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Dual furnaces


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I'd like to get some opinions on a set up that I've never seen before. There are two furnaces, set side by side and both controlled by the same thermostat.

Here's a little background. The original section of the home, which is about 45 years old was originally about 2700sf or so with a single HVAC system. About 25 years ago an addition was added, bringing the the square feet of the building to 3500, but they made no alterations to the HVAC system.

The seller, who bought the building about 17 years ago, realized that the single heating system wasn't adequate for the building and instead of zoning off the building, which would have been extremely difficult, decided to over-size the ductwork (from what was visible) and installed another furnace. Now when there's a call for heat, both furnaces fire up and deliver heat to the building through a single set of ductwork.

Has anyone ever seen this before?

Sorry for the long-winded description.


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Yes, I have seen this but usually installed with an outdoor sensor that only allows the second furnace to kick in when the temp is too low for the first furnace to handle the load. If the furnaces are identical in size the sequence can be changed from year to year to even out the stress on the primary furnace. Much more common in hydronic heat but stage fired systems have been around for many many years. As to the ductwork and the quality of the installation, that's your call.

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It's called Twinning Furnaces. Attached is a York PDF.

Proper a/c is typically more problematic than heating.

Download Attachment: icon_adobe.gif York Twining.pdf


Thanks for your replies. Terry, this is exactly what they did. Both furnaces are the same make, model, size and age, shared a common return duct and was set it up as a single-wire operation. They both fired up and shut down in unison. As far as ductwork sizing, almost none of it was visible. Just the plenum and about 5ft. of the trunk lines. Thanks again

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