Air Conditioning Cooling Coil or Evaporator Coil Diagnosis & Repair InspectAPedia® -
How to diagnose & repair problems with the air conditioning cooling coil or evaporator coil
Frost or ice build-up on cooling coils and its effect on cool air flow and mold
Frost or ice formation at air conditioning compressor/condenser units
Ice and condensate problems in air conditioning duct work, why it forms, how bad it can get, how to prevent it
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This article discusses the
problems that occur at the cooling coil (evaporator coil) in the air conditioning air handler unit. Our photo at page top shows the cooling coil in the attic air handler component of a central air conditioning system.
If your air conditioning system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS. See How to determine the cooling capacity of air conditioning equipment if the system seems to be working but is inadequate to cool your building. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Page top photo of an iced-up air conditioning evaporator coil are courtesy Guy Benfante.
Cooling Coil (Evaporator Coil) Basics for Air Conditioning & Heat Pump Systems
A cooling coil which is blocked by debris or ice and frost, or which is damaged can obstruct air flow and reduce air conditioning system output. The air conditioning system evaporator coil and problems include ice and frost build-up, dirt or debris blocking air flow through the coil, and damaged or leaky cooling coils.
We also discuss how cooling coils may be cleaned in-place and what to watch out for during that procedure. Cooling coils which are part of an air conditioning retrofit installation onto an existing warm air heating system can also present special problems of sizing and air flow, discussed further at ADDING A/C: RETROFIT SIZING. Sketch of heat transfer at the inside coil, also called the cooling coil or evaporator coil, courtesy of Carson Dunlop.
If ice, dirt, or damage block air flow across or through the cooling coil (evaporator coil) in an air conditioner, the cool air output will be substantially reduced or may even stop entirely.
Below we describe how the cooling coil works, what goes wrong with this component, and how its problems are diagnosed by simple visual inspection (inside of the air handler) or by some simple temperature measurements.
How To Inspect, Test, & Diagnose Cooling Coil (Evaporator Coil) Air Conditioner or Heat Pump Problems
Where to look for cooling coil problems: First, make a visual inspection of the cooling coil. Most air handlers provide an access panel or cover that can be removed to give at least partial view of the cooling coil surfaces. Turn off electrical power to the system to be safe from electrical shock. On opening an access cover or panel on the air handler you can recognize the cooling coil from our photos and sketches shown here and elsewhere on this website. You may need to use a flashlight and mirror to see the coil surfaces.
Remember to inspect the cooling coil from the incoming-air side - the side of the coil facing the blower fan assembly. That's because any dirt or debris entering the coil will come principally from this direction. If you inspect the wrong side of the coil it may look perfectly clean even though it is totally blocked by debris on its other surface. DIRTY COOLING COIL has photos of just how blocked a cooling coil can become in an air conditioner or heat pump.
Here are some common defects to look for at the evaporator coil (cooling coil) in an air conditioner or heat pump:
Ice or frost formation blocking air flow through the coil (FROST BUILD-UP)
Damaged cooling / evaporator coil fins over more than 10% of the coil surface, blocking air flow (shown in our photo at left in this case, the damage is to a condensing coil, not an evaporator coil). Small areas of damaged cooling fins can be straightened and cleaned-up using a cooling coil comb. Cooling coils with extensive physical damage such as shown in our photograph need to be replaced.
Evidence of refrigerant leaks (visual evidence may include stains from refrigerant oil left at the point of leakage) (REFRIGERANT LEAK DETECTION)
Evidence of mold growth on organic debris on the coil or elsewhere in the blower compartment (Mold Growth in Air Handlers)
Presence of unusual materials on the coil surface such as rodent debris, bird feathers and debris, fiberglass insulation, large trash fragments like paper or leaves confirming a duct or air filter problem. Some of these may indicate potentially serious health risks such as rodent or bird feces and debris which risk bacterial and viral hazards in building air. (Leaks, Rodents In Air Handlers)
Obvious coil-to-air-handler size mismatch of an add-on cooling coil onto an existing warm air system (ADDING A/C: RETROFIT SIZING)
Evaporator coil or cooling leaks or holes: if an evaporator coil is leaking (or also if the condensing coil is leaking) you'll find out pretty quickly as refrigerant will be lost and the cooling system will stop providing cool air. You'll need expert diagnosis by an HVAC service technician.
A lot depends on where the refrigerant leak has occurred and what caused the leak. If the cooling coil has a single point leak caused by some mechanical damage (one of our readers accidentally drilled a hole in his coil while trying to drill a drain hole in his air handler), it may be possible to find the hole and repair it using silver solder.
If the refrigerant leak is in copper tubing anywhere in the cooling or heat pump system that is not too close to an evaporator coil or condensing coil, it should be possible to solder a repair, then evacuate and recharge the cooling system.
If the refrigerant leak is in copper tubing in or close to the cooling coil (or in a condensing coil) a solder repair is hard to complete because the heat of the soldering process tends to de-solder other nearby connections. It might be possible if the technician is very expert and if s/he knows how to keep nearby surfaces cooled (we've used a wet rag).
