Air Conditioning Diagnosis, Inspection, Repair Guide

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Air Conditioning Diagnosis, Inspection, Repair Guide

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

What are the Parts of an Air Conditioning System and How do Air Conditioners Work?
Here is a simple explanation of how an air conditioning system works, with enough detail so that it isn't simply magic (the schematic of an air conditioner shown at left is compliments of Carson Dunlop) A detailed list and photos of air conditioner components can be seen at AIR CONDITIONER COMPONENT PARTS.

A air conditioning or heat pump compressor which compresses low pressure refrigerant gas into a high pressure, high temperature gas. Usually the compressor is in the outdoor portion of an air conditioning or heat pump system. The compressor is basically a high pressure pump driven by an electric motor. The air conditioning compressor is usually packaged in the outdoor compressor/condenser unit illustrated by our page top drawing. See COMPRESSOR CONDENSER, and see REFRIGERANTS


A condenser or condensing unit: typically a condensing coil inside which high temperature high pressure refrigerant gas flows, and over which a fan blows air to cool the refrigerant gas back to a liquid state (thus transferring heat from the refrigerant gas to the air being blown by the fan). The condenser unit is basically a coil of finned tubing and a fan to blow air across the coil. Usually the condenser unit is in the outdoor portion of an air conditioning system, often packaged along with the compressor motor discussed above. See COMPRESSOR CONDENSER and see our page top sketch too. The change of state of the refrigerant, from hot high pressure gas to a liquid releases heat, including heat collected inside the building) to the outdoors.


A metering device which dispenses liquid refrigerant into an evaporator coil. The metering device may be simply a thin section of tubing (a capillary or "cap" tube) or it may be a bit more sophisticated thermostatic expansion valve (TEV) which includes a temperature sensing control that can open and shut the device against refrigerant flow. See THERMOSTATIC EXPANSION VALVES .


An evaporator coil or cooling coil: typically the cooling coil is a section of finned tubing (it looks a lot like a car radiator) into which liquid refrigerant is metered and permitted to evaporate from liquid to gas state inside the coil. This state change of the refrigerant, from liquid to gas, absorbs heat, cooling the evaporator coil surface and thus cooling indoor air blown across the cooling coil. Usually the cooling coil is located inside the air handler. See AIR HANDLER UNIT and articles like DIRTY COOLING COIL


An air handler and blower unit which provides a fan to blow building air across or through the evaporator coil. The air handler blower fan unit moves building air across the evaporator coil surface in order to condition building air by cooling it (and thus also by removing moisture from the cooled air). See AIR HANDLER UNIT


A duct system which distributes conditioned air from the air handler in to the occupied space (supply ducts), and which takes air from the occupied space and returns it to the cooling system air handler. See DUCT SYSTEMS


Air conditioner controls and features, which include a room thermostat, electrical switches, fuses or circuit breakers, condensate handling system, and air filters. See OPERATING CONTROLS and AIR FILTERS for HVAC SYSTEMS

For photographs of these various air conditioning and heat pump parts, and for an explanation of where these air conditioning components are physically located, see A/C COMPONENTS which discusses Indoor A/C Components and Outdoor A/C Components

How to diagnose and fix an air conditioning system that is not working
Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING SYSTEMS.

If your air conditioning system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.

A/C REFRIGERANT LEAK DETECTION: how to use a TIF5000 to detect air conditioning refrigerant gas leak
A/C DIAGNOSTIC FAQs: air conditioning system diagnostic FAQs: Q&A about air conditioner repair - a detailed air conditioning system diagnostic checklist
AIR HANDLER UNIT: problems with the air handler, air filters, and the cooling coil itself
BACKUP HEAT: on heat pumps, types of backup heat; problems with backup heat; begin here if your heat pump is not providing enough heat or if your air conditioning system provides heat when it should be providing cooling.
COMPRESSOR CONDENSER: problems with air conditioner compressor/condenser units
CONTROLS & SWITCHES: air conditioner controls and switches - begin here if your A/C won't start. Here's an important tip: most refrigeration problems, in air conditioners, refrigerators, or freezers, are electrical, not mechanical. In air conditioning school, we used to drive out and collect abandoned refrigerators that people were tossing out during our community's spring cleanup week. Taking these appliances back into the shop we found that almost always the problem that had caused the owner to dispose of their air conditioner or freezer was in an electrical connection or electrical control. So it's worth checking out switches and controls on an air conditioner before replacing more costly components.
Dehumidification Problems - Air conditioner cools but does not dehumidify
DUCT SYSTEM DEFECTS: problems with the air duct system, air filters, supply registers, return air registers
Fire dampers, and Heating and Cooling Air Duct Controls such as manual and automatic duct dampers, zone dampers, and fire dampers are discussed and distinguished at DRAFT REGULATORS - barometric damper
LOST COOLING CAPACITY: what to do when not enough cool air comes out of the system
What to check first if there is no cool air or not enough cool air
Compressor failure diagnosis: checks of the air conditioner compressor
Ducts & Air Handler diagnosis: basic checks of the indoor air handler (blower), air ducts, and filter system
A/C Flow Too Weak - weak cool air flow can be caused by several problems, filters, icing, duct leaks, etc.
A/C Filter Problems - dirty or missing filters cause other operating problems
A/C Compressor Problems - quick check
A/C Off - Condensate Pan Switch - some overflow pans have a switch that will shut down the system
A/C Cooling Coil Icing - slow air flow, improper charge, etc. - eventually we get no cooling
A/C Not Dehumidifying - improperly-sized unit, other causes, and cures for lack of dehumidification
A/C Air Duct Problems - leaks, sizing, inadequate return capacity
Air Conditioner Won't Start - various causes including some simple switches to check
Air Conditioner Refrigerant Problems - too much is as bad as too little; leaks, cures.
Blower Fan No Start / No Stop - weird blower behavior can be diagnosed and fixed
Compressor Diagnosis: Diagnose & Repair - serious look into the compressor unit
Cooling Capacity of the Duct System

OPERATING DEFECTS: major air conditioning problem symptoms and how to get the air conditioning system working again,e.g. compressor or fan noises, failure to start, and inadequate cool air volumes
ZONE DAMPER CONTROLS . discusses manual and automatic air duct zone controls

1. A/C COMPONENT LIST - Basic Air Conditioning Components Inspection List
Conventional cooling systems include the following components:
Indoor Components of an air conditioning system (Details can be read and seen at AIR CONDITIONER COMPONENT PARTS)
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 = Cooling Coil
Supply plenum
Supply air ducts and registers
Return air ducts and registers
Air filter(s) and possibly other air cleaning/IAQ equipment
Electrical shut off switches, circuit breakers/fuses


Outdoor Air Conditioning System Components (Details can be read and seen at AIR CONDITIONER COMPONENT PARTS)
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 & circuit protection
All of the components, controls, switches of air conditioning systems and how to diagnose and debug cooling system problems are discussed in detail at this website.

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 air conditioning system 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.


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.

One ton" of cooling capacity, historically, referred to the cooling capacity of a ton of ice. Tons of ice does not explain a key ingredient in the comfort produced by air conditioning systems, dehumidification of indoor air - that is, taking water out of the air. Cool air can hold less water (in the form of water molecules or gaseous form of H2O) than warm air.

Think of the warmer air as having more space between the gas molecules for the water molecules to remain suspended.

When we cool the air, we in effect are squeezing the water molecules out of the air. When an air conditioner blows warm humid building air across an evaporator coil in the air handler unit, it is not only cooling the air, it's squeezing out some of the water in that air. Both of these effects, cooler air and drier air, increase the comfort for building occupants.

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