Clogged or dirty air conditioner or furnace filter troubleshooting:

This article explains the causes and effects of clogged air conditioning or heating air filters.

Air filter clogging may be the result of high indoor debris levels, abnormal condensate handling or indoor moisture, or even more subtle problems that need to be corrected.

A clogged HVAC air filter increases heating and air conditioning operating cost and in some cases can even be quite dangerous. We describe how to recognize a dirt-clogged air filter, and how to recognize replaceable air filters versus washable air filters.

Dirty Air Conditioner Filters Cause Multiple Problems for an Air Conditioning System

Dirty Furnace or Air Conditioner Air Handler Air Filters

Dirty or clogged HVAC air filters are a source of increased operating costs and poor cooling system operation.

Photos above and at page top show a very dirty air filter at a system inspected and repaired by the author.

While a small amount of building dust and debris on a air handler or blower unit air filter actually increases the air filter's efficiency at removing particles from air passing through the system, more dirty air filters can and in fact probably will have these bad effects:

1. reduce air flow into the building
2. increase the operating cost of the heating or air conditioning system
3. cause dirt to accumulate on the fan blades, further reducing air flow even if a new air filter is installed, thus also wasting your energy dollars -
   
   see DIRTY A/C BLOWERS
4. cause excessive dirt build-up inside the duct system, leading to mold or allergen problems in a building and to the need for more costly duct cleaning or replacement
5. block the cooling coil itself with dirt, further reducing system effectiveness and possibly leading to costly repairs
6. lead to frost build-up on the cooling coil and reduced or totally blocked air flow in the system
7. eventually permit dirt to bypass the filter where it soils and blocks the blower fan itself, leading to more costly repairs.
8. and in extreme cases a clogged HVAC air filter can be dangerous: a blocked air filter can be "sucked" out of position and right into the blower assembly.
   
   Watch out: If this happens the flower fan can jam, overheat, and the result is a risk of actual fire in the equipment. See HEAT RECOVERY VENTILATOR FIRE for an example.

The filters on an air conditioning or hot air heating system should be changed monthly when the system is in use. Discuss with your heating/cooling service professional the possible need to clean the blower fan and duct work.

Air Filter Accessibility

An air filter that is installed where it is not readily accessible may in fact be ignored completely by building owners/occupants, and may rarely if ever be serviced.

Be sure to inspect your HVAC system and ductwork thoroughly to be sure that you know where any and all air filters are located.

A forgotten air filter will mean poor heating or cooling in the building and higher energy costs.

See AIR FILTER ACCESSABILITY

Also see MISSING / LEAKY AIR FILTERS

 

Washable HVAC Air Filters: Washing a Split System Air Conditioner Plastic or Nylon Air Filter

Wall-mounted split system air conditioners as well as window air conditioner units like the one we show just above include one or more removable air filters that are designed to be simply rinsed clean using cold water.

Here you can see the author replacing the left-hand air filter of a wall-mounted split system air conditioner after having washed it clean.

You can wash these plastic mesh filters in the kitchen sink using a dish sprayer but I prefer to perform the operation outdoors (shown below). Outside avoids messing up the kitchen and eliminates any risk of drain clogs.

Watch out: We have read of washable air filters that can be sent through your dishwasher.

We do not recommend this step - as you risk clogging the dishwasher or its pump with dust and debris that wash off of the air filter.

Details of how to remove, clean, and replace these washable air filters are found

at SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS

...

---

---

ADVERTISEMENT

---

---

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Question: What's causing a damp, clogged air filter in my air handler?

Roger Cote said: Just had unit installed 2 weeks air flow was fantastic until Saturday started out great then got very little air flow.

Shut unit down and re-started Sunday am and got the same result. Looking to remove filter noticed electric box on Furnace blocked filter, also noticed filter has quite a bit of build up 1/8 inch thick.

I am assuming this could be my problem. After running an hour dirt gets damp and blocks air flow what is your take

Reply:

Roger, it would be unusual for an HVAC air filter to go from free flowing to totally clogged in two weeks without an unusual debris source or water source. You cite damp air filter - I agree that would certainly block air flow (and might collapse the filter for an even more dangerous problem if the filter gets sucked into the blower).

