Type of filter in Residential HVAC unit
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  1. #1
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    Jul 2008
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    Type of filter in Residential HVAC unit

    Hey guys, I had a house with a centralized HVAC system before, and I want to know what kind of filter is normally installed in a typical residential air handler. I don't know what kind of make or model or even the type of HVAC unit it was for, only that I had one. Are they usually high MERVs like HEPA/ULPA? The fact that it never required or wasn't cleaned is leading me to think it was HEGA or a poorly maintained electrostatic precipitator. Pardon my absolute lack of knowledge with these things but if anyone can give me an answer from the little that I provided I'd very much appreciate it. God bless.

    - Rick

  2. #2
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    Dec 2007
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    Afton, VA / Khorat, Thailand
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    If the AHU comes with a filter in it, we always disgard it and put one in the return ductwork. They come with cheapo filters, and we give them another cheapo unless they request otherwise. Never seen a unit come from factory with a HEPA or anything in it.

    But it just seems easier for HO's to change them if they are in the ductwork rather than in the unit itself.

    Never saw a filter that didn't require cleaning or replacement. If someone can patent that one........
    Tough times don't last...Tough people do.

    Midnight Sun Astrophotography

  3. #3
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    Central Maryland
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    Quote Originally Posted by WildWeasel View Post
    Hey guys, I had a house with a centralized HVAC system before, and I want to know what kind of filter is normally installed in a typical residential air handler.
    There are three types, basic fibre glass, paper, and fancy spun fibre. Merv goes from about 3 to 12 as you move up through the types and cost.

    The lowest level is only supposed to protect the equipment. The middle and higher levels are supposed to filter mildew spores, pollen, and even viruses.

    They come in 1" through 5" thicknesses in almost every imaginable size.

    High MERV filters in 1" sizes have very high flow resistance, which is bad.

    Low MERV in thick filters have very low flow resistance.

    -HF

  4. #4
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    Quote Originally Posted by hangfirew8 View Post
    There are three types, basic fibre glass, paper, and fancy spun fibre. Merv goes from about 3 to 12 as you move up through the types and cost.

    The lowest level is only supposed to protect the equipment. The middle and higher levels are supposed to filter mildew spores, pollen, and even viruses.

    They come in 1" through 5" thicknesses in almost every imaginable size.

    High MERV filters in 1" sizes have very high flow resistance, which is bad.

    Low MERV in thick filters have very low flow resistance.

    -HF
    Would'nt the same low merv rating have less resistance in a 1" than a 5"? Like a merv 4 or so?
    Key1

  5. #5
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    Mar 2008
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    Nordic Pure or Dirty Filters

    We use NP 12 MERV. Small loss of pressure, great filtration. They filter so well, we have to change out about every 3-4 weeks-filthy.

  6. #6
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    Mar 2008
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    Central Maryland
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    Quote Originally Posted by key1cc View Post
    Would'nt the same low merv rating have less resistance in a 1" than a 5"? Like a merv 4 or so?
    Key1
    No.

    A five inch filter is not 5 inches thick of filter material. It is the same thickness as 1 inch but bent, accordian style, many times. The result is much more surface area, essentially a filter 5 times the size pleated into a smaller space.

    The more surface area, the more places air can flow through, the lower the resistance.

    -HF

  7. #7
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    Jul 2008
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    Thank you all for your replies, they've all been very helpful. I'm guessing I had an upgraded AHU because the air seemed clean and comfortable. The fact that it was never cleaned however makes me think it was a HEGA as I've heard it requires less maintenance. Is that correct, and if so how often does it require servicing? What other filtration types don't require constant maintenance?

    There are three types, basic fibre glass, paper, and fancy spun fibre. Merv goes from about 3 to 12 as you move up through the types and cost.
    How often are these types of filter cleaned or maintained?

    Again, thanks for all your help and patience with dealing with someone who lacks the knowledge. God bless.

    - Rick

  8. #8
    Join Date
    Mar 2008
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    Central Maryland
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    Quote Originally Posted by WildWeasel View Post
    Thank you all for your replies, they've all been very helpful. I'm guessing I had an upgraded AHU because the air seemed clean and comfortable. The fact that it was never cleaned however makes me think it was a HEGA as I've heard it requires less maintenance. Is that correct, and if so how often does it require servicing? What other filtration types don't require constant maintenance?
    I have a car. I think it's a Chevy. It think it might have synthetic oil, because no one has ever changed it.

    How often should I change the oil, if ever?

    Keep in mind, I'm not really sure it's even a Chevy.

    In other words, until you give us some details... who knows?

    -HF

  9. #9
    Join Date
    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    Quote Originally Posted by hangfirew8 View Post
    I have a car. I think it's a Chevy. It think it might have synthetic oil, because no one has ever changed it.

    How often should I change the oil, if ever?

    Keep in mind, I'm not really sure it's even a Chevy.

    In other words, until you give us some details... who knows?

    -HF
    The garages I talk with all say every 3000 miles. The HVAC pros I hear usually say change the filters every 30 days.

    Some advanced technology cars use a computer to caluculate when oil should be changed, and the answer usually is far longer than 3000 miles. Some advanced technology AC systems measure ESP in order to tell you to change the filter when it loads up and pressure drop increases beyond a certain level.

    Carrier Infinity does this I am told. Does it have a timer to cap the change intervals at some maximum, or does it wait forever if the pressure drop does not change much? I really don't know the answer but would like to.

    For the great majority of homeowners, they have disposable filters accessible from the inside of the house and are supposed to change them monthly. Many people don't change them that often but they are supposed to.

    Hope this helps -- Pstu

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