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twinning furnace help

I'd like to use 2 gas furnaces as a 100% makeup air unit. They will sit on a common return air box drawing outside air. They will share a common large supply duct. I would like to be able to run 1 or both to get the required CFM as conditions dictate. I'm worried about short cycling the airflow if only 1 is running. Any simple ductwork solutions? Thanks


  • Ironman
    Ironman Member Posts: 7,114
    A Couple of Things...

    First, you cannot use 100% makeup on a gas furnace unless it's rated for it and standard residential furnaces are not. Most I&O manuals will specify that the return air be at least 55*. Any thing less and you've condensation, flue and heat exchanger issues.

    Second, when furnaces are twinned, both blowers must run at the same time or else the air flow will short circuit through the furnace blower that's not running instead of getting through the duct work. You can only cycle the burners separately.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Tim McElwain
    Tim McElwain Member Posts: 4,580
    Bob is correct

    if you are looking to do make up air you must use dedicated make up air units. If however you are pre-heating the air then it can run through the twinned units if it is 60 degrees or higher on the return. The units can also be staged furnace one low fire two added for high fire but the fans must be synchronized and also set the same on the EFT or Integrated Furnace fan on fan off dip switches.
  • Empire_2
    Empire_2 Member Posts: 2,343

    Are you using this application you speak of for a restaurant as a means of new MUA?  Make up air?
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