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Thread: Forced air furnace question

  1. #1

    Default Forced air furnace question

    I have a 2 story building with a shop and office downstairs and living space upstairs. The furnace heats the upstairs only. Could I install registers in the main supply trunk downstairs (to add some heat to the office) without upsetting the upstairs heat performance? The furnace is oversized by about 10K btu, It's the smallest one the company makes.

    Thanks alot

    Hugh

  2. #2

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    Size of the furnace is only one consideration. The balance and design of the ducts is the one that you could do some harm to by arbitrarily installing takeoffs to supply downstairs.

    Additionally, supplying heat to downstairs from above is obviously not optimal since heat rises...

    If cost isn't an overiding concern and there is some space, install another small furnace and it's own ductwork. If it is a concern, make sure whoever reconfigures the ducts does it not willy-nilly.

  3. #3

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    Thanks Johncap

    The installer didn't balance any of the duct runs. I don't have any dampers anywhere in the duct system. If I do upset the balance in the system, how will this show itself? The lower floor is slab on grade so I don't have any options at this point short of ripping the wall up. Thanks

  4. #4

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    Quote Originally Posted by tugzz View Post
    Thanks Johncap

    The installer didn't balance any of the duct runs. I don't have any dampers anywhere in the duct system. If I do upset the balance in the system, how will this show itself? The lower floor is slab on grade so I don't have any options at this point short of ripping the wall up. Thanks
    Well, typically no one these days does too much to formally balance much of anything. They duct tape (yes, that stuff is actually used on ducts too) the metal together and hope it works sufficiently. Generally if it's all straightforward there's not to much that can go wrong aside from the occasional room at the end of the run not getting much airflow. Which is precisely what will happen if you start adding takeoffs. The static pressure in the duct will be reduced (obviously depending on the runs now, how many takeoffs, etcs.) and perhaps you will discover farthest rooms not being serviced quite so well. You can manually adjust some of this by installing manual dampers and regulate, perhaps forcing more heat down in winter and cutting back on the lower floor in summer to allow more a/c airflow to the upstairs rooms where it is likely needed more.

    To heat a first level on slab from above may not work out too well. What is that level heated with now? Space heaters? Is there any place duct drops could be boxed in to drop supply registers down to above floor level around the perimter without taking up much needed space?
    Heat pumps are for chumps...

    Propane fired 97+ furnace with multi stage blower, backing up 15 SEER heat pump.... Yup, I'm a chump, but a mitigated chump....

  5. #5
    Computer Programmer Bill Arden's Avatar
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    The worst thing that can happen is that you get too much flow to one room and the adjustable lovers in the registers start to whistle when you close them.

    and even that can be fixed by using a short section of smaller duct.

    The hard part will be figuring out how to run ducts to where you want heat.
    Important note Ė I donít know man made laws, just laws of physics
    Disclaimer: I'm a big fan of Darwin awards.

  6. #6

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    I should add that too often not enough consideration is given to returns. Returns are critical, placement as well as size and number, in some cases. Especially for a/c but also for heat.
    Heat pumps are for chumps...

    Propane fired 97+ furnace with multi stage blower, backing up 15 SEER heat pump.... Yup, I'm a chump, but a mitigated chump....

  7. #7

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    Thanks for the replys. I will go back to the installer and see if they can modify the system for me.

    Hugh

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