Twinning 2 80 percent Armstrong Furnaces

LeftHandLuke said:

I am thinking not because the ductwork is sized for those furnaces. A co-worker seems to think they can be. Is that proper?

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You have to use a twinning kit that supports it that's recommended by the furnace manufacturer.

Use two-stage t-stat too.

The way it's done is that both blowers run when there's a call for first stage only. The air temperature out of the vents is at lot lower but volume remains constant. It has to be done that way, otherwise warm supply air from the furnace operating would short circuit through the furnace that's off, back into the return. The control alternates between furnaces so one doesn't wear out before the other.

Uses more electric this way but also provides more even heating, longer cycles for the furnaces, longer lifespan, possibly lower gas bills due to longer heating cycles. The 80% furnaces are really a little under 80 in the real world because of cycling losses.


------------not applicable to twinning but FIY----------
Duct sizing isn't that critical when it comes to staging single furnaces unless you have a system with long runs or the furnace is off to one side of the building. There are modulating furnaces that run at 40% capacity and airflow most of the time; two-stage units running at 66% capacity pretty much all the time. Have a long, narrow townhouse with the furnace at the back, bonus rooms and oversized ducts will become an issue on low fire.