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Air Conditioning Diagnosis, Inspection, Repair Guide AirCondAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

This website answers most questions about inspecting, troubleshooting, and repairing central air conditioning.

• How to inspect & repair central air conditioning systems
• What are the basic air conditioning components?
• Determining air conditioning cooling capacity & energy efficiency
• Troubleshooting air conditioning compressor problems
• Diagnosing air conditioning air handler problems
• Air conditioning condensate problems
• Duct system inspections, defects, repairs
• Cleaning air conditioning equipment & A/C refrigerants

This website answers most questions about central air conditioning system troubleshooting, inspection, and repairs. We describe how to inspect residential air conditioning systems (A/C systems) to inform home buyers, owners, and home inspectors of common cooling system defects. The articles at this website describe the basic components of an air conditioning system and then we discuss how to estimate the rated cooling capacity of an air conditioning system by examining various data tags and components. The limitations of visual inspection of A/C systems are described. 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.

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.

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

1. A/C COMPONENT LIST - Basic Air Conditioning Components Inspection List

Conventional cooling systems include the following components: Indoor Components Air Handler Unit (AHU) which typically includes the following Air filters - located at return registers or possibly at or in the air handler Return Plenum Blower fan in a blower compartment Evaporator Coil Supply plenum Supply air ducts and registers Return air ducts and registers Outdoor Air Conditioning System Components Compressor motor - on residential units this is normally a hermetic motor-compressor combined in a single sealed unit Condensing coil Outdoor cooling fan Electrical shut-off switch(es) for service

Rooftop combined units: While the list above describes the common components of a typical residential air conditioning system, other configurations and packaged units are also in increased use in both residential and commercial installations. Alternative designs may combine all components except for the duct work in a rooftop mounted unit such as the one shown above where it was mounted on a flat roof over offices at a commercial building.

Wall convector units (above) are often used for both heating and cooling in commercial installations and high-rise apartment buildings. The unit shown has its own compressor mounted right in the cabinet, visible at lower center in the photo. Wall-mounted heating and cooling convector installations may be designed with one central heater or cooling system which feeds multiple units with chilled or heated water or possibly refrigerant from a single remote heating and cooling heat pump. Another common residential alternative dispenses with duct work entirely, using a wall-mounted indoor evaporator/blower unit and a separate outside compressor/condenser. In this latter split design, one compressor/condenser may serve multiple wall-mount indoor units.

Do-It-Yourself Home made air conditioning systems such as this goofy example may actually work but not without problems. This system used a window air conditioner placed in a home's attic. Manhole ventilation duct (liberated from New York City) was used along with a home made hood attached to the air conditioner to blow cool air into the home through a ceiling register. The air conditioning condensate was collected in the blue plastic kiddie pool seen in the photo, and drained by gravity to a plumbing vent stack. Nothing about the system was proper, safe, nor very effective, and in addition, the attic moisture conditions were terrible as you can see from the blackened plywood roof sheathing.

CONTROLS & SWITCHES - Switches and Controls on a typical split system with indoor and outdoor components:

If the A/C system won't operate, before requesting a service call check that it is turned on and that the thermostat is properly set.

• Compressor circuit breaker: in the electric panel there will be a switch controlling power to the compressor/condenser unit
• Compressor service switch: at the compressor/condenser unit, typically on a building wall near the outdoor unit, may be a breaker, fuse block pull-out, or switch
• Air handler circuit breaker: in the electric panel there will be a switch controlling power to the air handler/blower circuit, perhaps two different circuits, one for the air handler unit, and a second for the compressor/condenser unit.
• Air handler service switch: at the air handler/blower unit, typically on the unit or on a building surface close to it
• Thermostat: which acts as a switch to turn on the A/C equipment, typically wall mounted in the living area. The thermostat may have both a temperature setting switch and a switch that can be moved to "cool", "off", or "heat" positions. For the thermostat to call for cooling it must be in the "cool" position and the temperature set to a level below the ambient air temperature at the thermostat location.
• Overload reset buttons: may be present on the blower motor in the air handler/blower compartment
• Air handler blower compartment access door switch: present on newer units, shuts off the blower fan as a safety control if the door is opened
• Notes: If the A/C system is not running check these switches. Not all of these switches will be present on every system; fuses may be used instead of circuit breakers; fuse pullouts may be used instead of a circuit breaker or fuse at some service switches.

