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Community and Q&A

Spotless HVAC filter

Chris Brown | Posted in Mechanicals on

Why would my HVAC filter show absolutely no dirt or dust? It fits tightly and there is no residual dust in the air handler unit. A couple of years ago I found that the top of one of the return runs was never closed in, leaving it open to the mechanical room in which that return was located. I covered the opening with duct board and sealed it with mesh and mastic. The majority of returns are panned (what a joke!) and are not accessable. Those that are accessable, I have also sealed appropriately. The filter may not display dust, but the house sure does! Climate Zone 5.

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    How long has the filter been in your furnace?

  2. Chris Brown | | #2

    Martin, the current one, only 3-4 weeks. I faithfully change the filter every 30 -45 days. Just to experiment, I have left a couple in for 60 to 90 days over the last couple of years. It makes no difference. When I pull them out, there is virtually nothing on them. The filters are typically fiberglass or the stronger fluted paper ones (1"). I have even tried different colors just to see if the filter medium was playing tricks on these well used eyeballs. I have checked the handler housing to make sure there are no unplugged openings and there aren't, and all of the mating panels in the housing are sealed up, so its not like the return air flow is coming from some indirect route to the handler. The unit appears to be functioning fine from a heating/cooling standpoint. Heating averages about $150/mo in heating season while cooling bills are about double that in July and Aug. Natural gas power vented furnace original to the house 10 years ago.

  3. User avatar GBA Editor
    Martin Holladay | | #3

    If the filter has only been in your furnace for 3 or 4 weeks, I don't think there's any mystery here -- except why you think it is necessary to change a perfectly clean filter.

  4. Chris Brown | | #4

    Nice little dig Martin! The mystery is this....I have owned 9 homes with central HVAC in 4 different states and in 4 climate zones over the last 35 years; some new builds and some existing....never have I had filters with no dust on them. If nothing is amiss, then fine, but it just doesn't make sense to me.

  5. User avatar
    James Morgan | | #5

    Your a/c bill seems very high, and I'm surprised that in Zone 5 it's double your winter heating bill, should be the other way round. It sounds like you may have a major air leak somewhere - that might also explain the lack of dirt on your filter. I'd suggest at the very least having your ductwork checked out by a knowledgeable professional, a full energy audit would be better still.
    By panned ducts I assume you mean ducts formed between floor joists by closing the bottom with sheet metal panels. A home old enough for this may also have diagonal boarded wall and floor sheathing installed with gaps between the boards as much as half an inch wide. If this is the case your return ducts could be pulling as much air from the exterior of your home as the inside. The cost of replacing them will have a pretty swift payback on your energy bill.

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