Why is my furnace filter alarmingly clean?

Question

I have central air and heat at my appartment I moved into about 6 months ago. Today I decided it was probably long past time to figure out how to change filters in the furnace so I poke around until I find the slot where the filter is.

However, when I pull it out, it looks like it was installed yesterday. It's perfectly clean, doesn't have a speck of dust on it. I've been using it for the past six months so I highly doubt it should look this way.

It's a really cheap fiberglass filter (Ace hardware brand), could it be defective? Could the landlord have placed it in the furnace incorrectly? Could something be wrong with the furnace? Or is this expected?

My last apartment had a permanent filter that had to be pulled out and washed every couple weeks or else it got filthy. Please help me figure out what's going on here.

Answer 1

The thin fiberglass filters (usually blue in color) are referred to as "rock catchers" by some of my fellow construction friends. They don't do much to catch dust, but they'll stop anything large from damaging the blower. It's entirely possible that it was changed before you moved in and just hasn't had enough time to catch the little dust that it will catch.

If you really want to filter the air, you'll want to upgrade this to the pleated filters. Just remember that these do require changing on a more regular basis.

The other possibility is that you may have filters at your return vents doing a majority of the air filtering, and the filter at the blower is simply redundant.

Answer 2

That is very unusual, is there a chance your landloard changed it for you recently? Mine seems to get dirty almost instantly.

Some things I'd check:

1. The airflow direction of the filter is correct. You can verify this by feeling the air flow and comparing it to the arrow that is usually printed on the filter.
2. Is the filter the correct size? If it's not then the air will just go around it. The filter should fill the space completely.
3. Is the fan running often?

Answer 3

If it still looks like new after six months, even though it is the cheapest and most inefficient filter available, then something serious could be wrong. Ensure that the return ductwork is all securely attached (so no unfiltered air is bypassing filter altogether). And as another asked, does the fan run often? If little to no air is going through the filter it's coming from somewhere else, and that's a problem.
When you get that figured out buy some new filters - they will cost a little more - that are pleated. Any a/c tech will tell you that. And for the fun of it look up MERV ratings so you can determine what filtration level you desire. The more you spend the more it filters. You can operate in your living room with the proper filters, air wise that is. Best of luck and see if homeowners will cover it.

Answer 4

I am an HVAC contractor with 9 years of experience on field. Too clean filter tells you that the dust that was supposed to be filtered out by this filter is still in the air that surrounds you. I am pretty sure that you've already checked filter size, correct installation, damaged air plenum as a way for air flow by-pass, etc. So, the filter is clean because very little of air flow happens trough the filter. I would say that in 90% it will be VERY insufficient air return. A lot of times for return air pathway builders use inside of drywall area just between two studs. That area is 3.5 x 14.5 inches and can carry ~100CFM (cubic feet per minute) of air flow. Lets say you've got 5 ton unit... Thus, 5 ton unit would need 2200-2400CFM of air return. This means that you should see 22-24 on these in-wall returns installed in your house. Does it happen in real life???? Absolutely not. Oh, keep in mind that metal girl on the return box would cut off ~30% of air flow. Thus, actual air flow though that return installed by your builder is ~70CFM... Keep in mind that longer the air duct, higher the air flow resistance. Long story short, if your A/C system return ducts are 6-7 times less than this system requires, you will see 6-7 times smaller air flow.... less air flow = less air filtration = clean filter. Two months ago I had to completely re-run entire air return ducts system in one house. Instead of using those inside of drywall spaces I ran 2 flex 14" ducts and installed 2 14x14 ceiling mounted air return boxes. A/C unit efficiency improved instantly and dramatically because initial air return for 5 ton unit was ~450 CFM and air return after duct work became ~2200 CFM. Before the duct work this air filter needed replacement every 8-10 months. It was looking only a little dirty after several months! Now, it needs replacement after only 1.5- 2 months! Thus, beside of improved system efficiency this A/C unit got better air quality. Before the improvement this unit had the "dirty sock syndrome". It is an official terminology for stinky air as a result of mold growth inside of the supply air plenum. After when air flow was corrected that "dirty sock syndrome" wasn't an issue anymore.