Combined Return Ducts

Garet Denise

What are the implications of combining return air ducts from 2 forced air furnace systems in the same house?

The issue is a new house (near the end of construction) that has 2 furnaces (independent thermostats) serving different parts of the house. The return air system is fairly well distributed throughout the two areas, but combine into a single trunk duct until just before the furnaces.

It seems like this will cause some odd air flows through the living space, no?

Garet Denise

Garet Denise

Combined Return Ducts

What are the implications of combining return air ducts from 2 forced air furnace systems in the same house?

The issue is a new house (near the end of construction) that has 2 furnaces (independent thermostats) serving different parts of the house. The return air system is fairly well distributed throughout the two areas, but combine into a single trunk duct until just before the furnaces.

It seems like this will cause some odd air flows through the living space, no?

Garet Denise

Brad White_9

If

The duct approaches are symmetrical and the spaces are kept at the same temperature and the same size and it operates as one large zone, I do not see an issue. But then, why have two zones?

(I have manifolded air handlers both on supply and return into loop systems to cover two stage heating in an open plan house; worked great using fabric backdraft dampers to close off the unused air handler.)

The downside is, if you keep the two zones at different temperatures appreciably apart, you will be pulling cooler air back to the furnace which increases the load to the operating one. This in turn pressurizes the "on" space and forces warmer air toward the dormant zone, much as you surmised. If the house is "tight" and zones separated, this might defeat the notion of zoning in my opinion.