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Thread: Furnace filter

  1. #11
    Join Date
    May 2011
    Posts
    251

    Default Re: Furnace filter

    Quote Originally Posted by Fencepost View Post
    Newer, high-efficiency furnaces tend to have lower fan speeds, which means the air isn't sucked through as powerfully. A 1" filter may restrict the air flow too much for the furnace to operate a peak efficiency.
    I thought I should make sure there isn't any misinformation or myths being spread. Many of the newer furnaces use Variable speed "ECM" motors. These motors match the airflow requirements more precisely than the older less efficient PSC motors. They can adjust to changing static pressure (airflow restrictions) and therefore maintain the roper airflow. Old furnaces might have often had their blower speed set higher than nessesary to make sure there was always adequate airflow.

    That being said... technically, a high efficiency furnace will need MORE airflow. Why? Because a 95% furnace puts out more heat than a 80% furnace with the same fire rate. The maximum temp rise is typically the same, so to move the extra heat, you need more air. Example... a 60k BTU 80% furnace puts out 48kBTU BTU, and at a 50F temp rise needs 810CFM. A 95% furnace the same size puts out 57kBTU and therefore needs 1060CFM at a 50F temp rise. ON an older furnace, that an entire fan speed faster on a 4 speed PSC motor. OR put this way, if fed the same amount of air, will be at a 65F temp rise, and could begin to damage the heat exchanger.

    A newer furnace might seem like it needs less airflow because those with VS fans are almost always 2 stage. SO on low stage the heat output is lower. Since most furnaces are oversized, they rarely go to high stage when maintaining temp. Actually low stage is 70% capacity, so even on a proerly sized system it won;t be in high stage very often either. why it might seem that it needs less
    1925 Two-Story Stucco Beaux Arts Neoclassical

  2. #12
    Join Date
    Nov 2011
    Location
    boston,ma
    Posts
    114

    Default Re: Furnace filter

    I use these filters 16x25x4 in my new carrier furnace and have found a case of 5 honeywell merv 10 filters at pex supply ****** for $104.95. So far for a name brand filter that seems to be better quality than the carrier merv 8 filters I was buying at $35 each from a local supply co. My furnace tells me when to change them, the default is set to 90 days, and measurers static pressure daily, and the filters do not seem that dirty, so I have gone to 4 months so far on current filter, also my local home depot has started carrying honeywell merv 8 filters in a 2-pack for 29.99. Thats only $15 each so you might want to give them a try if your have a home depot locally. These filters are expensive ,but the new high efficiency furnaces are expensive too so I guess it is all relevant in better air filtration. Good Luck!

  3. #13
    Join Date
    Jul 2009
    Location
    Northeast
    Posts
    661

    Default Re: Furnace filter

    Every Installer should have a checklist or report on the operation of there new furnace. On this checklist should be the allowable temp. rise by the manufacturer and the actual rise found by the installer. The homeowner should be given a copy of this for there records, at least that's what I have always done.

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