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Twinning 2 80 percent Armstrong Furnaces

2396 Views 7 Replies 5 Participants Last post by  beenthere
Can twinned furnaces be set up to be 1st stage then 2nd stage if it gets too cold and share the same duct work?
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I am thinking not because the ductwork is sized for those furnaces. A co-worker seems to think they can be. Is that proper?
It would have to be sized for the two stages together. Unless your current ductwork was already oversized.

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I am thinking not because the ductwork is sized for those furnaces. A co-worker seems to think they can be. Is that proper?
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.
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it may be helpful to post the model number of the furnaces and twinning kit.

You will need to reference your installation manual. Many modern furnaces do not require a twinning kit, but a single conductor from one board to another. If that is the case, no twinning kit is needed. Just use a 2 stage thermostat with each furnace being a stage.

Good luck!
...if u do that put some type of toggle switch to alternate between which furnace fires most of the time. In the typical oversized setup one furnace may meet the entire heating demand 90%+ of the time.
They should be the same model number furnaces. So that the blowers match each other.
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