COMPRESSOR CONDENSER - A/C Compressor & Condenser Inspection, Defects, & Compressor Replacement AirCondAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Air conditioner compressor defect diagnosis
• Air conditioning compressor noises
• Air conditioning compressor hard-starting
• Lost of cooling capacity
• When is a compressor at or near end of its life?

This chapter of "How to Inspect the Central Air Conditioning or Cooling System" discusses the detection of defects in air conditioning compressor and condensing units, including evaluation of air conditioner compressor noises, hard starting, lost cooling capacity, and detection of a burned out compressor or A/C compressors at or near end of their life. Maintenance tips including attention to compressor support pads and avoiding air conditioning refrigerant leaks are addressed.

If readers return to the first chapter or view the A/C chapter index, the major components of an air conditioning system are described, sketches and photographs are provided, and common defects for each component are listed along with visual or other clues that may suggest a problem or probable failure of A/C components. We explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects. We include photographs to assist readers in recognizing cooling system defects.

This paper describes the inspection of residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects. Example home inspection report language is provided to describe common air conditioning system defects. We continue to add to and update this text as new details are provided. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use the links at page left to navigate this document or to go to Other Website Topics. Green links at left show where you are in our document & website.

What are the Components of the Outdoor Portion of a Central Air Conditioning System - the Air Conditioning Compressor Unit?

1. The Air Conditioning Compressor Itself - on residential units the A/C compressor motor is most often a hermetic motor-compressor combined in a single sealed unit like the Carrier(TM) unit shown at above left. If a ductless split-system is installed an outside compressor unit is still required, typically looking like the Sanyo(TM) unit shown at the top of this page. The compressor is a basically a pump which moves refrigerant gas to the compressor via the larger refrigerant "suction line" returning it from the in-building air handler and evaporator coil. The compressor compresses the refrigerant to a high pressure gas and moves that gas into the condensing coil described just below. The refrigerant gas leaves the compressor at high pressure and at high temperature (since compressing a gas will raise its temperature.)
2. Refrigerant lines: the larger diameter refrigerant suction line connects the indoor evaporator coil outlet to the compressor inlet.
   The smaller-diameter high pressure refrigerant lines connect the compressor outlet and the condensing coil inlet and also move refrigerant liquid in it's cooled, condensed and now liquid state from the outlet of the condensing coil to the thermal expansion valve (basically a refrigerant metering device) and the evaporator coil inlet in the air handler unit in the building.
   Service valves or ports are usually present on the refrigeration lines near the compressor. to permit testing the condition of the air conditioning system and permit removal, replacement, or additions to the refrigerant in the system.
3. Condensing coil receives high pressure refrigerant gas from the compressor and cools this refrigerant gas back to a liquid state.
4. Outdoor cooling fan moves outdoor air across the condensing coil to cool it and assist in condensing the high pressure, high temperature refrigerant gas back into a liquid. It is this process which completes the transfer of heat through the refrigerant from indoor air to outdoor air as the compressor/condener unit compresses and then cools the refrigerant back to a liquid.
5. Electrical shut-off switch(es) for service at the unit are provided to permit maintenance and repair of the equipment. Circuit breaker(s) at the electrical panel protect the circuit supplying power to the air conditioning system.

These components are discussed in detail throughout this website using the links at the left of these pages.

Minimum Air Conditioner Compressor Unit Observations for an Air Conditioner Report

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

INSTALLATION ERRORS - Air Conditioning Compressor Installation Errors & Damage

Air Conditioning Compressor Location Problems

Inadequate clearance: Typical outside air-cooled A/C compressors require a minimal clearance around the unit to provide adequate air flow so that the condenser coils will be cooled efficiently. Units mounted too close to a wall, surrounded by shrubs, or multiple units located too closely together may not receive enough cool air flow to function properly. The result can be a shorter compressor life (expensive) and/or less efficient cooling operation (higher operation costs).

Refrigerant Line Defects

Absence of slack in the coolant lines at the compressor units can cause leaks: should the compressor move, perhaps because its supporting pads settle, there will be likely leaks at these lines. You should review this question with your HVAC service person. This item may be deferred until next maintenance or service.

Missing insulation on the refrigerant lines, particularly on the larger suction line, will cause condensation and drips from the lines in humid areas. I've seen very costly building damage where lines were not properly insulated indoors: condensate drips wet gypsum board walls, leading to a costly mold remediation project. Missing insulation also probably increases system operating cost.