A/C DATA TAGS - Air conditioner data tags

A metal tag is usually affixed to the outdoor compressor housing. Depending on the age and equipment manufacturer the format and content of data on this tag varies, but typically the tag will allow you to discover some or all of the considerable amount of data listed here:

• Manufacturer: The air conditioning equipment manufacturer
• Model Number and Serial Number: Some added data such as BTUH cooling capacity may be encoded in these numbers.
• Voltage or VAC for the compressor and separately for the blower fan. In addition to specifying voltage, the cycles (50HZ or 60HZ) and current phases (typically one phase for residential equipment) may be specified.
• Branch Circuit Selection Current- BCSC: recommended for determining the required size of the branch circuit conductors (wiring) supplying the equipment.
• Locked Rotor Amperage - LRA specifies the minimum circuit breaker or fuse size required and is the maximum current that the motor will draw when the motor's rotor is unable to move [locked]. [NEC Table 430-152-A]
• Minimum Circuit Ampacity - MCA: MCA can be used to determine the required size of branch circuit conductors (wiring and also control switches) supplying the equipment. [For example, using NEC Table 310-16 in the 60 degree column as required in 110-14]. However some experts recommend using BCSC. [Using the "minimum permitted" sized wire rather than the optimum-size to a compressor may save a few dollars at installation but may increase system operating costs and it may be less safe than using a larger conductor.]
• Maximum Fuse or HACR type Breaker: specifies the maximum overcurrent protection or MOP to be used to protect the equipment. The permitted ampacity of the equipment electrical circuit protection (fuse or circuit breaker amps) expressed as MOP or Maximum Overcurrent Protection. If MOP is specified, the breaker or fuse protecting the equipment should match this number. [A hermetic compressor draws varying amounts of current as its internal pressure changes during operation. Current draw is higher when starting the motor, and highest if the motor is starting against its highest backpressure such as when a unit is turned off and then back on in the middle of operation. Because fusing an air conditioning compressor at the minimum level can result in blown fuses or tripped breakers during these intervals of heavy current draw during compressor startup, compressors are either protected by a slow-blow fuse or a somewhat larger than minimum circuit breaker.] On some older equipment MOP is not specified. Only in the case that MOP is not specified can the overcurrent protection required be determined by the alternative means: [RLA OR BCSC whichever is greater x 175%], or if the compressor keeps tripping that device or blowing that fuse, RLA x 225% might be used. The National Electrical Code (NEC) specifies the degree to which a breaker or fuse may exceed the RLA.
• Rated Load Amperage - RLA, also called Rated Load Current or RLC on some equipment. This is the manufacturer's anticipated load during normal usage. [Typically about 64% of the maximum load current. See NEC section 440-2.]
• Full Load Amps - FLA full load motor current draw, level at which the motor can be operated without damage.
• Maximum Continuous Current - MCC not usually supplied, this is the most current that the compressor can draw without being damaged. [Typically about 150% of RLA.]
• BTUH Cooling Capacity: The A/C system cooling capacity, either explicitly stated in thousands of BTU's (British Thermal Units) per hour (BTUH) or implicitly given by other data, or coded in the unit's model number.
• The month and year of manufacture, possibly also encoded in the unit serial number. Since the typical life of an A/C compressor is about ten years, one would like to know the probable age of the equipment.
• The Energy Efficiency Ratio of cooling equipment is basically the amount of electricity you consume to obtain a given amount of cooling ability. It's expressed as (KW per hour of electricity used) / Thousand BTUs - this number is probably not going to be found on the equipment itself but may be in its documentation. Also see SEER at RATED COOLING CAPACITY.

A/C ENERGY SOURCES - Air Conditioning System Type by Energy Source

Air conditioning systems use four common energy sources:

• Air-cooled air conditioners (shown in the photo): this refers to the use of air to cool the compressor and the condenser coil used to return the refrigerant gas to a liquid state. These split systems usually have an inside evaporator cooling coil installed to work along with the blower and duct system which might also be sitting atop a heating furnace. The outside half of the equipment contains the compressor and condenser coil.
• Water cooled air conditioning systems: these work in a manner similar to the system listed above, but use water as a chiller to remove heat from the high temperature gas in the (usually but not always outside) compressor/condenser unit.
• Independent Systems or Split Systems: are air conditioning systems which do not make use of an existing blower and duct work installed for central heating. These include ductless systems mounted on roofs or in attics and wall-mounted units which may include an indoor fan and evaporator coil to produce cooled and dehumidified air, but which route refrigerant to an outside compressor/condenser unit.
• Gas Chillers: these systems operate by the same principles as the above units, but they use heat to cause the refrigerant gas to change states rather than compression and expansion by a compressor motor. (Some refrigerators, including ones used in recreational vehicles also operate on this principle, as they can cool without requiring electricity to operate a compressor. Ammonia was the traditional gas used for this type of system.)

2. RATED COOLING CAPACITY - How to Determine Air Conditioning Equipment Rated Cooling Capacity

The cooling capacity of an air conditioning system is expressed in BTU's or tons. One ton of cooling capacity equals 12,000 BTU's/hour of cooling capacity.

There are several ways to determine the rated cooling capacity of an air conditioning system's equipment:

Air Conditioning Equipment Age and Capacity from Equipment Numbers

Serial number formats vary by range of years over which equipment was manufactured, and may vary among countries of manufacture for a given company's equipment, for example between the U.S. and Canada for Carrier air conditioning equipment.

Example: a Carrier Compressor/Condenser Serial# 1389E54894 on a compressor unit.

