Photograph of a blocked corroded air conditioning evaporator coilAir Conditioning & Heat Pump Air Handler Cooling Coil Blockage

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Cooling coil blockage by debris, damage, or ice - diagnosis & repair:

This air conditioning repair article discusses the problem of dirt or debris blockage of the air conditioning system's cooling coil or evaporator coil in an air conditioning system air handler, how the dirt gets there, what problems it causes, how to clean a cooling coil (or evaporator coil), and how to prevent future dirt on the coil.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Blocked Cooling Coil: Air Conditioner Evaporator Coil Blocked by Debris

Photograph of a dirt blocked air conditioning evaporator coil

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?

[Click to enlarge any image]

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.

For A/C or heat pump cooling coil blockage problems, also see  DIRTY COIL CLEANING PROCEDURES


If your air conditioning or heat pump system has lost its cooling capacity or won't start see REPAIR GUIDE for AIR CONDITIONERS.

See COOLING CAPACITY, RATED of air conditioning equipment if the system seems to be working but is inadequate to cool your building.

Photograph of a dirt blocked air conditioning evaporator coil

Here is a close up photograph showing how fibers in building dust readily cross the blades of the cooling coil and how the fibers themselves then collect smaller particles to rapidly block air flow across the coil. The same principles that make an air filter work can also clog the cooling coil of an air conditioning system.

As a cooling coil (or evaporator coil) in an air conditioner becomes blocked with dust and debris the air flow across the coil is reduced.

This reduced air flow (in cubic feet per minute or CFM) across the coil means that the cooling capability of the whole air conditioning system is reduced.

How do we clean a dirty air conditioner or heat pump cooling coil or evaporator coil? 


Other Causes of a Blocked Air Conditioning Evaporator Coil or Reduced Air-flow Across the Coil

Dirty air filter: can be mistaken for a blocked cooling coil. Always check your air filter first. Replace it if it's clogged and dirty and check the filter regularly.

Air filter wrong way: depending on its location, an air filter installed "wrong way", that is its arrow is not pointing in the direction of air flow but rather against that direction can cause air filter materials to actually tear out of the filter and then to be drawn against the cooling coil, blocking it and causing coil clogging and cooling coil icing.

This problem happens if the air filter is located between the blower and the cooling coil and is installed facing the wrong way. See more at AIR FILTER LOCATION where we also discuss which way the air filter must face.

Watch out: for an air filter installed ahead of the blower fan, installing the filter the wrong way or installing a filter of the wrong size can cause the filter to be drawn into the blower fan, leading to overheating and even a fire. Install the air filter so that it is it is seated properly, not bent or damaged, and facing with the arrows on the filter edges matching the direction of air flow through the filter.

Air Conditioner Evaporator Coil Frost or Ice Formation: When the coil becomes sufficiently blocked with debris as to slow down the air flow enough, the coil may actually become so cold that the condensate forming on its surface freezes, completely blocking the coil.

That's because the rate of release of refrigerant into the evaporator coil was designed with an assumption of a sufficient volume of air moving across the coil to keep it from becoming too cold.

When the surface temperature of an air conditioning cooling coil drops below 32 deg F or 0 deg C, condensate forming on the coil surface begins to freeze, leading to sometimes some pretty weird behavior of the cooling system as we discuss
at FROST BUILD-UP on AIR CONDITIONER COILS where we explain that there can be more than one reason that a cooling coil ices-up but none of those conditions is desirable.

Air leaks or unnoticed duct openings: in the duct system can interfere with proper airflow through the duct system, reducing air delivery into the building, and can be mistaken for a blocked or dirty cooling coil.


Damaged air conditioner coil fins: can occur on both the evaporator (cooling) coil and the outside condensing coil.

See CONDENSING COIL REPAIR REPLACE for discussion of repairing bent or damaged coil fins. Only in extreme cases will fin damage be so severe that air flow across the cooling coil is severely blocked - enough to cause loss of function or coil icing.

Dirty air handler blower fan: see AIR HANDLER / BLOWER UNITS and its section titled DIRTY A/C BLOWERS This article describes the very significant reduction in airflow across a cooling coil that can occur if the squirrel cage fan blades are dirty on the blower fan in the air handler. A dirty blower fan can be the cause of reduced airflow across the cooling coil and can even lead to coil icing.

Loose Supply Plenum Insulation Falls onto & Blocks the A-Coil

AC coil blocked by fallen insulation (portrayal) (C) Daniel FriedmanInsulation, usually a fiberglass matt, is glued to the inside walls of the supply plenum of an air conditioner or heat pump. If this insulation comes un-glued from its surface the insulation can fall against the cooling coil, blocking air flow through the coil.

