cases, it can even be a damaged breaker. Proceed to Step 2: The Short-to-Ground Test.

NO: The issue is likely not a direct short to ground. Proceed to Step 3: Checking for Thermal Overload.

2. Performing the Short-to-Ground Test

If the breaker tripped, you must find the short.

1. SHUT OFF ALL POWER at the disconnect and verify that the circuit has no power.

2. Disconnect the compressor wires.

3. Set your multimeter to measure Ohms (Ω).

• Pro Tip: Use the manual range setting on your meter, not auto-range. “This is a critical step where mistakes are commonly made. Verify your work before proceeding.” Auto-ranging can give confusing readings by switching to different scales (kΩ, MΩ). Manually select the standard Ohms scale.

4. Check the compressor terminals for a short to ground. Measure the resistance from each compressor terminal (Common, Start, Run) to a clean ground point on the compressor chassis or copper pipe. NOTE: Not having a perfectly clean ground connection can throw off a mega ohms reading on your average handheld clamp meters.”

Any continuity or low resistance reading to ground? The compressor is shorted to ground and has failed. You have confirmed the diagnosis. Per Copeland, some compressors could read as low as 0.5 MΩ, depending on the compressor and meter combination.

No reading to ground (OL) or a high mega ohm reading? The compressor windings are not shorted to ground. The term OL stands for “open line” or “over limit,” indicating infinite resistance. The short likely exists elsewhere. Check other components:

• Condenser fan motor
• Crankcase heater wiring
• Any other wiring in the high-voltage circuit

AVOID THESE TOOLS FOR CONDEMNING 

Do not use a megohmmeter (megger) to condemn a modern scroll compressor. Again, according to Copeland's AE-1294 bulletin, a scroll can read as low as 0.5 MΩ and still be perfectly fine. Factors like refrigerant in the oil and winding temperature, along with moisture or acids, can give false “bad” readings on a megger. Many good compressors have been condemned due to improper tools or tool usage. A meter that says “bad” does not mean that the compressor is bad.

Here is the link to BULLETIN AE4-1294 R1, where Copeland discusses the efficacy of megohmmeters for testing scroll compressors.

A standard ohmmeter test to ground is the correct method.

As a final step to rule out other potential causes for a blown fuse or tripping breaker, you must isolate the compressor. Disconnect its wiring completely, then re-energize the unit. If the rest of the A/C system powers up and operates normally, you have definitively confirmed that the electrical short is internal to the compressor and not elsewhere in the system's wiring or components.

3. Compressor Won't Start: Checking for Thermal Overload

If the breaker isn’t tripped but the compressor won’t start, one of the most common scenarios is that the compressor’s internal thermal overload protector has tripped. This safety device shuts the compressor off when it gets too hot to prevent motor damage. You must identify and address this before proceeding to more invasive electrical tests.

How to Identify Thermal Overload:

Feel the compressor: Carefully touch the top of the compressor. If it is extremely hot to the touch, it has likely tripped on thermal overload.

Listen for a hum and click: When the thermostat calls for cooling, you might hear the compressor hum for a few seconds and then a distinct “click.” This is the sound of the motor trying to start and the overload protector tripping.

Cool-Down Procedures:

The internal overload will not reset until the compressor cools down sufficiently. This can take a significant amount of time naturally. Even if the compressor shell cools down, the heavy internal components retain a lot of heat, so it's possible for the internal thermal overload to remain tripped. To speed up the process:

1. SHUT OFF ALL POWER at the disconnect.
2. Use water: Gently spray cool water over the top and sides of the compressor shell. Use extreme caution to avoid spraying any electrical components, terminal connections, or the fan motor. Be aware that you are working with electricity in the presence of water, and if it's an RTU (Rooftop Unit), be cautious of water getting inside the building.
3. Use air: Place a fan to blow air directly onto the compressor to help dissipate heat.

What to Do After Cooling:

Once the compressor is cool to the touch, you can proceed.

If the compressor starts and runs: The overload was likely caused by an underlying system problem, not a failed compressor. You must now investigate why it overheated. Common causes include:

• Dirty condenser coil
• Failed condenser fan motor or capacitor
• Refrigerant charge issue (over- or undercharged)
• Low starting voltage

Proceed to the “Investigating the Root Cause of Failure” section.

If the compressor still won't start (hums and clicks again): The issue is more severe. It could be a failed run capacitor, a wiring issue, or an internally seized compressor. Now is the time to proceed with Step 4: Verifying Voltage & Components and Step 5: The Winding Resistance Test.

4. Verifying Voltage & Components

If the breaker was not tripped and the compressor is not coming on, we need to confirm that the compressor is receiving the correct power and that its supporting components are functional.

