Photograph of a dirty air conditioning filterAir Filter FAQs, Heating & Air Conditioning

  • AIR FILTER FAQs - CONTENTS: questions & answers about HVAC Air Filters Guide to Air Conditioner, Heat Pump, or Furnace Air Filter Location, Condition, Problems, Repairs.
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Air conditioner or heater air filter FAQs:

Questions & answers about air filters used on air conditioning or forced-air heating systems.

This article series answers almost any question you might ask about air filters for central heating or air conditioning systems. We explain how an air conditioning service technician will diagnose certain common air conditioning system failures or defects. In these articles we are referring to filters installed on central air conditioning or central heating systems that move air through air handlers and duct systems.

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Air Conditioner, Heat Pump, or Furnace Air Filter Questions & Answers

Dirty air filter (C) D Friedman

Question: which side of the air filter should face the room?

2016/11/26 Rev Patricia said:

My permanent filter says: "Black side receives dirty air white side gives you clean air." Which side faces the room? The filter is for the air duct that goes to the attic furnace.

[Click to enlarge any image]

Reply: Check the direction of air flow and then see the air flow direction arrows on your filter.


Thank you for asking about which side of a filter faces the room - it has pointed out that we need to be more clear about this point.

Recapping how air flows in an HVAC system:

Air flows in at a return register into and through return ducts
To the air handler (where it will be heated or cooled)
Out through supply ducts into the occupied space

I do not know for sure the answer to your question because I do not know where your filter is installed: a supply outlet or a return inlet or inside of an air handler.

Return Air filter's "black" or "dirty" Side Faces the Incoming Air - usually that's the room side

If you are installing the filter in a return register, then the “black” side of the filter is receiving “dirty” air (dusty air from your room) so it would be facing the room.

That is the usual case. Dusty dirty side faces the room because room air is moving towards and then through the filter.

If you are installing an air filter right inside the air handler itself, usually the filter is going to be placed at a point where it can filter air returning from the occupied space (“the room”) before that air gets on to the blower fan. That’s to try to avoid clogging up the blower fan with dust. So the filter’s “black” side intended to receive “dirty” air would be facing the incoming air - away from the blower fan and towards the return duct system.

Occasionally people install air filters right on the supply registers in their building (not the best practice because we’re sucking dust into and clogging up most of the air handling system: ductwork, blower fan, heat exchanger or cooling coil). But if someone were doing this naughty thing, then their filter’s “black” side would be facing away from the room and towards air that is moving towards the filter.

In general with any air filter:

Ask "Which way is air going to flow through this filter?
Then look on the filter or its edges. Usually there is an arrow showing the intended direction of air flow.
Put the filter into its holder facing such that the arrow points in the direction that air is going to flow through the filter.

If this is all less clear than ever, do let me know and I’ll take another stab at a shorter explanation.


Question: filters for a commercial AC unit never need changing? Says who?

(May 2, 2014) Raquel said:
I have questions regarding the filters for a commercial AC unit. We were told that our 2012 and 2013 units don't need filters inside our suite but rather the filter is inside the roof top unit. We also have a 2008 unit and for that we were told that we do need to change the filters inside our suite. We were also told that is because of new regulations. Is this true? So which filters should we be changing quarterly? We currently have filters at the roof top units and we also have filters in all the registers. Is this chocking our AC systems?



Really? I would very much appreciate it if whoever is quoting regulations would be kind enough to tell us the regulation authority, publication name, date, and where the regulation could be read. I'd be glad to study that information further.

It is possible that HVAC filtration is being performed in a rooftop unit, that's a common design. I'd prefer that all of the return ducts were protected by filters but that is certainly not a requirement and many systems filter only at the air handler.

What you might want to know is the maintenance schedule for the rooftop filtration system. Often remote filters suffer from being forgotten.

Yes it is a concern if you add too many layers of filtration as if you restrict the air flow beyond what the system is designed to handle it will not work properly and could fail entirely.

Question: dirty air filters and indoor air quality

Photograph of a filthy air conditioning air handler filter(June 2, 2014) DIRTY AC FILTERS said:

I moved into a older one bedroom Florida condo about 6 weeks part of my rental agreement I agreed to change the AC filter every 30 days. I am IMMACULATE. After about a week of living there I noticed the filter (the AC unit is located in a utility closet) was totally black....I immediately replaced it...two days later, again it is black....started to read about causes- had the coils cleaned (they weren't very dirty FYI) and paid out of pocket for a duct cleaning

mind you, the unit is only 730 SF so it only had one duct attached to the AC unit with a vent opening in the living area and the bedroom (FYI I also bought a mold test from Home Depot and taped it to the air blowing out as it directed to see if any mold spores grew-I did NOT take a sample from the duct itself-it looks like soot, not mold).