CONDENSING COIL FINS - Air Conditioner Condensing coil fin damage

If the condensing coil fins are extensively bent and damage, airflow across the coil is impeded, causing the same failures and operating cost issues described above for the compressor too close to the building wall.

Air Conditioning Compressor/Condenser Unit Support Pads

Outside A/C compressors are typically supported on a concrete pad, concrete blocks, or a vinyl pad. The pad should be level and secure against movement.

Compressors which are badly out of level may fail to function properly and need adjustment. Tipping and moving compressors can also cause can cause leaks in refrigerant lines.

COMPRESSOR NOISES - How to Diagnose Air Conditioner Compressor Noises

... We observed unusual noise at the compressor, a condition which could indicate either a service and adjustment requirement, or an upcoming costly repair involving replacement of this component. You should have the system checked promptly by an air conditioning service technician.
... Compressor noise could be due simply to loose hardware such as a loose shipping bolt, tubing, or a broken spring.
... Compressor noise could also be due to refrigerant liquid "floodback" into the compressor crankcase. If this is the problem the noise will appear only at the interval of compressor start-up. That's why it's useful for the inspector to be right at the compressor unit when the air conditioning system is first turned-on. If this is the problem, a crankcase heater can be installed to address this defect.
... Another source of compressor noise include a low oil condition in the unit - diagnosed perhaps by observing evidence of compressor oil leakage on or around the unit.
... Some compressors emit a high pitched noise during normal operation.
... Compressor noises, at least the costly ones, are usually due to a loose connecting rod, piston pin, crankshaft, or other internal part. Since compressors on most modern commercial and residential air conditioning systems are a sealed unit, the only repair is to replace this (costly) part.

HARD STARTING - How to Diagnose Air Conditioner Compressor Hard Starting

A "hard starting" compressor may stutter or begin to cycle-on then stop, then restart. If a compressor is frequently tripping the circuit breaker (or blowing the fuse) which protects its circuit, repair is needed.

Abnormally low line voltage may also be causing a compressor to "hard start".

When an air conditioning compressor has been running it has pumped refrigerant to a high pressure condition in the compressor head.When an A/C system compressor is running and is unexpectedly shut off, perhaps by a human testing a thermostat or switch, if the system is immediately turned back on, some compressors, particularly older window and wall units, may be unable to re-start against this high head pressure. Simply waiting a few minutes for pressures to equalized may be all that's needed. So if the compressor is not starting in this condition we simply turn it off and wait. If this problem is happening often, a service technician may install one of several versions of "hard start kit".

Do not just install a larger fuse or circuit breaker as doing so risks a fire or equipment burn up. Do not bypass fuses such as by installing copper tubing in place of fuses as some A/C service people recommend. This is a crazy electrical hazard risking fire, shock, and equipment burn-up. If new circuit breakers are to be installed to replace an older fuse-protection on an air conditioning circuit, the electrician should remove any obsolete, unused electrical equipment or devices to avoid future confusion or possible shock hazards.

A "hard start kit" is basically a capacitor which gives an extra electrical "jolt" to the compressor motor to get it moving. A starter relay may also need to be installed or replaced. Some air conditioning compressor brands do not usually need a hard start kit. GE, Trane, and Manurope are examples that do not usually take this kit.

A compressor which has difficulty starting might be fixed by installing a "hard start" kit, but depending on the reason for hard starting it's possible that the entire compressor will have to be replaced soon. On a more optimistic note, some hard start kit manufacturers assert that installing a "hard start kit" on a compressor will extend its life. "By assisting the compressor in starting at up to ten times faster than normal, Kickstart(R) hard start devices significantly reduce the amount of damaging heat that is generated in the motor windings with each and every start. Over time, this reduced stress on insulation, wiring, and other critical components of the compressor has the effect of increasing its reliability and extending its useful life." -- www.kickstartoem.com

How to Diagnose an Air Conditioner Compressor Which has Lost Cooling Capacity

A compressor which appears to have lost cooling capacity can be diagnosed by a service technician who can connect the appropriate test gauges to the system. Lower than normal discharge pressure and higher than normal suction vacuum will indicate this problem. But before assuming that something is wrong with the air conditioning compressor, some basic investigation is in order.

Cooling Capacity of the Air Conditioning Equipment

The cooling capacity of an air conditioning equipment refers to the ability of the compressor/condenser (usually outside) and the air handler/evaporator (usually inside) to deliver cool air to the occupied space. Briefly, the compressor/condenser draws refrigerant gas from the building air handler, compresses it and cools it back to a liquid refrigerant, and the air handler/evaporator coil permits liquid refrigerant to evaporate inside a cooling coil, across which the fan blows building air to cool and dehumidify it. The particular combination of this equipment has a cooling capacity, usually rated in BTUh or thousands of BTU's of cooling capacity per hour, documented on equipment data tags discussed above at "RATED COOLING CAPACITY" - see links at left.