1. SHUT OFF ALL POWER and verify that the circuit has no power.

2. Inspect the “Big 3” electrical components:

• Capacitor: Test the run capacitor. Is it within its rated MFD tolerance (+/- 5-6%)? A weak or failed capacitor will prevent the compressor from starting.

• Contactor: Are the points pitted, burnt, or welded? Manually press the contactor; does it move freely? Ants or even a small beetle can prevent the points from making contact. Check the terminals for discoloration, a sign of overheating.
• Wiring & Plug: Carefully inspect the wiring and confirm that it matches the manufacturer's diagram perfectly. There can be instances where someone before you may have miswired the equipment. Look for burnt or loose connections, especially at the compressor plug and contactor lugs but also at the disconnect. A bad connection is a common point of failure.

WARNING: COMPRESSOR TERMINAL INSPECTION BLOWOUT HAZARD: Exercise extreme caution when inspecting compressor terminals. The Fusite™ holding these terminals can become brittle and rupture under high compressor pressure, leading to a blowout. 

SAFETY PRECAUTIONS: Adhere to all safety protocols. Keep your face clear of any potential projectiles during inspection.

Inspect the wiring thoroughly. Look for nicks, cuts, or rubouts in the protective outer lining, especially where the wire enters the cabinet (knockouts can be rough). Also, check for contact points with refrigerant tubing; system vibrations can cause holes in the wire lining, exposing copper. Discoloration or stiff spots in the wire may indicate overheating.

A loose connection in the main electrical panel can sometimes cause problems. Depending on local regulations, working in the main panel might require an electrician's license, so always be aware of all applicable local codes.

If the components above are good, proceed to Step 5: The Winding Resistance Test.

5. Performing a Winding Resistance Test

This test checks for shorts or open circuits within the compressor's motor windings.

1. Ensure power is off and the compressor wires are disconnected.

2. Set your multimeter to measure ohms (Ω) on manual range.

3. Measure and record the resistance between the three terminals:

• Common to Start = _____ Ω
• Common to Run = _____ Ω
• Run to Start = _____ Ω

4. Analyze the readings:

Check the formula: (Common to Start) + (Common to Run) = (Run to Start). Do your readings add up? Note: this does not apply to three-phase or inverter compressors.

• If YES: The windings are likely okay, but compare with the manufacturer's specifications to be sure.
• If NO: The readings may indicate an open winding or an internal overload. If you get an OL (open line/over limit) reading from Common to Run and Common to Start, but you get a resistance reading between Run and Start, the compressor's internal thermal overload is likely open. This confirms the diagnosis you may have made in Step 3. If the compressor has been properly cooled and the overload does not reset, the overload itself has failed, and the compressor must be replaced.
• If you have an OL reading across all terminals, then you should double-check your meter settings, as this is a rare condition indicating that the wire has come loose inside the compressor.
• If you have an ohm reading across C-R but OL on C-S, then the start winding is internally broken.
• If you have an ohm reading across C-S but OL on C-R, then the run winding is internally broken.

Best Practice: The most accurate way to verify your readings is to compare them to the manufacturer's specifications. You can use a tool like the Copeland Mobile app or other manufacturer data for the exact winding resistance for the specific compressor model. For most three-phase compressors and variable speed compressors, the resistance readings between all three terminals should be equal. However, there are exceptions, so always consult the manufacturer's data.

6. Checking Low Voltage Controls

Many good compressors have been replaced due to a low-voltage control preventing the contactor from pulling in. Here are a few examples:

Low-pressure switches: Is there even refrigerant in the unit? There are a variety of low-pressure switches; some low-pressure switches are even on the high side and called loss-of-charge switches. Sometimes, these switches can fail even if there is refrigerant in the system.

High-pressure switches: These are designed to protect the compressor in the event of excessive pressure, such as a dirty outdoor coil or faulty condenser fan.

Discharge line thermostat: Can open to protect the compressor discharge line from overheating and damaging the compressor and oil. Some of these are used as a safety device; sometimes they are used to control a crankcase heater.

Always replace safety controls; never bypass them. Consider adding an additional safety control in a different location, if necessary.

Possible causes of compressor failure related to safety controls:

Time delay relay: A component that prevents rapid cycling (or short cycling) of the compressor. If the relay itself malfunctions, it can be the cause of the compressor short cycling.

Cut low-voltage wire: A damaged or severed low-voltage wire can interrupt the control circuit, preventing the compressor from starting or operating correctly.

Poor low-voltage connection: Loose or corroded low-voltage connections can create resistance, leading to intermittent operation or complete failure of the compressor.