the cleaning service said they could only think that the insulation inside the one duct (about 8 feet long) was covered in old cigarette smoke that was causing the filter to become filthy. Since the condo has been somewhat updated there is no odor or visible signs of smoking EXCEPT perhaps within the duct (unit were built in the 1970s) after they cleaned the duct I again immediately replaced the filter (this would be the fourth filter in 5 weeks) and within 12 hours it was pitch black again

. I am at a loss-I cannot get any indoor air quality testing done thru the state/county since its a residential unit and I am now at odds with my landlord (who plainly does not want to put even $1 into unit upkeep) So what does that leave me with?? Paying for air testing out of my own pocket or breaking my lease ??(which I would rather avoid due to moving expenses,etc) I am not a hypochondriac but if the AC filter is catching the dirty air and it is black within 12 hours that can't be good?!? I'm breathing that air. I really don't know what to do. .I live in Palm Beach COunty, Florida. .thank you for any assistance. Lee


Air testing as you suggest would be an unreliable approach in the first place and not diagnostic in the second.mif you want to do some useful testing I suggest sending a tape sample of the air filter debris to a lab for particle identification. That filter and the HVAC system have already made a huge air sample for you.


(June 2, 2014) Anonymous said:

Thank you- not one person that has looked at the situation can figure out what it is...I am sure the ductwork needs to be replaced as the duct cleaners stated bc the insulation is "contaminated" but in order to "force" my landlord to do so I have to prove what is actually causing the filters to turn black...I wish I could send an attachment for readers to view...are there any labs that you suggest? I have never dealt with anything like this

Just to let you know, I looked into lab testing and most quotes were in the thousand+ dollar range...wayyy out of my reach


Typically dominant particle identification of a tape sample of dust on an air filter surfac is $50 - $100. U.S. I sent you some lab names by email.

Keep in mind that even harmless fabric fibers could look quite dark on an air filter.

(July 22, 2014) a said:
Perhaps candles or other chemicals that are VOC's in the air.

Question: clicking air cleaner sounds

(July 29, 2014) kathleenmccallartist said:
I have a TRANE XV80 air conditioner, with a Dual Filtration Electronic Air Cleaner System by Perfect Fit.I have cleaned both the filters and the metal box that holds the fan like blades. I have done this when I hear clicking from the unit, knowing that its time to clean the filters and fan boxes out. The clicking has stopped. This time after cleaning it has not stopped. The clicking stays on the whole time the unit is cooling the house. What can I do to fix this situation.

Reply: electrostatic air cleaner noises are normal but a lot of clicks mean dusty air

Look for another dust source nearby.

Question: Smart Vent unit is not so smart at cooling the house - collapsed air duct

(Aug 26, 2014) Bobbie in Roseville said:
I was just told that our smart vent was not cooling the house due to a collapsed air duct and that the cause of the collapse was most likely a dirty filter. Due to an extended injury recovery period the filter did not get changed for about 5 months. Can a dirty filter really cause the ducting to collapse? The system is 15years old.



IF an air filter were very sorely blocked by accumulated dust and debris and if the return duct were made of flex-duct then yes that flex duct might collapse under the negative pressure created when the air handler was running.

Worse, if a section of duct or filter is sucked into the air handler the blower can jam and we might even risk a fire.

But a dirty air filter would not cause the collapse of a supply duct.

So we need to know which vent was collapsed and what it was made-of.

Question: where do I get the air filter size?

(Sept 20, 2014) Anonymous said:
what size filter do i need for a Oil furnace anthes modle LOD-115

Where do I get the right air filter when shopping at Loews - Annie 5Jan 2015


What kind of filter: air? oil? Air filter dimensions are determined by its mounting location.


You can probably read the filter dimensions in the owners manual for your heat pump air handler or blower unit. Then you can buy an air filter of the right size or dimensions at any HVAC supplier or at construction supply stores such as Home Depot or Loews or at your local building supplier.

Question: inaccessible or hard to access air filter

29 January 2015 Harry said:
I have a filter in the ceiling that is a pain in the neck to replace every 3 months. Is there a tool that will let me change it without getting up on a ladder. My mobile number is 240 671 8358.



A step ladder is de-rigeur. You need to be able to un-latch the cover as well as remove and replace the filter. Having just tried some overhead tools to change a recessed light bulb at a Chicago condominium I can testify that working overhead with a pole trying to do anything precise is difficult and often of marginal success. The bulb broke off in its socket. We were able to un-screw the broken bulb base from the socket using a rubber-tipped extendable pole but having to press hard enough to turn the broken base pushed the light fixture up higher into its recess, making bulb replacement impossible. Ultimately we had to go get a ladder. In my OPINION the same issue faces you.

