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Photograph of a blocked corroded air conditioning evaporator coilHVAC Cooling Coil Blockage FAQs
Q&A on causes, effects, cure

Q&A on A/C or heat pump cooling coil blockage by debris, damage, or ice - diagnosis & repair.

This air conditioning repair article series 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.

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Blocked Cooling Coil / Evaporator Coil FAQs

Photograph of a dirt blocked air conditioning evaporator coil

These questions and answers about the causes, effects, and cures of blocked evaporator coils or cooling coils were posted originally at DIRTY COOLING COIL / EVAPORATOR COIL - so you will want to review the advice given there.

[Click to enlarge any image]

On 2018-05-27 by (mod) - does omission of an air filter cause blower coil to get blocked?

Keith

The air filter in an HVAC system is not normally installed right "on the coil" but rather in the air handler at the return air end, or at the return air inlet openings in the building.

But yes, an HVAC system operated with no air filter, or with one that leaks badly, has nothing removing the dust normally found in indoor air. As a result the dust will collect particularly at the finned surface of the evaporating coil in the air handler or "blower unit".

The dust also collects in the blades of the blower fan, reducing the air flow rate through the system and thus increasing system operating costs.

Please see MISSING / LEAKY AIR FILTERS

On 2018-05-25 vby Keith

If a filter was not installed on the coil would that cause the coil in the blower unit to get blocked?

On 2012-09-18 by Kapil

Is a dirty evaporator a cause of Evaporator leakages. Is there any connection between dirt on the evaporators and Leakage of evaporators.( Mainly Automobile evaporators)

On 2012-08-08 by (mod) -

Greg, the condensate drain may be blocked or if the A/C has been running for a long time you may have dehumidified the indoor air enough that the rate of condensate production has slowed.

On 2012-08-08 by Greg

Air still is blowing out cool but water is not coming out the tube and is running down the side of the furance.

On 2012-07-29 by (mod) -

Dave where is the filter in relation to the cooling coil and the blower? One wonders if

- your system is splashing water onto the blower fan that is then spraying it onto a nearby filter on the intake or return plenum side

- the condensate system is backing up leading to deep water where there should be none

Make sure that condensate is draining

On 2012-07-22 by Dave

AC filter in attic has water on it causing rust to appear on the filter and then dripping onto the side. The secondary pan has no water in it. The condensate line with the p-trap and runs to the sump pump(I think)seems to work well. The house is comfortable when the unit is running. Appears to be one clean coil. Haven't been able to check the coil pan for blockage.

On 2011-08-15 by (mod) -

Sal if the holes are small enough and a protruding sheet metal screw wouldn't contact anything you can sometimes immediately seal small holes using a stainless steel sheet metal screw and rubber washer; galvanized screws and washers of larger diameter are also sold for patching holes in water tanks using this method. If conditions permit, I prefer to clean and dry the surface and use silicone or a similar product.

On 2011-08-14 by sal

As always thank you DJF. MY new dilemma - there are 2 small holes slightly larger then screw size in the metal drain pan on the lip. Its at the part of the pan lip that buts up against the outer sheet metal of the air handler.

It's almost like there are two plugs for these holes that got lost in the shuffle. Could this be? This is where my leaks are coming from. PIcture a backwards letter L. At the top of the L is the drain line which is clear. At the right angle part of the L are the two holes.

Just wanted to say that had I initially followed DJF's tips instead of listening to the lazy tech from the gas company I could have avoided ALL of the unnecessary cutting, and all of the related trouble I'm having now. DJF is the real deal.

I apologize if this posts twice, as my initial post has seemingly disappeared.

On 2011-08-13 by (mod) -

Good work Sal. At AIR HANDLER / BLOWER UNITS (find the link in the ARTICLE INDEX) we illustrate the effects of dirty blower fans on reduced airflow and recommend blower cleaning.

I have added some tips on reduced airflow across the cooling coil - things that may be mistaken for a blocked coil, just above.

If your condensate drain line is leaking by overflow it's blocked and needs to be cleared. If it's leaking at a joint or trap that's a plumbing repair. If the pan is leaking it needs to be dried, cleaned, patched, or replaced.

On 2011-08-13 by Sal/Anon

Hello Don Joe Friedman. After imposing on a buddy (an HVAC) tech we acted on the previous info given me by the tech who stated it 99,9% had to be a dirty coil causing poor airflow we cut away and checked the coil.

Low and behold - not dirty at all. It was a combo of a caked up dirty fan blower (yes the tech told me it was clean, go figure) and the fact that the unnecessarily open vent in my garage was sucking most of the airflow from the rest of the house.

We used shurtape (HVAC type) to seal up the air handler temporarily as I plan to put the ancient singer out of its misery come september/october.

Low and behold, the unit is leaking inside the house. I carefully shop vac'd out the drain line yet the unit is still leaking like crazy. Cooling house better than ever, but any suggestions are welcome. Thanks in advance.