Air conditioning equipment age from serial number for the example above, the equipment was made after 1980. The first four digits of the serial number are week and year of manufacture, in this case, week 13 of 1989.

FROM MODEL # - Air conditioning equipment capacity from model number

Example: Carrier A/C Compressor Condenser Model# 38XD12400 (same unit as used for the serial number example above), there is variation in how Carrier assigned these numbers but typically the numbers indicate either tonnage or MBTUH. This example has digits in the 4th and 5th positions (right hand 5 digits), so the rating is in MBTUH for this number and "24" signifies 24 MBTUH or 2 tons of capacity.

Guide to Heating and Air Conditioning Equipment model numbers, serial numbers, age, and capacity: See Technical Reference Guide, published by Carson Dunlop Weldon & Associates, Ltd., Toronto, 2006 for a $69.00 book which translates air conditioning equipment model numbers and serial numbers into date of equipment manufacture and rated BTUH capacity.

FROM EQUIPMENT RLA # - Air Conditioning Equipment Capacity from Equipment RLA Numbers

RLA Rule of Thumb: RLA, Rated Load Amps, or in some older texts, mis-named as "Running Load Amps" is the manufacturer's specified rated current draw when the equipment is operating, excluding the current draw during startup, but when the compressor is under load.

On a single-phase 240V circuit feeding an A/C compressor/condenser unit, the equipment will draw typically 5 to 6, (7 in some cases) RLA per ton of cooling capacity. So if the data tag on a compressor shows its RLA rating=21.2 I would rate the system as 21.2/7=3 Tons. Translating Tons into BTUH, 3tons x 12 MBTUH/ton = 36,000 BTUH estimated Cooling Capacity. Details of this and related calculations are in the "Guide" book cited above.

COOLING RULES OF THUMB - determining Cooling Capacity Requirements - Rules of Thumb

A home inspection does not involve the calculations of heat gain necessary to decide if the cooling capacity on a building is adequate, but the inspector is expected to examine and report on the rated system capacity (such as "36,000 BTUH") and on the presence or absence of cooling sources in the habitable rooms of the building.

Air conditioning capacity requirement: A simple rule of thumb for relatively cool climates such as the Northeastern United States: one ton per 400 sq.ft. (Commercial) or one ton per 500 to 1000 sq.ft. (Residential) or one ton per 400 to 800 sq.ft. (Space Pak Systems). . Or a 3000 sq.ft. house may require a 5-ton unit. Or count the supply outlets: 10 outlets @ 100 cfm (estimated) = 1,000 cfm = 2.5 tons needed.

Oversized Air Conditioning Systems: Can an air conditioning system have too much capacity? Yes. If a system is over sized for a building it may be able to drop the indoor temperature so rapidly that the cooling cycle is too short to permit adequate reduction in the humidity level. Remember that indoor comfort is a function of both temperature and relative humidity. Also, since an oversized air conditioning system will be cycling on and off more frequently, not only is the building actually less comfortable (temperatures are swinging up and down unnecessarily quickly and frequently) but it may also be harder on the equipment, thus shortening its life. (Turning electric motors on and off is hard on them.) If the "on cycle" of the A/C system seems unusually brief, or if the indoor humidity is not dropping this question merits further investigation.

SEER RATINGS EXPLAINED - determining Seasonal Energy Efficiency of Air Conditioning Systems

SEER stands for Seasonal Energy Efficiency Ratio. This is a measure of the energy efficiency of the air conditioning system. SEER ratings permit consumers to compare operating costs of various cooling systems and products.

SEER = [Total Cooling Output Over the Cooling Season] / [Total Electrical Energy Input Over the Cooling Season]

Older air conditioning systems are likely to have a lower SEER (perhaps 5 or 6) than a newer more efficient system (perhaps SEER=10). But beyond comparing SEER ratings, a look at the building insulation, air leakage, and the layout, insulation, and adequacy of the air conditioning duct system are likely to have a very large, usually determining effect, on the operating cost of air conditioning systems in buildings.

Air Conditioning SEER - New DOE Air Conditioner and Heat Pump Efficiency Standard offers more details about SEER and the phase out of older refrigerants (like R-22)

3. INSPECTION LIMITATIONS - Air Conditioning System Inspection Limitations

COOLING SYSTEM INSPECTION LIMITATION: We check for normal temperature differential between input and output air, unusual operating noises, visible damage or defects, and a variety of other possible defects. This inspection is not technically exhaustive; a more thorough inspection, also not technically exhaustive, can be performed by a qualified HVAC service professional, and is recommended when any defects or malfunction are suspected.

Central Air Shut Down, Not Tested, May be Not Useable

*** The central air conditioning equipment is in questionable condition and may not be operable. As the equipment had clearly been shut down for a considerable time
... we were not able to test-operate this equipment and we recommend that you make no attempt to turn it on before having it examined by a qualified air conditioning service person. Operating equipment which has been "shut down" without proper preparation risks costly damage to the compressor or other components. If replacement of major components is required, repair may involve significant expense.

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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