Float switch (indoor unit): Typically found in condensate drain pans, a float switch shuts off the system if the pan overflows, preventing water damage. A stuck or faulty float switch can falsely indicate an overflow, shutting down the compressor. Note: There are some instances where certain brands of compressors will run backwards if the float switch is bumped.

Thermostat malfunction or miswiring: A malfunctioning thermostat won't accurately sense the temperature, leading to improper compressor operation. Damage can also cause the compressor to run erratically or not at all.

7. Inspecting Compressor Protection Components

Surge protector: This device safeguards the compressor from sudden spikes in electrical voltage, such as those caused by lightning strikes. A damaged surge protector indicates it successfully absorbed a harmful surge, preventing damage to the compressor.

Buck-boost transformer: This component regulates voltage, either increasing (“boosting”) or decreasing (“bucking”) it to ensure the compressor operates within its optimal voltage range.

Voltage monitor: A voltage monitor continuously tracks the incoming electrical voltage to the compressor, alerting if it falls outside acceptable limits.

Utility demand response systems: The electricity company might have a system in place that limits the compressor's operation during periods of high electrical demand to reduce strain on the power grid.

8. Checking Minimum Voltage at Startup

It is important to check the minimum voltage at startup. If the voltage drops too low, the compressor's startup time is increased, and it can trip on internal thermal overload.

Solutions for low starting voltage:

Electrical supply: A larger wire size may be needed. Electrical components may need to be replaced, such as breakers, disconnects, or connectors. In some cases, an electrical service upgrade is required.

True soft start: For systems operating on generators or with undersized wiring, a true soft start kit that uses a processor to limit amps at startup can help overcome low starting voltage. A great example of this is the Micro-Air EasyStart.

Factory hard start: A factory-matching hard start kit can also help overcome low starting voltage.

9. Inspecting Hard Start Kits

Hard start kits can sometimes add more life to a failing compressor, but they can sometimes be the root cause of compressor failure. It’s important to inspect these components and replace them if necessary.

A hard start can also add life to a compressor that is suffering from internal copper plating or bearing failure. They can specifically benefit reciprocating and rotary compressors under specific conditions, like those with long linesets, solenoids, and hard shut-off TXVs. Typically, scrolls don’t need hard starts, even with those design considerations, because they start unloaded. But because reciprocating and rotary compressors don’t unload internally like the scroll, a hard start can help them overcome low starting voltage—and in some cases, they can prevent the compressor from running backwards.

Failure Example 1: If the run capacitor fails, the hard start kit will continually engage. This can cause the compressor's start winding to burn out. A CPT (current protection terminal) can help prevent this from happening.

Failure Example 2: A faulty potential relay can fail to disengage, keeping the start kit energized. This can lead to a blown start capacitor or a burnt-out start winding.

How to check: You can test the coil terminals and inspect the points in the relay for signs of damage. Thermal disks are sometimes used in relays to reduce costs. However, these disks depend on temperature and resistance, which can compromise their effectiveness and make them challenging to inspect.

To confirm if a hard start kit is functional, measure the amp draw on its lead to the run capacitor's “HERM” terminal during compressor startup. A reading of zero amps means the kit is not working and is failing to assist the compressor, making replacement necessary even if the capacitor's MFD reading is within specification.

Installation best practices: Never reuse an old hard start kit on a new compressor. It’s highly probable that the new compressor will not require a start kit. If a hard start kit is needed, always use one recommended by the compressor manufacturer to ensure proper function and warranty compliance.

Diagnosing a Running but Underperforming Compressor

This is a specific diagnostic for a compressor that appears to be running but is not providing the expected cooling or heating capacity. In these cases, the compressor has often been mistakenly condemned when the actual issue lies elsewhere.

Is this a two-stage compressor?

YES: A two-stage scroll compressor typically operates on low stage at about 67% capacity. When the second stage is energized, it shifts to 100% capacity. This is where many issues can be misdiagnosed. To confirm the diagnosis, follow these steps:

1. Check the call for the second stage. Use your multimeter to check for 24V AC at the Y2 terminal. Use the common terminal (C) as your reference, not ground. This confirms that the indoor unit is attempting to activate the second stage. If you are not getting 24V, you must find where you lost the voltage, from the thermostat settings all the way to the wires and connectors outside.

2. Amp check both stages: Confirm both stages with an ammeter. The current flow should change from 35–45% of RLA for single-stage operation then 50–60% RLA when operating in the second stage.