If the refrigerant leak is in an aluminum part, soldering aluminum is more tricky and may not be feasible. Ordinary procedures using a torch, for example, just melt the aluminum. Expert welders use inert gas welding methods.
If the refrigerant leak is due to severe corrosion anywhere in an HVAC system we're not optimistic that a solder repair is possible. The conditions that caused a corrosion-related leak are likely to have thinned and weakened other parts. The cost of an attempted repair may be wasted.
Replacement of the cooling coil (or condensing coil) is more often going to be recommended by your HVAC technician because of these difficulties.
Temperature measurements at the cooling coil: See OPERATING TEMPERATURES for a discussion of where and how air temperature measurements are made to diagnose cooling coil or other air conditioner operating problems.
Below we introduce some of the more common air conditioner or heat pump cooling coil or evaporator coil defects and repairs.
Air flow requirements across the air conditioning evaporator coil: if airflow is weak for any reason (dirty coil, duct system defects, blower fan defects, dirty
blower squirrel cage fan), the air conditioning system will not operate properly. Some experts write that there should be between 350 and 400
cubic feet of air per minute (CFM) moving across the evaporator (cooling) coil for each ton of air conditioner capacity.
One ton of cooling or heating capacity = 12,000 BTUH so if your AC unit or heat pump is a 24,000 BTUH unit it is a "two ton" unit and needs to see 700 to 800 CFM of air across the evaporator coil.
Some home inspectors and air conditioning service technicians carry a small airflow meter that can actually measure this number with fair accuracy.
(The same tool is nice for comparing air flow and balancing air flow at various building supply ducts and registers.
How Air Conditioning & Heat Pump Evaporator Coils (Cooling coils) are Cleaned
Evaporator coil cleaning often requires cutting refrigerant lines, removal of the coil and
other components for cleaning, and reinstallation, pulling a vacuum on the
refrigerant lines, and recharge with refrigerant. Such service and repair may involve significant expense, although there are some
"in place" cleaning methods using foams and sprays that are a simpler procedure.
FROST BUILD-UP - Frost Build-up on the Evaporator Coil in an Air Conditioner
The ice or frost formed on a cooling coil in an air conditioner air handler unit is usually caused by an improper refrigerant charge, possibly by inadequate air flow across the cooling coil, or by a thermostatic expansion valve (TEV) or other air conditioner or heat pump control defect.
Ice blocks air flow through the coil, thus reducing air conditioner output; if the ice formation is extreme nearly all of the airflow across the coil is blocked and the air conditioner system runs but does not produce cool air flowing into the occupied space.
Frost and ice can also form on refrigerant tubing at other locations, and frost and ice can form inside air conditioning duct work itself, leading to troublesome leaks into the building.
Details of what causes frost on air conditioning equipment, what problems that creates, and how to diagnose and repair icing or frost on cooling coils or other air conditioner parts are provided at FROST BUILD-UP. There we discuss locations and causes of condensate, frost or ice formation in air conditioning systems, air handlers, compressor/condensers, refrigerant lines, and in air ducts.
BLOCKED COOLING COIL - Air Conditioner Evaporator Coil Blocked by Debris
Ice is not the only (nor even the most common) cause of blocked air flow in an air conditioner. This photograph shows how easily debris can stick to and clog the inlet side of the cooling coil in an air conditioning system. This evaporator coil was nearly totally blocked with dust and debris.
How does this happen?
There was no air filter installed in the system. Ordinary house dust is comprised largely of fabric fibers and skin cells.
These and other debris in building dust such as soot and organic particles like pollen and mold spores all join to form a gray mat on the
fins of the cooling coil in an air handler.
Debris sticks particularly quickly to this surface because of the combination of close spacing of the cooling fins (about 1/16" apart) and the fact that condensate forming on the coil keeps the surface damp.
Details about the detection and cleaning of dirt and debris which block an air conditioner cooling coil are at DIRTY COOLING COIL.
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Additional technical contributors & reference sources for this article are listed below.
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Thanks to to Guy Benfante, Chesapeake, VA 8/26/07 for the photograph of an ice-blocked air conditioning system evaporator coil and for his suggestion that we provide an air conditioning system troubleshooting FAQ.
Thanks to Mark Cramer, Tampa Florida, for assistance in technical review of the "Critical Defects" section and for the photograph of the deteriorating gray Owens Corning flex duct in a hot attic. Mr. Cramer is a Florida home inspector and home inspection educator.
Thanks to Alan Carson and Bob Dunlop, Carson Dunlop, Associates, Toronto, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education, publications, report writing materials, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Carson Dunlop, Associates, Toronto, have provided us with (and we recommend)
Carson Dunlop Weldon & Associates' Technical Reference Guide to manufacturer's model and serial number information for heating and cooling equipment ($69.00 U.S.).
Wikipedia provided background information about the definition of HEPA and airborne particle interception.
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