On 2011-08-08 by (mod) -

Anon, if your whole AC system is going to be condemned anyway because nobody wants to try to access and clean the coil, you don't have much to lose in trying to find a way in.

First remove the humidifier and see what you can see - send along some sharp photos of your whole system from each side and I might be able to suggest other approaches.

Second, it's common for a service tech to cut away the side of the cooling plenum on an air conditioner to expose the coil - and working with care to cut just sheet metal, after inspecting from the humidifier opening you may see which side is going to give access to the coils for cleaning.

Don't bang around the coil with a shop vac - it's too easy to bend over the cooling fins making the coil unusable. Any vacuuming has to be very gentle and with a good view of what you're doing so as to not smash the fins.

On 2011-08-07 by Anonymous

Dan Joe Friedman, Thanks for your advice. After having someoneone else check out the basics, they too said it's most likely a dirty coil causing the greatly reduced airflow. They too said they wouldn't touch it because it involves cutting etc.

Needless to say I have no idea where to cut or what to do as it's seemingly sealed off behind sheet meatal above the air handler. Just an idea, there is an ancient, non working humidifier attached aboved the air handler way up top.

Do you think if I remove it at least temporarily (as it serves no purpose), I would be able to access the coil from above?

Also, how do you feel about delicately shop vac' ing and or using a foam spray cleanser IF I'm able to access the coil at all?

The worst part of this is I'm willing to pay a pro and nobody wants to go through the ordeal of getting to and cleaning the coil. Thanks for your time, SP.

On 2011-08-05 by (mod) -

Tom a partially-cooling coil can be tricky to diagnose without looking at it carefully. In quite a few air conditioner / heat pump designs, the manufacturer may actually have built multiple "coils" that fit into the same space - you will see the incoming (small diameter) refrigerant line split in to two or more lines that connect to different parts of the coil structure.

If this is what you have, perhaps one of those sub coils is blocked and the other is working. I'm assuming a single refrigerant metering device is feeding all coil segments.

Take a look.

On 2011-08-04 by Tom R

I have a cooling coil where only part of the coil cools. The upper half of both sides stays hot and only the bottom half of both sides cools and has air flow. I thought it could be blockage but that should stop the system eventually.

Could that be a dirty coil or is it another problem with the coil?

On 2011-08-02 21:26:42.882991 by (mod) - what to check if heat or A/C air flow is weak

If all the supply registers show weak airflow out and the blower fan is "running" I would check

- for dirty blocked coil

- for dirty squirrel cage fan blades

- dirty clogged air filter

- loose drive pulley on the blower fan (if a belt/pulley are used)

- crushed, obstructed main supply duct

- inadequate RETURN air - blocked or clogged

- low voltage, slow fan rotating speed or a defective control running the fan on low speed only

- cover left off of the air handler blower compartment (and maybe a bypassed blower door safety switch)

On 2011-08-02 by sal

sorry, should mention that everything else on unit was checked and is ok, with the exception of the airflow.

Tech checked system - outside unit good (only 3 years old, freon good, inside blower/fan clean and working. When he measured air flow is when he stated it's very week - even vents that are closest to unit. According to the tech it's extremely low, and the tech said it's most likely a dirty coil.

Unit is ancient Singer that's still going (thank God)and I can't upgrade the system till next year. Thanks, and sorry for the novel

On 2011-08-02 21:16:18.323943 by (mod) -

If the unit itself is physically accessible but there is no access opening, an experienced HVAC tech will simply CUT an opening on one side, giving access to the cooling coil for cleaning.

If the whole unit is flat inaccessible, you're in a tough place. I'd look hard at doing whatever is needed to provide access, even if it means cutting a wall and making a removable panel.

Access to service and clean the equipment is necessary if it's to be useful and maintained.

On 2011-08-02 by sal

sorry. touch, not tough.

Hello Mod. Just like peg I can't gain access to my coil. Had our local gas company come check, and not only won't they tough it, but they are of the opinion that nobody else will clean coil either as its so inconvenient to access.

Unit is OLD singer that sits atop a warm air furnace. Outside unit was changed 3 years ago, no leaks, and freon level is good.

They flat out told me its more than likely a dirty coil and that nobody's gonna tough it. Called another place left detailed message with no call back. Please share your opinion?!

On 2011-06-05 by (mod) -

Peg:
Quite right: on some indoor air handler units the evaporator coil is not readily accessible - depending on just where and how the unit was installed, such as a retrofit that sits atop a warm air furnace.

If there are no covers that can be removed to give access to the coil then the technician actually cuts away metal on one side of the A-coil housing to expose the coil. After cleaning s/he uses self-tapping sheet metal screws to reattach a piece of sheet metal over the opening. The seams are sealed with gasket or with metal foil tape.

On 2011-06-05 by peg

How do you clean the evaporator coil if it seems you cannot gain access to it (sealed plenum? above furnace)


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