3. Check the molded plug. The second stage of the compressor has a separate two-wire molded plug, which often contains a diode to convert AC to DC voltage. With the unit calling for the 2nd stage, set your meter to read DC and check the plug itself for 24V DC. Switch the terminals and check again.

4. Analyze the plug readings.

• If you read 24V DC at the plug: Ohm out the terminals on the compressor side and compare them to the manufacturer's specifications. If you read an open line (OL), the DC coil inside the compressor is bad, and the compressor needs to be replaced.
• If you do NOT read 24V DC at the plug: Follow the two wires back to the other end and check for voltage there. Some units supply 24V AC to the cord, and the diode in the plug converts it to DC. Others supply 24V DC from a control board directly. If you are getting 24V AC or DC at one side of the wire but not at the plug, it's likely a bad diode in the plug. In this case, you need to replace the plug and diode, not the compressor.

Note: A diode acts as a one-way check valve for electricity. It converts 24V alternating current (AC) to a direct current (DC). If the voltage is already supplied in DC, the diode will not make a difference. Many good compressors have been replaced because of faulty plugs.

NO: If the compressor is a single-stage model, or if the two-stage check was inconclusive, you can check the compression ratio and internal safeties. (Note: This does not apply to variable-speed compressors or digital scrolls, which have wide operating ranges and require manufacturer-specific diagnostics. Also, note that two-stage reciprocating compressors, which often run in reverse with separate contactors, have not been commonly used in recent years; always consult the manufacturer for specific instructions for these.)

Using the Copeland Mobile App for Advanced Diagnostics

For the most complete diagnostics on operating capacity, use the Copeland Mobile app.

Process:

Open the app, go to Performance, and select Dynamic Performance. Enter the pressures and other readings under the specific conditions, and the app will provide the exact expected discharge temperature and amperage for those conditions.

Interpretation:

If the amps are very low compared to the expected values, you likely have a compressor capacity issue.

If the amps are very high for those conditions, you may have an internal electrical short.

If everything else is within range, but your discharge temperature is very high, it could be an indication of inadequate oil return to the compressor.

Calculating the Compression Ratio

Formula: (Discharge Absolute Pressure) / (Suction Absolute Pressure) = Compression Ratio

First, calculate absolute pressure: PSIG + 14.7 = PSIA

Example: (Discharge 350 PSIG + 14.7) / (Suction 75 PSIG + 14.7) = 364.7 / 89.7 = a 4.07:1 ratio.

• Suction Absolute Pressure: __ PSIG + 14.7 = __ PSIA
• Discharge Absolute Pressure: __ PSIG + 14.7 = __ PSIA
• Compression Ratio: __ PSIA / __ PSIA = _____

Typical ranges:

• Air Conditioning: 2.3:1 – 3.5:1 (Note: it is possible to see slightly lower than 2:3:1)
• Medium-Temp Refrigeration: 3:1 – 5.5:1
• Low-Temp Refrigeration: 6:1 – 13:1

Interpretation:

• A low compression ratio may indicate a compressor capacity issue.
• A high compression ratio suggests that the problem is not the compressor itself, but this condition can still cause damage to the compressor.

Checking for Activated Internal Safeties

Many compressors have built-in safety mechanisms to protect them from damage due to high pressure or temperature.

Internal pressure relief valve: This valve can open to dump discharge pressure into the suction side, which you might identify by a hissing or spraying sound. This high-pressure discharge will eventually trip the internal thermal overload. To reset, the compressor must be allowed to cool. If this occurs, check for air and refrigerant restrictions both inside and outside the unit. Internal pressure relief will be much more than just overcharged. Here are some examples:

•  Dirty condensing coil
•  Bad outdoor fan motor
• Incorrect outdoor fan blade
• Bad condenser fan capacitor 
• Fan blade set at the wrong height 
• Fan blade turning backwards 
• Condenser air recirculation
• Restriction inside the condenser
• Excessive ambient temperatures 
• Non-condensables
• Wrong refrigerant

Thermal operating disk: Some scroll compressors have a valve that opens to redirect refrigerant when internal temperatures become too high, also dumping discharge gas into the suction or crankcase side. Check for air and refrigerant restrictions inside and outside the unit.

Floating seal: If the compression ratio becomes too high, a floating seal can open, allowing the scroll plates to bypass and effectively disengage, similar to pressing a clutch. Repeated overheating from this condition can lead to permanent damage to the seal, causing it to leak like a faulty reciprocating valve.

How to Perform a Pump-Down Test

Some technicians perform a pump-down test to check a compressor mechanically.

Procedure:

With the system running, close the liquid line service valve. Pull the disconnect before the compressor reaches 0 PSI or when the compressor starts making unusual noises. Do not run the compressor into a vacuum.

