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  1. #1
    Join Date
    Aug 2007
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    273

    Static pressure test questions

    I have found out from this board that a static pressure test is advisable for retrofitting new a/c ,furnace systems especially in an older house such as mine. Are static pressure tests difficult ,time consuming etc. to preform? And if the test comes back with a negative result what can be done besides installing new ducts which for a retrofit in an older house would be impossible with out riping the whole house apart. Thanks for your help

  2. #2
    Join Date
    Dec 2004
    Location
    Chicago area
    Posts
    1,439
    Performing a static pressure test is fairly simple. Static pressure is one part of calculating air flow. The other part is duct size.

    All that is required to perform the test is a magnehelic gauge or a manometer which are pressure gauges that read in inches of pressure.

    The trick is interpreting the results. For every ton of cooling you need to move 350 to 400 cfm so a 2.5 ton a/c needs 875 to 1000 cfm of air to work propperly.

    @ .08 W.C. static pressure you need a 14" round duct to move 900 cfm air which will move at a velocity of 850 feet per minute.

    but lets say you only have a 9" duct so to move the same 900 cfm, you will need to increase the static pressure to .7'' W.C which will move at a velocity of 2100 feet per minute.

    The higher the static, the larger the blower needs to be, and the faster the air will need to move which will cause the system to be louder.

    I typically size for .08" for residential application .5'' is the highest recommended static.

  3. #3
    Join Date
    Dec 2004
    Location
    Chicago area
    Posts
    1,439
    To know options of what can or would need to be done to correct duct problems, you would be best having a contractor or a few of them to give you estimates as to what your best options are.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,763
    I think your confusing FR, with static pressure.
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    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Aug 2007
    Posts
    273

    Blower size and ducts

    Thank you all for your responses. Are you saying that if i have smaller ducts i need a larger blower.Would the larger blower cause any problems. my contractor has specked a infinity gas furnace 58MVB 100-20. Do you know the size of its blower from this model number. Thanks again for all the input and help.

  6. #6
    Join Date
    Mar 2007
    Location
    Myrtle Beach, SC
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    2,919
    Quote Originally Posted by heatingman View Post
    The trick is interpreting the results. For every ton of cooling you need to move 350 to 400 cfm so a 2.5 ton a/c needs 875 to 1000 cfm of air to work propperly.

    @ .08 W.C. static pressure you need a 14" round duct to move 900 cfm air which will move at a velocity of 850 feet per minute.
    That assumes exactly 100 feet total equivalent feet of duct. Throw some elbows and a tee and takeoff in the duct and now it is undersized. With only a short run, that duct would be too big.

    Normally the static pressure test should be done with the furnace blower charts in hand. That will tell your total air flow.

    Or you can use a flow hood or anemometer to measure actual air flow.
    Remember, Air Conditioning begins with AIR.

  7. #7
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    ron,
    Sorry but I'd ignore heatingman's comments,for the most part.

    They need to use the fan data charts for your existing furnace ,to determine the current air flow,at the static from their test.Then compare that to the required air flow of the new furnace,to see if the ducts are adequate.

    If your current equipment has a dirty coil,blower,filter or cooling coil,the tested static needs to be adjusted for that situation,experience is the only guide.

    The duct fittings at or near the furnace can usually be modified ,if the static is too high.Adding turning vanes to all 90° elbows will help,as will adding another return trunk,if that's possible.

    Once the Infinity is installed you can read the static on it's control,but it's best to deal with any issues beforehand.

    Another way to deal with undersized ducts is to improve the envelope of your home,insulation,caulking,windows,etc.,this can reduce the equipment size needed ,and thus the amount of air flow required in the ducts.

    Manual J load calculation will be needed,to determine the load,for heating and cooling.

  8. #8
    Join Date
    Aug 2007
    Posts
    273
    Thank you for your response. what should the static reading be on the infinity control if the system is to work properly? Thanks again

  9. #9
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    IMHO, .7 in. wc ,or less.

  10. #10
    Join Date
    Feb 2003
    Location
    Huntsville,AL
    Posts
    4,125
    Dash:
    YOU are entitled to drop "humble" !!!!
    harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!

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