Photograph of attic air conditioning air handler, condensate drips on floorAir Conditioning Cooling Coil or Evaporator Coil Diagnosis & Repair

  • COOLING COIL or EVAPORATOR COIL - home - CONTENTS: How to diagnose & repair problems with the air conditioning cooling coil or evaporator coil [found in the indoor air handler] . 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. Frosting & non-frosting evaporator coil designs. How frosting type evaporator/cooling coils are defrosted. Dirt or debris-blocked cooling coil/evaporator coil cleaning & repair methods. Types of evaporator coils/cooling coils: dry vs flooded evaporator coil design differences; frosting vs non-frosting cooling coils & frosting-coil defrosting methods & controls. How cooling coils are changed-out or replaced
  • POST a QUESTION or READ FAQs about cooling coils (evaporator coils) in air conditioners, heat pumps, refrigeration systems
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Air conditioner & heat pump cooling coil or evaporator coil diagnosis & repair:

This article discusses the diagnosis and repair of cooling coil or evaporator coil problems that occur in the air conditioning or heat pump air handler unit such as frost or icing, dirt, blockage, refrigerant leaks, or improper sizing. Our photo at page top shows the cooling coil in the attic air handler component of a central air conditioning system.

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Cooling Coil (Evaporator Coil) Basics for Air Conditioning & Heat Pump Systems

Schematic of an air conditioning cooling or evaporating coil (C) Carson Dunlop AssociatesThe cooling coil or evaporator coil is where building indoor air cooling actually takes place.

The liquid air conditioning refrigerant entering the cooling coil through the metering device (a capillary tube or THERMOSTATIC EXPANSION VALVE) is increasingly changed to gas form as it "boils" or evaporates as the liquid refrigerant flows through the cooling or "evaporator" coil, so that at the end of the cooling coil the refrigerant is totally in gaseous form.

[Click to enlarge any image]

This state change (liquid to gas refrigerant) absorbs energy, cooling the tubing and fins of the cooling coil and thus indirectly, cooling and dehumidifying indoor air that is blown across the coil.

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 Associates.

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

Damaged fins on an air conditioning coil (C) InspectAPedia.comWhere 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 / EVAPORATOR 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:

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. See DIRTY COIL CLEANING PROCEDURES for details of this topic.

FROST BUILD-UP - Frost Build-up on the Evaporator Coil in an Air Conditioner

Photograph of attic air conditioning air handler, condensate drips on floor

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 on AIR CONDITIONER COILS. This article explains locations and causes of condensate, frost or ice formation in air conditioning systems, air handlers, compressor/condensers, refrigerant lines, and in air ducts.

Note that frost formation at some cooling coils (not air conditioners or dehumidifiers) may be normal. We discuss frosting and non-frosting cooling coil types and coil defrosting methods further at Frosting vs. Non-Frosting Types of Evaporator Coils

BLOCKED COOLING COIL - Air Conditioner Evaporator Coil Blocked by Debris or Dirt

Dirty filter blocks air flow (C) Daniel Friedman Photograph of a dirt blocked air conditioning evaporator coil

Watch out: something as simple as a blocked or dirty air filter or dirty cooling coil can lead to frost build-up on the cooling coil. That's because the reduced airflow over the cooling coil may permit it to run colder than normal.

Clogged Air Conditioner filters can lead to lost cooling capacity first, because the clogged filter reduces the air flow through the system, meaning that you'll feel less air flow at the supply registers than was previously present, and second because the low air flow may lead to partial or completed frost blockage of the cooling or evaporator coil in the air handler.

See AIR FILTERS for HVAC SYSTEMS for details.

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 / EVAPORATOR COIL.

Air Filter problems: not enough air coming out of air conditioning ducts?

Check for a very dirty, blocked air filter or blower fan. See AIR FILTERS for HVAC SYSTEMS and AIR HANDLER / BLOWER UNITS

Temperature Measurements at the Cooling or Evaporator Coil

Reader Question: what should the air temperature be when leaving the cooling coil of an air conditioning system?

What is normally the air temperature leaving the evaporator?

I never looked into it but I think it should be as close to 32F (freezing) as possible (-- with out reaching freezing - Not to cause ice accumulation on the evaporator fins).

I want to put a thermometer by the evaporator fins and see how good the air cooling is, its probably an good indirect way to see if there is sufficient refrigerant in the system or if there is air or other gases mixed in with it -- making the cooling inefficient. - E.K.

Reply: look at the air temperature drop across the cooling coil rather than looking for an absolute or specific air temperature

Air temperature leaving the evaporator: there may be some standards that I don't know (probably are) but the way I look at it, because of variation in refrigerants, air speed across the evaporator, and temperature of the incoming air aimed at the evaporator, we look more at the temperature difference across the evaporator to see if it's doing its job.

Figure that 15-20 degF would be a good temperature drop across the coil for a typical air conditioning system. Other experts add that the temperature difference across an evaporator (cooling coil) may be as little as 14 degF or as much as 22 degF.

To a beginner HVAC service tech [DF] it was striking to see how dramatic and critical was the effect of airflow across the evaporator coil on coil behavior and coil frosting or icing.

During an actual case of refrigeration system diagnosis [1] (the coil was icing over and the system was not cooling) I sought to adjust the TEV to bring the frost line to the end of the cooling coil where it belonged. But one learns immediately that only if the blower fan was sending air across the cooling coil could one expect the system to behave as designed.

Without that airflow, at just about any TEV setting of refrigerant flow rate into the evaporator coil the coil would ice up quickly.