Important notes: A pump-down test should not be performed on a microchannel coil without a liquid receiver. Some commercial refrigeration systems are designed to operate in a vacuum. A scroll compressor has a floating seal for internal protection and may not pull down below 40 PSI. The rattling noises are associated with the scroll sets bypassing and a lack of lubrication, so the compressor should not be run beyond that point.

Interpreting the results:

If a reciprocating or rotary compressor cannot pull down near 0 PSI, or a scroll cannot pull down below 40 PSI, it could indicate one of three things:

1. There is too much refrigerant in the system for the condenser to hold.
2. A low-pressure switch opened to protect the system.
3. There is internal compressor damage, such as bad valves or a bad floating seal.

After the disconnect is pulled, the suction pressure should rise slightly and then level off. If the suction pressure continues to rise back to normal operating pressures, the compressor has an internal failure, such as bad valves in a reciprocating/rotary compressor or a bad check valve in a scroll.

Investigating the Root Cause of Failure (“The Why”)

Now that you've confirmed the compressor is bad, you must find out why it failed. Installing a new compressor into a faulty system will only lead to another failure.

Checking for Signs of Overheating

Overheating can quickly kill a compressor. The most common cause is a high compression ratio, which results from a system imbalance. This can be caused by:

Low refrigerant charge: The most frequent cause of a high compression ratio is a system that is low on refrigerant due to a leak. Recovering the refrigerant and weighing it against the manufacturer's specified charge is a quick way to confirm this.

Suction side restrictions: A restriction on the low-pressure side of the system can cause a high compression ratio. Examples include:

• An undersized suction line
• A suction drier that was left in the system
• A clogged filter drier or multiple filter driers
• A clogged screen before the metering device
• A kink in the refrigerant lines
• A TXV that is stuck closed

Dirty condenser or poor airflow: Issues on the high-pressure side of the system can also lead to a high compression ratio. Examples include:

• A dirty condenser coil
• A condenser fan motor failure
• A condenser fan that is out of alignment with the shroud or is turning the wrong way
• An incorrect fan motor RPM
• Recirculating air around the condenser

Discharge Superheat

Discharge superheat is another tool to evaluate compressor health.

It is measured by taking the discharge temperature and subtracting the liquid saturated temperature. If your suction superheat to the compressor is normal but your discharge superheat is high, it can be a sign of a compressor low in oil in the crankcase. Roman Baugh will soon publish a tech tip that does an in-depth review of discharge superheat.

Performing Oil Diagnostics after Overheating

Proper oil return is essential for a compressor's longevity. If a compressor failed due to overheating, it is vital to determine if it was running low on oil.

How to test for oil issues:

Drain and measure: Drain the oil from the old compressor and measure the quantity. This also allows you to inspect the oil's condition for signs of sludge or burning. Compare this amount to the manufacturer’s data (found in the Copeland Mobile app) to determine if it was low.

Weigh the compressor: Weigh the old compressor and compare its weight to the new compressor. If you have removed all copper from the old unit, the weights should be similar. A significant difference can indicate an oil issue.

Use a thermal imager: Using a heat gun and a thermal imager, you can heat the base of the compressor. Areas that heat up quickly have no oil, while areas that heat slowly contain oil. Compare these results to a new compressor.

What if the oil is low?

If the compressor is low on oil, the oil likely migrated to and is logged in the evaporator coil. You can try to flush it out of the evaporator coil to be sure. If it is, the next question is why. This could be from a number of issues, such as short cycling, low indoor temperatures, or, a significant cause, oversized suction lines.

What if the oil is high?

If the compressor has too much oil, it may be a result of a previous technician adding oil to a system with an oil-logged evaporator coil or multiple compressor changes over time. Note: Some new compressors may come with slightly less oil than the original, so always verify the correct charge.

Important note on line sizing: Refrigerant line size and the need for traps are application- and manufacturer-specific. Do not add traps or change line size without checking with the manufacturer first.

Checking for System Contamination

Contaminants act like poison in a refrigerant circuit, leading to chemical reactions and physical damage.

Perform an oil and acid test:

When you remove the compressor, take an oil sample. Inspect the oil for a dark, sludgy appearance or a burnt smell. There are two main types of acids that can be found in a system: organic and inorganic acids. Organic acids typically form due to moisture contamination, which can turn the oil into an acid, especially with hygroscopic POE oil. This acid etches copper from the tubing walls, and the copper flakes can cause arcing across internal compressor terminals and lead to copper plating on moving parts. Inorganic acids form from refrigerant decomposition due to electrical arcing. This acid is much more powerful and can attack the varnish on the insulation of a new compressor; as little as 50 PPM can cause a new compressor to burn out in days. It is important to note that quick acid test kits using litmus paper typically only check for organic acids. For a more comprehensive test, an oil sample can be sent off for testing that will give specific details on contamination. This type of test is more common for inverters and commercial applications.