Take a look at the condensate handling system in your blower unit to be sure it's draining and not blowing water or leaking water where it doesn't belong.

Take a look at BLOWER LEAKS, RUST & MOLD

and WATER & ICE IN DUCT WORK

and WET CORRODED DUCT WORK

More obscure: if an HVAC system is oversized for the space to be cooled, it might drop the temperature very nicely but not dehumidify - that'll be a bigger problem to fix. If you suspect that problem see if your blower fan can be run on a lower speed - as a test and stopgap measure. We explain air conditioner oversizing

at DEHUMIDIFICATION PROBLEMS.

Your system is new, surely under warranty - let us know what the installer and service tech report.

Question: what happens if I install an air filter that is too small?

(6 days ago) acourtland said: Guys. I placed a filter in the unit in the attic that was the wrong size; it was too small. Now, the compressor outside is not working, and I am not getting any cool air. Can anyone tell me what I need to do, or what did I actually break, and what my next steps should be?

Reply: Acourtland, A too-small air filter at the air handler would not be expected to break the outside compressor/condenser. At least not directly.

But severe airflow restriction in the air handler could slow down airflow across the cooling coil (indoors in the air handler) enough that it might ice-up, blocking some or all airflow indoors.

In turn I could SPECULATE that the effect could be unusual refrigerant pressures developing in the whole system and speculate further that in turn that might activate a pressure safety switch that could shut the system down - though it's not exactly what I've seen in residential HVAC systems.

More often the outdoor compressor/condenser continues to run but no cool air is being delivered indoors.

Shut off the system for an hour, or more, and put in the right filter, then restart (turn everything back on) and let us know what happened.

Question: my new central air conditioner doesn't cool very well - what might be wrong?

(July 10, 2011) larry said: I bought a new central a/c new duct work n all, but it doesn't cool that great n the day time when its really hot. the repair man said its because its so hot outside. I've gone to my friends houses and its always nice. what could b the problem

Reply: Larry there are several possible problems with your new system. First, was it installed properly in all regards and second was it sized properly for the home.

I'd start by taking a look at the article linked-to at the ARTICLE INDEX the bottom of this article and

titled LOST COOLING CAPACITY

and LOW VOLTAGE TRANSFORMER TEST

Question: Air is only coming out of one of my air supply registers - what might be wrong?

(July 8, 2011) sara said: i turned on my a/c recently for the first time in quite a while. i had replaced the filter prior to starting it up. it seemed to take forever to cool the house and i went around to each room and found only one of the supply vents had any air coming out of it. what could the problem be?

Reply: Since we don't think a clogged air filter is at fault, if you have good air flow at even ONE supply register then I'd look for

• a damper closed in the ductwork somewhere
• individual registers are in the "closed" position
• ductwork has come disconnected or crimped or squashed somewhere
• If even the ONE air vent had just WEAK air flow, then provided that your blower unit is actually running I'd look for:
• a coil that has iced over and blocked air flow
• a very dirty blower squirrel cage fan assembly
• a blower that has a loose fan belt or otherwise is not pushing air
• on an older system a dirty evaporator coil in the blower unit

Question: New central air system doesn't cool; tech says "it's hot outdoors"

I bought a new central a/c new duct work n all, but it doesnt cool that great n the day time when its really hot. the repair man said its because its so hot outside. ive gone to my friends houses and its alway nice. What could be the problem? - Larry

Reply:

Larry there are several possible problems with your new system.

First, was it installed properly in all regards and second was it sized properly for the home.

I'd start by taking a look at LOST COOLING CAPACITY

 

Question: wrong size air filter installed, too small, now outdoor compressor stopped

I placed a filter in the unit in the attic that was the wrong size; it was too small. Now, the compressor outside is not working, and I am not getting any cool air.

Can anyone tell me what I need to do, or what did I actually break, and what my next steps should be? - (Aug 1, 2011) acourtland said:

Reply:

Acourtland,

A too-small air filter at the air handler would not be expected to break the outside compressor/condenser. At least not directly.

But severe airflow restriction in the air handler could slow down airflow across the cooling coil (indoors in the air handler) enough that it might ice-up, blocking some or all airlflow indoors.

In turn I could SPECULATE that the effect could be unusual refrigerant pressures developing in the whole system and speculate further that in turn that might activate a pressure safety switch that could shut the system down - though it's not exactly what I've seen in residential HVAC systems.