Cooling Capacity of the Duct System

Even if a very high BTUH capacity cooling system is installed, if the duct system is defective the ability of the system to deliver cool air to the occupied space can be severely or even totally lost. Duct System Efficiency (in percent) describes the percent of cooled air produced by the A/C equipment which is actually delivered to the occupied space. This number is less than 100% because of air flow restrictions and losses in the duct work. Duct Delivery Effectiveness is the percent of cooling capacity which is delivered through the registers into the occupied space. Registers themselves restrict air flow. See "DUCT SYSTEM DEFECTS" - see links at left.

Cooling Capacity of the Whole Air Conditioning System

So the ability of the entire A/C system to cool a building or rooms in it requires that both the cooling equipment and the duct system be in proper working order.

How to Diagnose a Burned-Out Air Conditioning Compressor

An air conditioning compressor can "burn out" by shorting of internal components - which means it will fail to start completely. This failure is detected by disconnecting all power and wiring from the unit and measuring resistance (ohms) between the motor start/common and run/common terminals. If there is zero resistance the winding is open or broken.

If you measure the resistance across a compressor winding and your meter's needle is stuck over at infinity, or "OL"/"OVER" on a digital meter, that would indicate the compressor winding is open (burned through). The same effect can be observed from simply connecting the meter to absolutely nothing.

If the resistance measured across the air conditioning compressor winding is too close to 0 ohms, it's shorted. The compressor should blow the fuse or trip the circuit breaker when power is turned back on. But watch out: we get field reports of equipment burn ups and even fires when the air conditioning circuit breaker for the compressor happens to be an old FPE Stab-Lok or Zinsco unit.

If there is resistance between the motor terminal and the motor casing, the motor has become shorted to ground internally and the unit needs to be replaced. If there is no resistance between the start and run terminals to common, but there is resistance between the start and run terminals, this means that the internal motor overload protection circuit is open. In this last case, allow the motor to cool and re-test it before replacing it.

Air Conditioning Compressor Motor Capacitors

Capacitors are usually present on large single phase compressors, as found on home A/C units. They're only very rarely present on small compressors, such as those in refrigerators, and as far as we know, never present on 3-phase power systems. The capacitor gives the compressor motor a boost for starting.

Capacitor safety warnings:
When testing a compressor, one must discharge the capacitor first! It'll otherwise have enough power stored on it to be at least very painful. (Author and others have been zapped!) Some systems will automatically discharge the capacitor, but shorting its leads with a screwdriver (after verifying that the power's off) is a safe way to ensure that you won't get shocked. Motor starting capacitors can hold a charge for days!

If oil has leaked out of a capacitor: Don't touch any oil that leaked out: old capacitors may contain PCB oils, an extremely carcinogenic (cancer causing) material which require special disposal.

Once the capacitor has been discharged (as described just above), then it can be tested with a multi meter. Either use the meter's built in capacitor test function, or use this trick: Charge the capacitor by using the sense current the meter puts out when set to ohms. You should observe a rapidly rising resistance before the meter indicates over range/infinity. Disconnect the test leads, and switch over to volts. Then, reconnect the test leads. A voltage reading should be observed, approaching zero.

If the capacitor doesn't hold a charge, or the resistance reading never approaches infinity, it probably needs replacement.

Also, the capacitor may be defective if the compressor hums but does not start. Visual inspection may reveal it to be bulged, or have a blown out safety plug.

References and More Reading:
"Troubleshooting Compressor Problems," Henry Puzio, Fuel Oil & Oil Heat with Air Conditioning Magazine, June 1993, p. 39
Tom Morris, Engineer, capacitor discussion and correction to the original data. Email to D Friedman 5/29/2006 - Thanks Tom for critical editing. The text above explaining about capacitors was suggested by Mr. Morris. The original text of the 1993 compressor diagnosis article had the resistance explanation backwards.
Federal Pacific Electric (FPE) Website Stab-Lok Circuit Breaker Panel Hazards Latent fire hazards
Zinsco Electrical Circuit Breaker Failures: overheating, failure to trip, burn-ups involving Zinsco and certain Sylvania electrical panel components

AIR CONDITIONING SYSTEMS Chapter Index

To continue reading this air conditioning inspection guide, use links to the document chapters at left or below. Links shown in green font indicate where you are in this document.

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