If acid is present, acid cleanup procedures are needed. Copeland has a step-by-step procedure; most of them include suction line filter-driers. 

Identify the contaminant:

• Moisture: The most common contaminant, leading to acid formation and copper plating.
• Non-condensables (air, nitrogen): Increase head pressure and system temperatures.
• Non-condensables test: After you recover the refrigerant, let the tank settle in a shaded area of constant temperature. Measure the pressure and temperature of the tank where both liquid and vapor are present, and compare these readings to a pressure-temperature (PT) chart. If the measured pressure is higher than the chart's value for that temperature, non-condensables are likely present. Note that this test can be more challenging with high-glide refrigerants, but using the mean or average temperature is the best approach.
• Solid debris: Debris from a previous failure or installation can cause blockages and wear. Check the old filter-drier and screen at the metering device for solid debris, oxidation from brazing, carbon from burnt oil, copper shavings, soft solder balls, flux, etc. A magnet should be used on the suction line near the compressor to ensure that no metal debris remains.

Checking for Liquid Refrigerant Damage

Liquid refrigerant will destroy a compressor, which is designed to compress vapor only. This happens in two main ways: floodback and flooded starts.

Check for poor airflow & metering device issues:

It's important to actually measure the airflow to find the cause. Causes of poor airflow include:

• Dirty evaporator or secondary heat exchanger coils
• Dirty filters
• Dirty blower wheel
• Undersized or blocked returns
• Undersized supply ducts
• Closed vents

Metering device issues: Investigate the metering device. Is the piston or TXV the correct size? Is the TXV bulb mounted correctly and insulated?

Check for flooded starts (off-cycle migration to the compressor):

Flooded starts happen when liquid refrigerant settles in the crankcase during shutdown and violently boils at startup, washing oil away from critical parts while simultaneously filling the compressor with liquid that it tries to compress.

Inspect the crankcase heater: Is it working? Is it wired correctly? A common mistake is miswiring a heater after replacing a single-pole contactor with a two-pole, rendering the heater useless.

Liquid solenoid valve: Does the system have a liquid line solenoid valve that is not closing all the way?

Evaluate system design: Are there long refrigerant lines without proper traps? Is the system overcharged? These factors increase the risk of flooded starts.

Inspecting the Accumulator

If the system has an accumulator, there are additional steps. An accumulator is beneficial to protect the compressor from liquid floodback.

The accumulator has a small screen and orifice in the bottom to allow oil return. If that screen gets clogged up, it can starve the new compressor of oil.

The accumulator will need to be replaced if it is rusted, leaking, or holding oil. You can use a heat gun and a thermal imager to check to see if it is holding excessive oil.

You can also remove the accumulator and try to drain the oil out to check the level and check for any debris or carbon buildup that could clog up the screen. See the HVAC School article about confirming liquid levels in accumulators with a little thermodynamics to learn more.

Installing the New Compressor

A quality installation is the best guarantee for a long compressor life. Follow these best practices meticulously.

How to Remove the Old Compressor & Prep the Line Set

Safety first: The safest way to remove the old compressor is by using tubing cutters. Heating a compressor with a torch can cause remaining refrigerant trapped in the oil to boil out and carry oil with it, which can ignite when it crosses the torch flame. This is especially dangerous with A2L, A2, and A3 refrigerants. Make sure to have enough couplings and copper on hand to replace the section of the line set that you cut out.

Swaging vs. couplings: While swaging the suction line is often acceptable, a coupling on the high side is recommended. The higher vibrations and temperatures of the discharge line make a swaged connection more likely to fatigue and crack over time. Note that the coupling will need to be purchased ahead of time.

Following Brazing Best Practices

Proper brazing prevents internal contamination and ensures strong, leak-free joints.

Purge with nitrogen: Before brazing, you must remove the oxygen from the lines to prevent internal oxidation (black scale). Use the 3X pressurize-and-dump method:

1. Pressurize the lines with dry nitrogen.
2. Vent the nitrogen completely.
3. Repeat this process at least three times.

Flow nitrogen during brazing: After the purge, flow nitrogen at a very low rate (2–3 SCFH) through the lines while brazing. While it is not possible to flow nitrogen directly through the compressor, exchanging it three times reduces the amount of oxygen and reduces the likelihood of copper plating.

