Thread: Single Unit Dual Zone System

I inspected a new townhouse today with dual zone duct system with a single gas furnace in the attic. There was a supply duct to each floor with a zone damper and thermostat. There was also a by-pass duct from the supply plenum to the return plenum with no damper installed. With both floors calling for heat the gas furnace was being cycled on and off at 150 degrees by the high limit setting on the zone controller. What I normally see is the gas furnace coming on and running continuously until both spaces are satisfied with a discharge temperature of 125-140 degrees.

My question is....has anyone seen a gas furnace on a dual zone system being cycled by a zone controller as normal operation? My first thoughts are that the gas furnace is to large for the house.

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Not doubting you, but I have never seen a high limit or any thermostat as part of a damper motor on a FAU. I would suspect that the limit is the furnace high limit switch that is shutting off the burner as the air is recycled through the furnace. Put in a barometric damper assembly in the bypass duct and I'll bet everything would work out fine. The only air that should be flowing through the bypass is the excess when one zone is off.
That said, the furnace could be too large for the duct system but that is beyond the scope of the standard HI.

Bottom line, the furnace was cycling on a safety switch and needs to be fixed.

This zone controller has a high limit and low limit with an adjustable setpoint. There is a discharge air sensor that provides the signal to the zone controller. The zone controller also displays the supply air temperature. The purpose in my opinion is this is a safety feature and not an operating controller.

The HVAC contractor told me that the reason they had the furnace setup to cycle using the high limit on the zone controller was because if the high limit on the furnace shut down the furnace three times the furnace would lock-out and the homeowner would call saying they had no heat.

I agree with you about the damper in the bypass.