More often the outdoor compressor/condenser continues to run but no cool air is being delivered indoors.

Shut off the system for an hour, or more, and put in the right filter,
then restart (turn everything back on) and let us know what happened.

 

Question: Diagnose poor air flow out of new air conditioner system

Just had unit installed 2 weeks air flow was fantastic until Saturday started out great then got very little air flow.

Shut unit down and re-started Sunday am and got the same result. Looking to remove filter noticed electric box on Furnance blocked filter, also noticed filter has quite a bit of build up 1/8 inch thick. I am assuming this could be my problem.

After running an hour dirt gets damp and blocks ari flow what is your take. Roger Cote

Reply:

Roger,

it would be unusual for an HVAC air filter to go from free flowing to totally clogged in two weeks without an unusual debris source or water source. You cite damp filter - that would block air flow (and might collapse the filter for an even more dangerous problem if the filter gets sucked into the blower).

Take a look at the condensate handling system in your blower unit to be sure it's draining and not blowing water or leaking water where it doesn't belong.

MOre obscure: if an HVAC system is oversized for the space to be cooled, it might drop the temperature very nicely but not dehumidify - that'll be a bigger problem to fix. If you suspect that problem see if your blower fan can be run on a lower speed - as a test and stopgap measure.

Your system is new, surely under warranty - let us know what the installer and service tech report.

Reader follow-up

Roger Cote said:

My error on the length of time with dirty filter, 3 years of build-up and 2 weeks with air conditioner. Moved outlets and pulled air filter and cleaned now air flow back to normal and system work fine, star-up and shut down normal. Did have water on the floor but not sure where it comes from will check where you recomended to check.

Could it have been from condensation where it was very humid over the weekend.

Could the water be from ice build-up from the reduced air flow and melting when system was shut down?

Will recheck blower area and check for leaking water, thanks for your help.

Moderator reply: find the condensate leak, look for ice-blocked cooling coil

Thanks for the follow-up Roger, that's helpful. Water on the floor around the A/C system is usually a condensate leak.

Check the pan drainage.

Also you are right, if there is a lot of ice build-up in the system, when it melts, depending on just where it is, that can overwhelm the condensate drain or even accumulate in and leak out of HVAC areas that are not right over the drain pan.

 

Question: rapid clogging of air filter with black stuff

What would cause a 9 yr. old Lennox AC to suddenly have a filter blocked with what looks like black charcoal every two weeks? - Nancy

Reply:

Nancy: it's a bit speculative to try to name what black stuff is clogging your HVAC air filter, but I'd start by looking carefully at the air handler to be sure that nothing has fallen into the blower fan assembly where it'd be getting ground up (and could risk a fire).

I've examined some HVAC dust samples whose particles were traced to disintegrating plastic bearing assemblies in moving equipment parts.

But ultimately if a visual inspection of the ductwork and air handler don't show a probable debris source,
and if you are certain that the debris is not coming from the building itself, then you'd need a dust analysis of the particles to make a better guess at what it is and where it's originating.

 

Question: water droplets on the fan control unit

What is wrong with my FCU, it has water droplets? harold sazuke said:

Moderator Reply:

Look for missing insulation and moist air condensation on the cool surfaces

 

Question: Can a dirty air filter cause a crack in an evaporator pan and cause debris there?

Renting a year-old 'manufactured' home (1 of 8) that was left standing w/the roofs exposed for approximately 2 years to 'the elements', including horrendous rainstorms;

I changed the air filter once within 11 months -- HVAC unit, condensor, evaporator, etc. -- housed indoors within a closet. What developed was a huge problem of mold infestation in and outside the closet area where the HVAC unit is housed, accompanied with lots of water on the floor of the closet housing the unit.

The 'landlord' came out, looked at the damage, in and around the closet housing the unit, removed the filter without me seeing him do so, returned inside the house and told me the damage was caused because I hadn't changed the filter -- which he refused to show me.

Anyway, that same day I called out an HVAC guy who's been in the business for 30 years, who did a thorough inspection and found, among other things, a crack in the evaporator pan as well as insulation debris and some kind of cork debris inside the evaporator!