Use the right alloy: Compressor stubs are often copper-plated steel. If the copper plating burns away, you'll need a high-silver (e.g., 56%) flux-coated brazing rod to create a strong bond with the steel underneath.

Protect components: Use wet rags or a heat-blocking putty (like Viper WetRag or HeatShield) to protect the compressor body paint, electrical terminals, and nearby components from heat damage.

Performing a Leak Test

Never skip a thorough leak test. Finding a leak now is far easier than coming back for a refrigerant leak call.

Nitrogen pressure test: After the joints have cooled, pressurize the system with dry nitrogen to the pressure specified on the unit's data tag. To ensure accuracy, perform a temperature-compensated pressure test built into the HVAC school app, the measureQuick app, or many others.

Bubble test: Apply a quality leak reactant solution (like Refrigeration Technologies Big Blu) to all brazed joints and fittings. Use a mirror to inspect hard-to-see areas. Watch carefully for any bubbles or foam “cocoons” to form, which will reveal even the smallest leaks. Microbubbles can go unnoticed without careful inspection.

Performing an Evacuation and Decay Test

A proper evacuation removes moisture and non-condensables, ensuring system efficiency and longevity.

Pull a deep vacuum: Evacuate the system to below 300 microns.

Perform a decay test: Isolate the system from the vacuum pump. The vacuum level should not rise above 500 microns after holding for at least 10 minutes.

Troubleshoot a failed test: If the vacuum rises quickly or goes above 500 microns, you either have a leak or remaining moisture in the system. If it jumps past 1000 microns, you most likely have a leak that needs to be found and repaired. If the ambient temperatures are below 40°F, heat will need to be added to the system to prevent moisture from freezing. The lower the temperature, the more heat that will need to be added.

Completing Wiring and Final Electrical Checks

Incorrect wiring can damage the new compressor or prevent it from starting.

Correct terminal wiring:

• The compressor's Run winding connects to one side of the contactor.
• The run capacitor's “C” terminal is fed from that same side of the contactor.
• The compressor's Common winding is fed from the other side of the contactor.
• The compressor's Start winding connects to the “Herm/H” terminal on the run capacitor.

Replace components:

Always replace the run capacitor and contactor with a new one when replacing the compressor.

Always replace the compressor terminal plugs. Be gentle when tightening connections on the glass Fusite terminals to avoid causing damage.

Remove any aftermarket hard start kits. Only install a factory-approved start assist if the manufacturer requires it, especially on a system under warranty.

Charging the System and Performing Final Checks

The final steps involve carefully charging the system and verifying its operation to ensure the new compressor starts its life under ideal conditions. Correctly charging the system is critical to prevent a repeat failure from an overcharged or undercharged condition.

Initial holding charge: Begin by introducing a partial charge into the liquid line with the system off. Place the refrigerant cylinder on a scale and purge the hoses (unless a vacuum was pulled on the hoses during the evacuation process). Weigh the charge into the liquid line. For package units, this will be the factory charge with a small variation for a larger filter-drier. For a split system, an adjustment needs to be made for longer or shorter line sets. Depending on the conditions, it's common not to get the complete charge in the system at once. This can be done with a tank heater or with the next step.

Operational charging: Once the compressor is running, slowly add the remaining refrigerant into the suction line by throttling in the refrigerant, allowing time for any liquid to flash to a vapor before reaching the compressor. Blends must be charged as a liquid, while single-component refrigerants like R-22 or R-32 can be charged as either a vapor or a liquid. Charge to the manufacturer's specifications using the superheat, subcooling, or weigh-in method. Be patient, especially on systems with an accumulator, allowing the system time to react and balance out. The closer you get to the target, the slower you should go to prevent overcharging and having to recover again.

Verify the charge: Verify the charge after running for 15 minutes without a change and record the final weight used in the panel for the next technician. Systems with a liquid receiver should not be over 80% full when pumped down.

Commissioning & Verifying the System Post-Installation

After installation, verify that the system is operating as intended and create a performance baseline. This ensures the root cause of the failure was fixed and provides a reference for future service calls.

Confirm system airflow: Low airflow is a major cause of compressor failure. Confirm the actual CFM using a tool like a TrueFlow grid, not just by measuring static pressure. Investigate and correct any issues like dirty filters, blocked coils, or ductwork problems.

Clean and inspect: Ensure the condenser and evaporator coils are clean, and install new air filters.

Verify operational parameters: Allow the system to run and stabilize before taking readings.