The result of insulation falling against the A-coil or cooling coil is reduced air flow through the system, reduced cooling, and possibly ice formation on the cooling coil (due to reduced air flow) that in turn further blocks air flow.

Our photo illustrates what can happen if an insulating panel falls from the upper surface of an air handler down onto the surface of the evaporator coil or cooling coil in an air conditioner or heat pump.

The red arrows show the direction of normal air flow while the yellow arrow points to the location of an insulating panel that could fall from above.

Indicators of Insulation-blocked cooling coil

HVAC service techs note that if the cooling coil is blocked by fallen insulation that problem won't be visible until the plenum is opened for visual inspection. But several operating tests may point the service tech in the direction of this problem:

To repair this problem the service tech may use spray contact cement to re-bond the (dried) insulation back in place in the supply plenum. But in our OPINION if the insulation is torn, damaged, soggy, or moldy it should be replaced and the new insulation glued in place.

Watch out: do not just tear out and throw the supply plenum insulation away. If it were not needed the manufacturer wouldn't have put it there.

Insulation glued to the inside walls of the supply plenum of an air conditioner or heat pump is there to control noise and to avoid un-wanted heat gain at the cooling coil. If this insulation comes un-glued from its surface the insulation can fall against the cooling coil, blocking air flow through the coil.

Question: Trane's A-Coil containers insulation falling down and blocking the air flow ?

Thanks for the great website. Can you tell me if you have had any experience with Trane's A-Coil containers insulation falling down and blocking the air flow and reducing cooling by blocking the air flow through the coils.

One Trane tech says he sees this insulation problem on about 30% of his new installs. I assume that if the insulation blocks the cooling, the refrigerant does turn to a gas state after passing through the cools, and then the condenser unit tries to compress a fluid - and freezes up.

About 18 months ago, an HVAC company installed an extra return air duct to our two southwest bedroom area. Since we have had the 4 ton XR15 system installed back in 2012, those two rooms were stuffy / slightly warm in the summer. With the additional return air duct, the rooms are better.

Note: Since installation and additional ducting added, I have changed HVAC companies, to Xtreme Air, in the D/FW area. However, since the insulation above the A-Coil has been tacked up (strapped up) to the top of the A-Coil cabinet, allowing full air flow through the system and allowing proper exchange on the evaporating coil, the 1000 plus square feet that this system heats and cools is a lot better.

Much cooler with a fresh feel during this July. If the HVAC tech is correct that the insulation had probably come unglued by the time of installation, I think it would be reasonable that Trane would have statistics on this problem.

The Xtreme Air's HVAC tech said his evaluation was the refrigerant was freezing up and not taking additional refrigerant (28 degrees versus 54 degrees, not changing from a liquid to gas while going through the A-coil) and since the system wasn't leaking, a full charge was not made at the time of installation of the new system.

During the evaluation with hoses and gauges hooked up, the HVAC tech coordinated his diagnosis with another Xtreme Air Tech. They both agreed the first thing to check was the insulation, so this must be somewhat common in the field of HVAC repair (ie, seen a lot of times.)

This HVAC tech said he saw the A-Coil insulation problem on about 30% of new installs. That is a lot, and agree that it probably isn't just Trane. On an older Trane system at the house we used to live in, I found the insulation that is suppose to be glued to the top of the cabinet that holds the blower, laying on the blower. I took it out of the cabinet and taped on top (on the outside) of the cabinet.

So I question whether the additional return duct was required, but agree that it probably does enhance the air flow.

I have two 4 ton systems that cool 3300 square feet in a single floor house: one dedicated to the living areas (about 2000 square feet, kitchen, den, study, dining room, and living room), and the other one (with inslulation problem) dedicated to three bedrooms (about 1000 square feet).

I think this issue probably causes a lot of extra repairs, extra cost for electricity, and frustration by homeowners.

Hope InspectApedia can research, collect data, and put some light of just how bad the problem is. - Anonymous, Plano TX 2017/07/20


Thank you for the more-detailed write-up of the A-coil insulation blockage.

My initial research finds that it happens but I've not found a scholarly nor authoritative study on how frequently this problem occurs.

There is no reason to think this problem is peculiar to just Trane nor any other single manufacturer. - Editor 2017/07/20


Continue reading at DIRTY COIL CLEANING PROCEDURES 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

Or see MISSING / LEAKY AIR FILTERS - lead to clogged evaporator coil

Or see these

Air Conditioner or Heat Pump Cooling Coil Articles

Suggested citation for this web page

DIRTY COOLING COIL / EVAPORATOR COIL at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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