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Gas furnace: Twinning vs. 2 independent furnaces

ClaireL Member Posts: 1
edited October 2021 in Gas Heating
Hi, I own an 8-apartment converted Victorian house that is approx. 6,000 square feet, spread out over 3 floors. My two oil tanks are ancient and need replacing and I am using this as an opportunity to convert my old oil furnace to gas. The entire building is run off 1 thermostat (in the LR of Apt. 2 which is the left side of the house) and I have played around with dampers and such to give each apartment/room sufficient heat (but not too much). My old oil furnace is 230,000 BTUs but I have been told I only need 150,000 BTUs with a new gas furnace. I have two main trunks coming off the furnace, each one heating apartments either on the left or right side of the house. I have returns coming back to the furnace, but only from one large apartment on the left side and one large apartment on the right side. My question is--because I have 2 separate trunks of existing duct work, should I run 2 independent gas furnaces or 2 twinned gas furnaces? I have read that twinning lowers the efficiency to 85%. I realize that if I put in 2 independent furnaces I would need to put a thermostat in one of the apartments on the trunk that leads to the right side of the house. I also realize that my current duct work is hugely oversized and it needs to be smaller. As an amusing note, I had an HVAC guy quote split systems for 5 of the apartments and the cost was $$,$$$.

So, to twin or not to twin, that is the question.

Thanks, Claire


    EBEBRATT-Ed Member Posts: 13,457
    The best job would be two independent furnaces each with it's own thermostat.

    Next would be one furnace (preferably a furnace with a two stage gas valve) and 2 motorized duct dampers with a damper on each supply duct (1 FFloor) & (1 2d Floor). With individual thermostats

    Twinning two furnace would be the same as the 2d option above.

    Weather you twin or use a 2 stage furnace depends on what size range you fall into
  • Ironman
    Ironman Member Posts: 7,119
    Many of the newer furnaces with variable speed ECMs cannot be twinned.

    Twinning is usually utilized when one large residential furnace is not sufficient to provide enough heat and it’s cheaper to install two residential than one commercial furnace.

    I’d separate it into two systems, but beware: ECMs cannot share a common return duct.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.