• Check superheat at both the evaporator outlet and the compressor inlet.
• Check subcooling at the condenser outlet.
• Measure suction and head pressures.
• Measure delta T across the evaporator coil.
• Measure evaporator temperature difference (TD)
• Check the discharge line temperature about six inches from the compressor outlet. High temperatures can indicate oil breakdown or other internal issues.
• Measure the condensing coil TD, also known as CTOA (condensing temperature over ambient).
• Record the return air wet bulb, supply air wet bulb, and the outdoor temperature.
• Record the amperage and the voltage.

Create a full system profile: Record all operating temperatures, pressures, and electrical readings (amps, volts). Using a tool like measureQuick can help streamline this process and provide a comprehensive report.

Considering Protective Devices & Electrical Upgrades

Consider valuable add-ons that protect the new compressor and improve system reliability.

Surge protection: Install surge protection to prevent damage from transient voltage spikes. While standard single-phase compressors are robust, power surges can still cause damage. Not all surge protectors are the same. There are many to choose from, but we like the DTK-120/240CM+.

Contactor upgrade: Discuss upgrading to a premium contactor like the White-Rodgers SureSwitch. These offer benefits such as brownout protection, built-in short-cycle delays, and sealed contacts that are less prone to failure due to insects or pitting.

Note: The image above only represents the wiring required for the compressor and crankcase heater. It does not show the fan or low-voltage wiring.

Other accessories: Other beneficial upgrades include high- and low-pressure switches and a discharge thermostat to further protect the compressor from unsafe operating conditions.

Overvoltage protection: For sites with consistent steady-state overvoltage problems, a device like the DITEK KoolGuard2 can be an option, though it provides the most value for systems with inverter drives or VFD motors.

Gathering Tools and Supplies

To replace a compressor successfully, you’ll need the right tools. Here is a list of tools and materials needed for a residential single-phase compressor installation.

Safety & Protective Gear 

☐ Safety glasses (tinted if using oxy-acetylene) 

☐ Gloves 

☐ Other standard PPE

☐ Fire extinguisher

 ☐ Fire blanket

Tools

 ☐ Acid test kit 

☐ Leak detector 

☐ Thermal imager, heat gun (optional) 

☐ measureQuick or a similar system profiling tool 

☐ TrueFlow grid (or similar airflow measurement tool) 

☐ Multimeter 

☐ Manifold gauge or probe set 

☐ Digital thermometers/probes 

☐ Recovery machine 

☐ Recovery tank 

☐ Vacuum pump 

☐ 2x Vacuum hoses (one large diameter) 

☐ 2x Valve core removal tools 

☐ Dedicated recovery hose 

☐ Nitrogen cylinder and regulator 

☐ Micron gauge 

☐ Scale (for weighing old/new compressors and refrigerant) 

☐ Wire strippers, crimpers 

☐ Screwdriver set 

☐ Adjustable wrenches 

☐ Pliers 

☐ Tubing benders 

☐ Level 

☐ Compressor lifting tool 

☐ Service valve wrench 

☐ Socket set 

☐ Strong magnet 

☐ Pipe cutters 

☐ Deburring tool 

☐ Brazing torch/kit

Supplies 

☐ 15% silver phos brazing rods 

☐ 56% flux-coated high-silver brazing rods 

☐ Heat shields (such as the Viper WetRag HeatShield) 

☐ Wet rags

☐ Suction line filter drier (HH model for burnouts) 

☐ New liquid line filter drier 

☐ Ball valves (for isolating driers) “optional” 

☐ Standard piping (to replace temporary suction line filter-drier) 

☐ Various fittings and piping as needed 

☐ Refrigerant 

☐ New air filters 

☐ Electrical tape 

☐ Sandpaper or emery cloth 

☐ Leak detection bubbles or spray 

☐ System labels 

☐ Cleaning rags/towels

Replacement Components & Electrical 

☐ New compressor 

☐ Replacement plugs and connectors for compressor terminals 

☐ Factory-approved hard start kit (if specified) 

☐ Run capacitor 

☐ Contactor (or upgrade) 

☐ Large gauge wire (if needed for rewiring) 

☐ Crankcase heater 

☐ Wire nuts 

☐ Zip ties 

☐ Surge protector (recommended) 

☐ DITEK KoolGuard2 (optional)

Conclusion

Replacing a compressor is more than just a mechanical swap; it's a complete system overhaul. By following this checklist, you move beyond simply fixing the immediate problem. You become a system detective, a skilled installer, and a proactive technician.

This methodical approach—from confirming the failure and diagnosing the root cause to performing a meticulous installation and commissioning—is the difference between a callback and a satisfied customer who keeps coming back. Taking the time to do it right ensures the new compressor has the best possible chance at a long, efficient life, ultimately providing the most value and reliability for your customer.