Shared Return

[Jim Robinson]

Today's house had two pretty new Carrier single speed furnaces. Both furnaces shared a 20" x 30" return duct and filter. One furnace was heating the upper level of the house, and the other was heating the lower level. They are controlled by separate thermostats. I have not come across this before. We talked about the consequences of not having enough return air, and the possibility of blowing air through the other furnace if the filter got clogged. What other issues could come up from this?

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[Markus Keller]

Without pics I can't say for sure. However based on your description they could be run in tandem and they could have zone dampers. If that isn't the case then that is a problem.
Of course return air will come from the path of least resistance meaning that if this is one continuous return trunkline the units may pull plenty of air from the lower level but not much at all from the upper level. During heat mode this tends to lead to air stratification since there won't be much of a circular air path. During cooling mode it tends to be a bigger issue. The upper level could stay abnormally warmer since you aren't sucking any of the hot air out.
I would also prefer to see a separate filter rack at each furnace rather than one filter for the return line as a whole.

[Jim Robinson]

Not in tandem and no cooling, heat only. Pretty much the way we thought about it at the inspection today. My biggest concern would be that when it was extremely cold the system may not heat the whole house very well.

[Jim Luttrall]

Today's house had two pretty new Carrier single speed furnaces. Both furnaces shared a 20" x 30" return duct and filter. One furnace was heating the upper level of the house, and the other was heating the lower level. They are controlled by separate thermostats. I have not come across this before. We talked about the consequences of not having enough return air, and the possibility of blowing air through the other furnace if the filter got clogged. What other issues could come up from this?

From what you describe it is a problem. First the filter is likely undersized when both furnaces are running then you will get reverse cycling of air through the other furnace when only one is running.
The big problem is that air reversing from the other furnace will bypass the filter and will be pulling air from the other zone causing improper temperature control.

Assuming the filter and return ducts are large enough, the units might be wired to run simultaneously with one thermostat or zone dampers if return/filter placement correction is difficult.