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  1. #1

    When to change my filter?

    Hello All

    I am new here, I did search for an answer to this question but did not find one.

    I have a 5 ton HVAC system with a 25x20x5 filter (marv13) I have a magnahelic (sp?) between the return and supply it reads .07 (inches of water) with out a filter, .31 with a new filter and .34 with a 2 month old filter. An 8 foot long, 5 inch ridged duct closest to the supply of the HVAC unit has 420 ft/ min speed with out a filter and 380 ft/min with a new AND the 2 month old filter. (We run the HVAC fan 24/7 so the filter gets dirty way faster than if it was in intermittent use.)
    My question:what is the best way to determine if the filter needs to be changed? Pressure differential increase or air speed decrease?



  2. #2
    Join Date
    Sep 2007
    Boise, ID
    IMO, when the DP (differential pressure) hits .5 and no later. Could do sooner a 0.4 if you want. Some of the determining factors is how much 0.5 affects system performance. If you notice it's not heating or cooling as well at 0.5 then replace sooner. Your system is affected by total static pressure, not just the pressure drop across the filter.
    Last edited by Gib's Son; 09-10-2011 at 02:43 PM.
    If a nation expects to be ignorant and free, in a state of civilization, it expects what will never be. (Thomas Jefferson 1816)

  3. #3
    Join Date
    Jul 2004
    My first comment is that absent a filter, your total static seems to be quite low, too low for my design criteria. Anything less than a .2 TESP indicates a lack of air movement and an inability to control the air. Are you measuring the static with the blower on high speed?

    In order to determine the static loss of the filter you'd need to position the high and low static sensors on each side of the filter, rather than on the supply and return. Does your system have a variable speed blower? If so, airflow is a useless measurement overall as the blower will adjust to produce a constant airflow. It's static that will vary according to resistance in the distribution system. Therefore, determining what the pressure drop is across the air filter would be the critical determination of when it should be replaced. If you have a variable speed blower I'd not want to see the TESP higher than .6 IWC. Ideally that would mean inclusive of the filter. So if you're seeing a .25 increase in static with the filter in place I'd lean toward a drop across the filter of .35 IWC maximum. That would probably be a good average even if you have a PSC motor.

  4. #4
    Join Date
    Jul 2008
    Rochester NY
    Skip, my partner is an NCI certified air balancer. They recommend designing duct to .08 static. Most furnace manufacturers want to see no more than .5 esp.

    Are we talking about the same thing here? I thought higher static on psc meant LESS flow, but you imply it means more?

    These ecm's attempt to deliver flow until they burn up, which high restriction causes, so I'm told. You seem to be completely contradicting these things. What am I missing? BT, help us out here?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to, or RESNET, and find an auditor near you.

  5. #5
    The Magnahelic is measuring the difference between the return in front of the filter and the supply side of the filter before the fan.

    I have been changing the filter at .35-.40. I left it in longer once and the supply air speed was lower (don't remember what it was) the pressure differential was closer to .45-.50 at the time.

    I figured that if the air speed remains the same the filter is still able to pass air efficiently and if the air speed drops off it is restricted.

    I just thought it was funny that the pressure differential was higher but the air speed was the same. (could be that the anemometer can not measure fine enough resolution.) It just seems to me the higher the pressure differential across the filter would = lower air speeds?

    The blower measurements were done at low speed of a NON variable speed blower.

    Thanks again to all!

  6. #6
    Join Date
    Jun 2009
    The variable speed blowers we have worked with will start to "surge" if static gets too high. That's typically above 1" TESP. Manufacturers typically *rate* their equipment at .5 TESP but that's very difficult to achieve. Look at the tables provided with the system manuals and you'll usually find ratings up to .8 or even 1".

    It's not clear to me what your reference point for static is, so it's hard to pick a number. I'd look at your temperature differential across the heat exchanger and the cooling coil (along with several other things) before I'd even guess at the "change me" reading. I will say that if you're running 24 hours a day (I do the same thing) that you should probably expect to change that filter at least 3 times per year. If you have pets, pet doors, kids that leave doors open, or leaver your windows open a lot it could be even more.

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