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  1. #14
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,778
    No. Its the resistance to air flow in the duct that pushes the air out the supply branch and register.

    So if a supply trunk has a static pressure of .3" at 1200 CFM, at 300 CFM it only has a static pressure of .02". So the air isn't forced out the close supplies like many thing.
    So the furtherest supply and the closest supply will have the same percentage of reduced air flow.
    When a supply near the furnace/air handler doesn't have as much as it should at the reduced air flow rate, it can usually be traced back to an ell or kink or something else causing more restriction in the run then originally thought. But the higher static is able to over come it.

    Ran into it a couple times on 2 stage heat pumps. A near supply doesn't get enough air, and 2 of the furtherest away now keep their respective rooms more comfortable then they ever were in 20 years.

    Installing mod furnaces, you just have to check the balance better before and after you install one.
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  2. #15
    Join Date
    Aug 2002
    Location
    Westlake, Ohio
    Posts
    2,470
    300 cfm @ .02 static equals 410 fpm velocity
    1200 cfm @ .3 static equals 1500 fpm velocity

    Remember I am a Combustion guy not one of the Air guys. Agreed that low pressure steam travels faster than higher pressure steam but there is only a small percentage of difference in BTU's.

    300 cfm @ 30 degree rise is about 9,000 btus.

    1200 cfm @ 50 degree rise is about 60,000 btus.

    These are fairly comparable readings at factory settings. At 3-1/2 times less velocity the 9000 will disipate even more by natural convection. If we do get any heat to the far runs then I don't see any available for the close ones. This would be a good study in the field, which I have yet the opportunity to do. May this coming winter we can get some people to make some measurements as far as temperature and velocity out of each register.

    Not disagreeing with you, just having trouble grasping the how?

    I bet even my Boss hasn't done this one yet!
    captain CO

  3. #16
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,778
    I have one mod in a 3 story, all floors are fairly even temp, within 2° of each other.

    The longest linear truck run I have a mod on is about 45, maybe 48', (can't tell you the TEL anymore)its a 2 story house in a heavey shaded area. It eliminated the uneven temps in the bedrooms and the familyroom now gets warm.

    Uneven air distribution is caused by trying to move the air too fast.

    And if your moving 1200 CFM at 1500 FPM, your duct is undersized, and you have uneven room temps without major balancing damper adjustments.

    I believe you are confusing static pressure with FR on a ductulator. They are not the same thing.
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