Results 1 to 11 of 11
  1. #1
    Join Date
    Jun 2004
    Posts
    247
    I have been trying to determine how long to let the filter go before changing it. With new 'stats that measure run time, I have an answer that some might find useful. What follows involves some anecdotal data.

    MERV 8 Pleated Filter life Calculations.

    Based on watching a filter get dirty to where the flow was noticeably restricted (too much dirt)

    Household family of 4 without pets

    Filter was a MERV 8 pleated filter.

    Only one filter grille with a 32X16 filter

    120 hours of use based on digital thermostat timer

    Blower set to 800 cfm

    Total cfm of air that passed through filter was--

    120 Hrs X 60 Min/Hr X 800cuft/min = 5,760,000 cu ft of air filtered.

    Filter area= 16 X 32=512 sq in.

    Volume of filtered air/Sq In is 57600000/512 = 11,250 cu ft/sq in

    Since the filter was too dirty, I'm going to use 10,000 cu ft/sq in as my ideal volume per sq in.

    Now, the ideal filter is sized for 2 cfm/sq in of filter area.

    Therefore, the expected life of a filter on a properly designed system would be--

    10,000cu ft/sq in divided by 2 cfm/sq in= 5,000 minutes or 83 hours.

    A more messy household would shorten this value. A retired couple could lengthen it.

  2. #2
    Join Date
    May 2005
    Location
    lawn guyland, ny
    Posts
    27
    you have way too much spare time.

  3. #3
    Join Date
    May 2002
    Location
    Colorado
    Posts
    697
    Originally posted by madmark
    you have way too much spare time.
    Maybe he should use it to vacuum a couple times a week.

  4. #4
    Join Date
    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
    Posts
    3,304

    How to know when to change

    I can see you are trying hard to analyze this problem, and taking lots of measurements and providing lots of facts. However it seems to me you still are not talking the language that the pros speak, sorry to say.

    It is my understanding that a filter needs to be changed when its pressure drop exceeds a certain amount due to loading up, no sooner and no later. All the things people say, be it 3 weeks or once a year, are at best attempts to estimate this pressure drop without knowing any measurements. At worst they are marketing mis-truths designed to sell a product (usually their filter).

    If there is any way to measure the ESP ("External Static Pressure") of your system, that would give you numbers you could use to answer your question. Many AC techs have the tools to measure this, I suspect they could be used more often to good advantage. If your tech would measure ESP before and after your filter change, that would give you some useful info.

    The proper way to measure is a manometer or gauge with one tube leading to before the filter (i.e. in the house air) and the other tube leading to after. I have done this with a Dwyer red-oil manometer bought on Ebay -- but I am a nut about this stuff and don't expect you to do the same. Dwyer professionally documents this application for commercial installations, where there is more money at stake.
    http://www.terrauniversal.com/produc...iltergauge.php

    There is also a little "G-99" gauge intended to measure pressure drop changes across the filter, specifically intended to tell you when filter change is needed.
    http://www.filters-now.com/products/dar.html

    It is cheap and simple, and looked good to me -- but somebody on the board told me they disapproved of it, I forgot what they said but it sounded like a professional opinion at the time. Still, how could this be a waste of $15?

    Hope this helps -- P.Student

  5. #5
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    if it looks gray, change it -- with family of 8 + 2 dogs, I used to change ea week -- el' cheap o -- Yankee country = COLD

  6. #6
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,241

    Exclamation Filter It Requirement is HIGHLY SUBJECTive

    Originally posted by wendel
    Filter was a MERV 8 pleated filter.
    .. life of a filter on a properly designed system would be--
    A more messy household would shorten this value. A retired couple could lengthen it.
    ------------------------------
    NICE calc absolutely requires validation and modification:

    Is 83 hours or changing a MERV 7 ( or 8 ) filter ~ every 10 days realistic?

    What FILTER dP and AIR FLOW did you measure
    with Clean Filter? Dirty?
    ------------------------------

    http://www.dwyer-inst.com/htdocs/pre...s2000Price.cfm

    ........
    Use Merv 7 ... Measure and Track over an extended period.

    Seems like you would have to decrease air flow > 20% to make significant, noticeable difference in A/C performance.

    ------------
    Actually, retired couple may be MORE SENSITIVE & at HOME 3 times more than others, so MORE frequent filter change may be required.
    ------------
    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

  7. #7
    Join Date
    Jun 2005
    Location
    GA
    Posts
    30

    Cool

    Refreshing to see an engineering mind at work!

    Your calculations (maybe a bit overdone) seem to be acceptable. However, I would think they are ONLY useful within your specific house AND assume that you don't do anything drastically different, like have the windows open during pollen season.

    One thing that I've learned from the Pro's around here is to CHANGE THE FILTER. So, I use once a year, but check quarterly for abnormal changes method (big 20x20x5 filters). Others use the monthly, or the "does it look dirty method". If this works for you, I think you are ahead of the game.


  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,727
    So people with PSC blowers that run the fan 24/7 should change their air filter twice a week, and VS once a week.

    The location of the house, and amount of air infiltration will greatly vary the life of the air filter.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  9. #9
    Join Date
    Jun 2004
    Posts
    247
    Thank you for your replies. The intent was to devise a simple way to calculate a reasonable filter life expectancy for a homeowner to use. Of course each house is different and you'd make adjustments for pets, open windows etc. When you're in a house I'm sure you have a sense of its air quality. I determined a (my) MERV 8 filter can handle 10,000 cu ft of air per Sq In in a relatively clean air home before needing replacement. Since I knew the (my) blower air flow, the time was simple division. If a house has great filter area relative to air flow, the calculations would extend the life of the filter. If the filter is small, then the filter life would shorten. These are calculations that can be done easily and programmed into the thermostat's "reminder" feature. The best part of these new thermostats is they keep track of 'run time'. We homeowners don't have fancy pressure equipment nor do we want to be bothered checking the filter all the time--we're lazy. So a simple calculation from data you have at your finger tips, specific to each home you're working in, would be of great value to a homeowner, IMHO.

  10. #10
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    6,241

    Exclamation Scatter

    Originally posted by wendel
    Based on watching a filter get dirty to where the flow was noticeably restricted (too much dirt)

    Since the filter was too dirty, I'm going to use 10,000 cu ft/sq in as my ideal volume per sq in.
    It can't be KNOWN to be "Too Dirty" unless there are definite symptoms of significantly decreased flow or __ you MEASURE it!
    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

  11. #11
    Join Date
    Jun 2004
    Posts
    247
    That's true, Dan. In my case the evidence was the formation of condensation on a supply register. After the filter change, no more condensation.

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