Options are:

- relocate the return plenum or grille to a more accessible location (costly, probably not reasonable)

- switch to a longer-lasting air filter, a deeper plated unit (possible but may require removal and replacement of the ceiling register itself with a different design that accepts a different filter. Be sure the new filter does not impact air flow rate or you'll foul up the HVAC system operation.


(May 31, 2015) Jo said:
I just bought filtrete filters for my furnace. They are very thin and I was wondering if they worked as well



And see

Question: OK to add a secondary air filter downstream and a pre-filter before the air conditioner?

(July 16, 2015) Filter downstream the A/C said:
Is there any issue to have filter (secondary filter) downstream the A/C unit and prefilter before the unit.

Reply: maybe

Filter downstream:

It's ok (as long as you are not creating too much of an air flow obstruction) except you are creating additional filter locations to find and maintain; the risk is that a future occupant fails to find and change a filter.


Question: A/C fan with 2.5 static pressure and a MERV 16 - is this static pressure OK?

(July 16, 2015) External Fan Static Pressure said:
I have A/C Fan with 2.5 Static pressure and have two filters to put in the way MERV 16 and prefilter. the duct to the area is no longer than 15 ft. Is that Static pressure enough to have these two filters in the way.
Thank you

Reply: no


Your static pressure reading is more than twice what is likely to be found and four times recommended.



Question: where to buy the right-sized air filter

Photograph of a home made air filter.(Sept 4, 2015) Silvia M Rapacz said:
Where can I find an air filter that's 15-3/4" x 17-1/2". There was one filthy one in place, but I can't find a replacement. I purchased one from Home Depot that consists of a plastic grid with a mesh like cover that can be cut to size. It's the only thing I could put in its place. The management office was of no help.


Either at your local HVAC supplier - a more specialzed company than Home Depot - or online at air filter suppliers. Don't forget to also specify the thickness.

Most HVAC equipment is designed to use one of the standard air filter sizes. If your equipment uses an odd filter you'll have to go to the manufacturer.

Watch out: there are kits to build air filters of any size, but beware of cutting and modifying a store-bought air filter to fit your system. Such home-made filters can collapse, get sucked into the blower fan and cause it to overheat, possibly causing a building fire.


Question: air flow issues - is the cold air return adequate?

(June 8, 2016) AKH said:
We are having an air flow issue. We have had 3 HVAC people out and get different answers. One said that our cold air return is undersized, others said it was fine. I am trying to do the calculations and having issues. The a/c unit is a 3.5 ton. The filter size is 16x25. Is this undersized? Also I would love help on how you calculated this?


See INCREASING RETURN AIR where we show some very simple field tests that can help you determine if your return air is adequate or not.


Question: which side of the air filter should be dirty?

(Aug 19, 2016) JDH said:
Removed an air filter and the outflow side of the filter was dirty and the inflow side was
clean. I would have expected the reverse? Was it in wrong the whole time?


"In wrong" wouldn't affect which side of the filter got dirty unless somebody took out a dirty filter and flipped it around to expose its cleaner side to the incoming air.

Check the airflow direction in the air handler.

Now look at your NEW filter (throw that old one away) and spot the airflow direction arrow on the edge of the filter.

install the new filter so that the air flow through the filter will be in the same direction as the arrow.

One obscure and unlikely situation: sometimes a power outage or an electrical problem can cause a blower motor to run backwards. THAT would dirty up the "wrong" side of the air filter and when the motor is running you'd feel air coming OUT of the return register instead of moving into it.

Let me know what you find.

Question: wrong sized air filter, moldy damp smells - will right filter fix the mold smell?

(Sept 18, 2016) alison said:

My landlord has the wrong sized filter in the HVAC and the system is in the basement where there is a moldy damp smell, that damp smell is filtering into my living area above the basement when the AC is on, will the right size filter combat the smell, I told him but he doesn't seem to get the problem?

Reply: no

These problems may have different causes and effects. Wrong filter size - if too small - means air leaks and ineffective filtering; - if too large: perhaps poor fit, bending, leaking; if a filter gets drawn into the blower it can cause a fire.

Moisture and dampness are not problems likely to be caused directly by a filter problem; I'd look for the water or moisture source as well as for its effects (such as mold).



Harry Bayo said:
Central conditioning work perfect when new, but wait until they start getting faulty, they may cost you a lot and be an embarrassment to you not high power bills nor noise producing from the faulty system. Try looking at the alternative cheaper and convenient way of managing the air in your home,



We deleted Mr. Bayo's advertisement for his own product, but left his remark.




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