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  1. #1
    Join Date
    Jun 2005
    Posts
    66

    Are two sets of filters needed?

    We have a new house with a media air filter in attic and fiberglass filters at return air vents. Are both sets of filters needed? It seems like running both would create a flow restriction.

  2. #2
    Join Date
    May 2006
    Location
    east central indiana
    Posts
    1,117
    Originally posted byaw
    Are both sets of filters needed?
    No.
    If this is a new house as in 'just got done being built', I suspect the filters at the return air grilles were to keep drywall dust out of your new system. If this is the case, these filters did not serve their intended purpose.
    Please make sure your HVAC professional has thoroughly cleaned the drywall dust out of the furnace and cooling coil, otherwise your factory warranty is now void.
    This space for rent.

  3. #3
    Join Date
    Nov 2005
    Location
    Charlotte, N.C.
    Posts
    960
    No, remove the 1 in filters.
    Those who restrain desire do so because theirs is weak enough to be restrained. (William Blake)

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,245

    Thumbs up Pleated Filter

    Quote Originally Posted by aw View Post
    We have a new house with a media air filter in attic and fiberglass filters at return air vents.
    No,
    remove the one in the attic and
    keep 1" (or 2", depending on actual grille) pleated MERV 8 filter
    in the return air filter grille to maintain clean ducts.
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  5. #5
    Join Date
    Nov 2002
    Location
    VA
    Posts
    296

    Be certain to consult the manufacture’s specs

    AW;

    Where are you located? What time of year was the home built?

    You've received excellent advice about a concern that your HVAC system was run during the construction of your home. This is a constant issue that doesn't appear to be close to resolve. Articles in our professional publications on this very issue go back decades...

    You might want to have the static pressure measured with/without the filters in place. Consult the specifications for your unit to see what it is rated for, and then you will be making intellect decisions about the air distribution system, make sense?

    Next question, what are you looking for in the way of IAQ (indoor air quality)? Possibly that should drive the equation, not the other way around.

    Do consider a service contract for you system, wise to choose a dealer of the same brand since they tend to know the equipment better. After installing and commissioning hundreds of units one typically gets a good handle on where potential problems occur.

    My personnel preference is a large (four inch) pleated media filter that can go six to twelve months between changes. Why? Because that’s how often our customers seem to change ANY filter... must be something to do with the seasons, and certainly not with the cycles of the moon.

    God Bless...

  6. #6
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Maybe....

    I prefer measuring the actual air flow with a flow hood before I decide what filters are best.

    To measure air flow:

    1) you can measure with a flow hood like I do.

    2) or do a duct traverse with a hot wire anemometer or pitot tube, but this requires a long straight duct (12 duct diameters minimum) with no takeoffs.

    3) You can measure each grille with an annemometer then multiply the velocity at each grille times the grille Ak factor, which is the percentage of net free area times the grille size in square feet. You must have the correct chart for each brand and model of register or grille.

    4) you can measure the static pressure across the furnace or indoor unit with a manometer and static pressure probes and use a manufacturer supplied chart to convert that to air flow. You myst have the correct chart for each brand and model and fan speed.

    Then, if the air flow is sufficient, you can leave both filters in. If the air flow is low, you must replace the filters with clean filters or remove one or make other changes.

    The 1" fiberglass filter used as a prefilter and changed every month can make the media filter in the attic last longer by catching the lint and other large lumps so the media filter does not load up as fast.

    If you can find a contractor who can actually MEASURE air flow, then you can get a good answer. Those of ul on this site can only guess, because we cannot measure over the internet.

    Kevin

  7. #7
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
    Posts
    2,919
    Note in my previous post, in number 3, You must add all the supply register air flows together OR all the return grille air flows together to get the TOTAL air flow for the system.

    Kevin

  8. #8
    It depends. If the system ws designed with pre-filters in mind, then leaving the filters in place will not be restrictive because airflow considerations that would allow them have already been made.
    Check with the contractor who designed your duct system and see if the added filters were a part of his/her design.
    If not, remove them.

  9. #9
    Join Date
    Jan 2007
    Location
    Mn the state where absolutey nothing is allowed
    Posts
    1,333
    keep in mind people are generaly lazy(sorry if this is your home,i should say customers are lazy)... ,who grabs a ladder every month to check the filter in the attic?,apply KISS principle,itll get changed more often if its easly accessible

  10. #10
    Join Date
    Nov 2004
    Location
    Naples, Fla.
    Posts
    1,403

    2 Filters

    On systems we design or systems we modify to operate properly (duct sizing) I recommend the use of low density filters at the return grilles to keep the dust, dog hair & lint out of the ducts --

    Then for IAQ - we use a Merv 10 or 16 at the HVAC equipment

  11. #11
    Quote Originally Posted by chillbilly View Post
    It depends. If the system ws designed with pre-filters in mind, then leaving the filters in place will not be restrictive because airflow considerations that would allow them have already been made.
    Check with the contractor who designed your duct system and see if the added filters were a part of his/her design.
    If not, remove them.
    The HVAC material aside, that "A liberal is a person whose interests aren't at stake at the moment" is sure an on target and succinct way of stating it. I appreciated it.

  12. #12
    Join Date
    Jun 2007
    Location
    Round Rock
    Posts
    3,520
    Quote Originally Posted by 4l530 View Post
    No.
    If this is a new house as in 'just got done being built', I suspect the filters at the return air grilles were to keep drywall dust out of your new system. If this is the case, these filters did not serve their intended purpose.
    Please make sure your HVAC professional has thoroughly cleaned the drywall dust out of the furnace and cooling coil, otherwise your factory warranty is now void.

    This is complete bull. The dust should be cleaned out, but it does not void any warranty unless your contractor decides to play ch1cken**** with you and void it. Which will surely lose you as a long term customer and any possible resulting referrals, pretty much professional suicide. I've taken components to distributor to warranty with dust and filth on it and never had a problem with warranties. Change the filters in your filter frames with inexpensive filters to filter out large particles and change your media filter out every 6-12 months to filter out fine particulates. We do this all the time with no problems at all. I do not know why so many people shy away from media filters on this site, they are a great product and keep your equipment a lot cleaner. If you use a very high quality filter at your filter frame and use a media filter as well, you could run into problems, just buy the cheapos at Wally World, change them once a month and be done with it.

  13. #13
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,614
    call the installing contractor to come out and inspect the hvac system(s) if they are dirty have the builder pay to have them cleaned.This is a common problem in custom homes as they want the ac to remove the humidity as the wood work is being installed

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