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Changing two air filters after 3 months: one was dirty but one was quite clean

I changed the filters last weekend. The big 30x20 filter in the lobby was quite dirty but the smaller 20x20 in the master bedroom was really clean (almost like new). Is this normal? Thanks!


  • pedmec
    pedmec Member Posts: 807
    make sure the duct to the 20 x 20 is connected
  • Jamie Hall
    Jamie Hall Member Posts: 22,081
    and any dampers are open and grilles unobstructed
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • mattmia2
    mattmia2 Member Posts: 8,826
    It can depend a lot on where the returns are and what is in the area. High returns sometimes don't pick up much.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,683
    I have found throughout life that nothing is ever equal, or fare, or just. I guess that goes for filter too!
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • patrickl
    patrickl Member Posts: 3
    Lobby has more door openings perhaps.
    Cleaner in bedroom.
  • tomtomtom
    tomtomtom Member Posts: 35
    Thanks everyone! A tissue paper was sucked to the grille when I tested it so it's definitely working. Maybe it's cleaner in the bedroom which is closed most of the time
  • leonz
    leonz Member Posts: 957
    edited September 2022
    You have to think of your homes scorched air heating system like a farm tractor air filter.

    It is pulling in huge amounts of return air into the intake plenum at a low pressure gradient and the
    more dirt, dander and mites it pulls into the filter the more resistance and the more the increase in pressure gradient Hg occurs.

    This reminds me of the place where I used to work. They were using an air handler to heat the building
    and they had a full pallet of water valves in front of the air handlers intake air screen inlet.

    They were told to remove the pallet of valves and store it somewhere else and for two days the air
    in the shop was cleaner and warmer.

    Then they put the pallet of valves back 2 days later blocking the air handler again saying: "We had no place to store the valves". It was absolutely "Genius".

    You would be very surprised how changing a homes air filters would improve the air flow and air quality if they were changed every three weeks in a heating season or air conditioning season.

    The only time the air ducts were cleaned in my parents home was just before dad sold it after 50 plus years so you can just imagine all the dust that was blowing through the "dust" work.