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  1. #1
    Join Date
    Feb 2007
    Posts
    999

    Beenthere: related ? to Mod vs 2 stage

    Quote Originally Posted by beenthere View Post
    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.
    Rather than intrude on the original thread, I'm posting a question prompted by your (above and others) post.

    During the last few days, I have been 'changing the dampers' for cooling season. The changes are mostly to open the upstairs runs fully. They were slightly closed as heating was more than adequate. Downstairs adjustments were slight, but the tendency was to err on the side of closing them down (to optimize upstairs flow).

    After reading your posts, I'm thinking I should open up the downstairs runs which originate closer to the furnace and (if I understand correctly), would reduce static and possibly send more air to the farthest register (upstairs).

    Do I have that right? If so, should this procedure be limited to the first trunk section (2 transitions downstream). Also, current furnace and a/c is single stage; will be replaced shortly by 2 stage a/c and Mod furnace. Any difference in strategy?

    AM

  2. #2
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,793
    Seasonal damper adjustments are somewhat different then the discussion your refering to.

    You most likey won't get more air flow upstairs because your blower isn't slowed down like a 2 stage or mod furnace would slow it down. So the air flow to the upstairs won't increase in a gainfull percentage that you would notice.
    Your system is basically trying to jam the air through the ducts and pipes.
    If you had your static tested, it would probably be fairly high. Opening the first runs would lower the supply static at both the beginnig, and the end of the trunk line. Although total CFM would increase, it would not neccesarially be proportional for all supplies.

    Once you have your mod, and 2 stage A/C, you'll be able to leave the first floor open further in cooling, and shouldn't have to close the second floor as much in heating.

    On single stage, non VS systems. Using the balancing dampers for the seasons is common. I usually mark them summer, and winter on the pipe/duct.
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  3. #3
    Join Date
    Feb 2007
    Posts
    999
    Quote Originally Posted by beenthere View Post
    Seasonal damper adjustments are somewhat different then the discussion your refering to.

    You most likey won't get more air flow upstairs because your blower isn't slowed down like a 2 stage or mod furnace would slow it down. So the air flow to the upstairs won't increase in a gainfull percentage that you would notice.
    Your system is basically trying to jam the air through the ducts and pipes.
    If you had your static tested, it would probably be fairly high. Opening the first runs would lower the supply static at both the beginnig, and the end of the trunk line. Although total CFM would increase, it would not neccesarially be proportional for all supplies.

    Once you have your mod, and 2 stage A/C, you'll be able to leave the first floor open further in cooling, and shouldn't have to close the second floor as much in heating.

    On single stage, non VS systems. Using the balancing dampers for the seasons is common. I usually mark them summer, and winter on the pipe/duct.
    Thanks, Been. Gives me food for thought. I have the 'winters' marked, but just some pencil marks (temporary) for the cooling. Will probably have to redo all of them.

    Thanks again.

    AM

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