He opined that the (substantial) leak was either caused by the crack in the pan or a drain blockage . . .

after all of that, my question is can a dirty filter -- in this case, 6 months old -- cause a crack in the evaporator pan as well as cause the debris found there -- which I kept, of course?

FYI, I had been a homeowner of 5 homes, three of them brand-new, from 1978 to 2010, when I found myself in the position of having to be a renter. Any information or opinions from anyone will be greatly appreciated, and I thank you. Cynthia C.

 

Question: What pests attack un-maintained air filters?

What kinds of pest attack the system when the filter is not maintained? - Suresh

Reply: mice, m old, etc.

Mice can enter duct work and chew on anything.

Moisture landing on a dirty air filter (that thus is nicely coated with house dust that includes organic matter) can lead to mold growth.

 

Question: air filter is not trapping dust - looks clean. What's wrong?

For the past few weeks or so i have observed that my a/c filter is not trapping dust ,it appears to be as clean as ever, what could be wrong with my a/c? - AIKLAVYA

Reply:

Aiklavya, if air is moving through your system and out the supply ducts, and in at the return duct, unless the filter is a very bad fit, most of the air is going through the filter. But if your indoor air dust level is already pretty low it may be taking longer to show up on the filter.

Hold a bit of tissue in front of the return duct to convince yourself that air is moving into it.

 

Question: ok to reinstall a wet washable air filter?

We have one of those washable air filter for our air conditioner and my husband will put it back in after washing even it's still wet. Is there any harm to doing this? Could this create mold in the duct?
Thanks, - P.D.

Reply:

This shouldn't be a problem as long as you're not soaking nearby insulation or wetting electrical components; moving dehumidified air conditioned air should dry the washable filter very quickly.

 

Question: what are the effects of a too-small air filter?

My new furnance width is 17.5" but they installed a filter size 16x20x1 inches.

My old one was 20x20x1 is it the new filter too small for air return? And if so, what trouble will that cause? Anonymous

Reply:

If the replacement air filter does not fully enclose the passage where the filter is intended to be placed air will simply bypass the air filter, circulating dust throughout the HVAC system.

The result is lower indoor air quality and eventually reduced cooling when debris clogs the blades on the air handler's blower fan.

 

Question: air conditioner stopped turning on - filter was really dirty

Keith said:

My air conditioner stopped turning on. I noticed my furnace filter was really dirty. I replaced it but the ac is not working. Isn't ideas what may have broke?

Reply:

Sure Keith

Try the diagnostics for "air conditioner won't start"

found at

inspectapedia.com/aircond/Air_Conditioner_Wont_Start.php

and keep us posted

 

Will clogged air filters cause the room thermostat to shut OFF

if filters are clogged will it cause the thermostat to shut off? - by Jim -

Replhy by mod: no but ...

Keep in mind that if your air filters are clogged that will reduce air flow so that the room thermostat may never be satisfied.

The heating or cooling system may run longer than normal (increasing energy cost) or may simply never shut off.

See DIRTY AIR FILTER PROBLEMS

...

Continue reading at ENERGY SAVINGS by AIR FILTERS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIR FILTERS for HVAC SYSTEMS
  • AIR FILTER EFFICIENCY
  • AIR FILTERS, FIBERGLASS PARTICLES
  • AIR FILTER LOCATION
  • INSULATION BLOCKED COOLING COIL
• AIR FILTER OPTIMUM INDOOR
• AIR FLOW IMPROVEMENT, HVAC
• DIRTY A/C BLOWERS

Suggested citation for this web page

DIRTY AIR FILTER PROBLEMS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING INDOOR AIR QUALITY IAQ

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Or see

INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CLEANERS FILTERS PURIFIERS

Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia

Ask a Question or Search InspectApedia

Try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Note: appearance of your Comment below may be delayed: if your comment contains an image, photograph, web link, or text that looks to the software as if it might be a web link, your posting will appear after it has been approved by a moderator. Apologies for the delay.

Only one image can be added per comment but you can post as many comments, and therefore images, as you like.
You will not receive a notification when a response to your question has been posted.
Please bookmark this page to make it easy for you to check back for our response.
Our Comment Box is provided by Countable Web Productions countable.ca

Citations & References

In addition to any citations in the article above, a full list is available on request.

---

ADVERTISEMENT

---