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Response from HVAC contractor regarding air filter

nvman | Posted in Mechanicals on

Is this HVAC contractor trying to bs me with this comment regarding a Lennox Pure Air filter box with a MERV 16 filter?

I chose the Pureair because the rated pressure drop was only 0.42″ of water column and the air resistance is rated at 0.11″ of water column of air resistance at 800 cfm. The stand alone MERV 14 filters have a slightly worse pressure drop and a much worse air resistance (0.47″WC and 0.35″WC respectively). For this reason I wanted to give you the best filtration with the lowest pressure drop possible. The stand alone MERV 16 filters have a much worse pressure drop and air resistance but because the Pureair is modular, they can achieve a much lower pressure drop and air resistance while maintaining a higher filtration rating.

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  1. Expert Member
    Akos | | #1

    Blowers only care about the pressure drop across the filter assembly at the flow rate of the system. Not sure what you make by rated air resistance.

    High MERV filters will always have a higher pressure drop than equivalent standard filter. So if you want low pressure drop, which you want, you need to installer a larger filter. This means going with 4" filter and if possible a larger filter.

    On a typical furnace around me is a 16x25x1 MERV8 filter, but if you step that up to 4" MERV13, it is the same pressure drop. If you increase the filter size to 24x24x4 MERV13, you half the pressure drop, which means reduce blower power and lower operating cost.

    1. nvman | | #2

      Thank you Akos.
      The filter he is recommending is 16x26x5 Merv 16.
      I am confused when he mentions two measurements,
      “rated pressure drop was only 0.42″ of water column and the air resistance is rated at 0.11″ of water column of air resistance at 800 cfm.”
      I never heard of air resistance.
      Also, if my furnace runs at 1600 cfm, does that mean the pressure drop is .21 or .84?
      Thank you

      1. Expert Member
        Akos | | #3

        Just because it says that it is rated for 1600CFM, it doesn't mean it should be used at that flow rate. Sticking a high merv filter on an air handler not designed for is asking for trouble. I recently went with it when I replaced a filter with a unit I had laying around from a project (you know the "might as well use it instead of throwing it out"). This filter had enough resistance that it caused the furnace to shutdown and lock out. Not what you want in the winter.

        If the unit he is talking about:

        The 16x25 is pretty restrictive. Most systems are sized for around 0.5" WG pressure drop, so having a filter that is 0.4" doesn't leave all that much for the rest of the ducting in the house. You want to be less than 0.2", so at 1600CFM that is a minimum of 24x24x4 MERV13. MERV16 is probably a bit larger than that.

        There is absolutely no drawback to a larger filter. Less restriction, longer life and minimal replacement cost difference.

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