I was recently asked to look into a failing furnace. It is one of five furnaces in a large multi-zone home. Upon looking I found that the failing furnace is sharing its central return with another furnace adjacent to it.
Rather than being mounted in the furnaces, the furnace filters are stacked, two in-a-row in the shared return duct before the tee to the two furnace inputs.
This results in zero air flow resistance between the two blower inlets and 2 filter resistances between either fan and the common return.
The result is that when one blower is on, the blower that is off is spun in reverse by the suction from the on blower. When the off blower then attempts to turn on it gets stuck in reverse, even when the other is turned off.
This may all sound confusing...and believe me, it is!
My question is: Is there any practical explanation for why someone would have installed a shared return duct between two separate furnaces?
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