Temperature measurements at the cooling coil: see OPERATING TEMPERATURES for a detailed discussion of where and how air temperature measurements are made to diagnose cooling coil or other air conditioner operating problems.

Types of Evaporators or Evaporator Coils or Cooling Coils: Dry vs Flooded Evaporator Coil Designs

Evaporator types (C) D FriedmanThere are two types of evaporators used in refrigeration systems: flooded and dry evaporator coils.

Dry Evaporator Coils: in a dry evaporator coil design, all of the refrigerant entering the evaporator coil enters as a vapor (or gas).

In a dry type evaporator coil (or cooling coil) the refrigerant oil travels constantly in the system along with the refrigerant, and some oil is discharged into the condenser. That is, only liquid refrigerant can actually carry oil.

In the evaporator the refrigerant is vaporized and the oil travels through, but the vapor is less capable of actually carrying the oil through the coil.

For the oil to pass through we need refrigerant gas velocity and turbulence in the evaporator coil, so we do not want much pressure drop across the evaporator coil.

Therefore dry evaporator type coils are usually short - to avoid much pressure drop.

Otherwise we get oil traps at the bends in the evaporator coil.

These are the more common type of evaporator coil or cooling coil in refrigeration systems. So, for example, for a small air conditioner that has to be packed into a small space, to keep the evaporator length short the manifold system may used to run several short evaporator loops in parallel - to avoid long individual tubing runs that might cause an ensuing refrigerant pressure drop and oil traps in the system.

[An oil trap will clog or prevent refrigerant flow through the evaporator and thus will prevent the system from working. A symptom might be loss of cooling and high refrigerant pressures on the high side]

Flooded Evaporator Coils: in a flooded evaporator coil design, the evaporator is constantly full of refrigerant, whether the cooling system is "on" or "off". See our cooling coil sketches shown here.

Frosting vs. Non-Frosting Types of Evaporator Coils

Commercial refrigeration system (C) D Friedman

Our sketch shows the basic layout of a commercial refrigeration system. Here we detail the difference between frosting and non-frosting evaporator coils and we explain how frosting-type systems must be defrosted to keep working.

Non-Frosting Evaporator or Cooling Coils - No Defrosting Needed

Dehumidifiers are examples of non-frosting type cooling coil designs. These devices are basically little "air conditioners" or cooling systems in their design (though their warm air output is exhausted directly into the same space). The dehumidifier system is a refrigeration system designed such that the coil will never form ice or frost.

Room air conditioners (portable or window or through wall units) are also examples of non-frosting type cooling coil designs.

These "frost-proof" or non-frosting systems (in normal operation) ar more difficult to charge: you must use a precisely measured charge or a temperature-sensing device and matching gauge with the temperature-sensing device - you find where the liquid ends in the evaporator line - where there is no further change in temperature in the evaporator coil tubing, there is no more liquid refrigerant present.

If you see ice or frost on these cooling coils it's an abnormal condition that needs to be diagnosed and repaired. See our diagnostic advice at FROST BUILD-UP on AIR CONDITIONER COILS

Defrosting Methods for Cooling Coils (Evaporator Coils) in Refrigeration Systems

Frosting Evaporator or Cooling Coils Require a Defrost Cycle

Examples of frosting evaporator coils or cooling coils include refrigerators (or freezers). When more than 1/4 of the surface is ice or frost that condition acts as an insulator that reduces the efficiency of the appliance, so the appliance will have to defrost itself - either automatically or manually.

There are two defrosting methods commonly used in frosting-evaporator coil designs:

Defrost by electrical resistance heating (common on refrigerators, including frost-resistance for door faces and jambs using extra resistance heating elements in those areas too);

Refrigeration system defrost methods (C) D FriedmanDefrost by hot gas: a solenoid in the compressor discharge line shuts [sketch above left] down vapor from the condenser and deposits high pressure/high temperature refrigerant gas directly into the evaporator coil, bypassing the refrigerant metering valve (TEV or cap tube).

The problem with dumping high temperature refrigerant vapor into the cold (iced, needs defrosting) evaporator is that it causes it to begin to condense - back pressure of the gas goes up and head pressure at the condenser goes down - now liquid refrigerant can back up to the compressor (where it would cause damage).

To avoid compressor damage from liquid refrigerant during this defrost cycle we add heat at the end of the evaporator coil (cooling coil) to insure that refrigerant reenters the compressor as a vapor, never as a liquid. Typically, setting a maximum of 20 minutes of defrost cycle adds protection against warming up food in the refrigerator or freezer where this design is used.

The refrigeration compressor continues to run during the defrost cycle in the hot gas method case, but the compressor will not keep running during the defrost cycle in the electrical resistance defrost cycle method.

When is cooling coil frosting abnormal?

Note that on dehumidifiers and air conditioners or heat pumps frost or ice formation on the cooling coil is not normal and is an indication of the need for repairs. See our diagnostic advice at FROST BUILD-UP on AIR CONDITIONER COILS

How cooling coils are changed-out or replaced

When an evaporator coil or cooling coil needs replacement (perhaps because the old one is damaged or leaky):

If your air conditioning or heat pump 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.

Page top photo of an iced-up air conditioning evaporator coil are courtesy Guy Benfante.

Air Conditioner or Heat Pump Cooling Coil Articles


Continue reading at DIRTY COOLING COIL / EVAPORATOR COIL or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see COOLING / EVAPORATOR COIL REPAIR to diagnose cooling coil leaks or to replace a corroded, leaking coil


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