Is an air handler compatible with gas backup?
If a heat pump/air handler system has gas as a backup for winter sub-zero temperature periods, does that necessitate replacing the air handler with an evaporator coil?
My son has recently purchased an above-energy code house in central MA (zone 5A), built in 2008. It has the original high efficiency (natural) gas furnace coupled with a Carrier AC unit that needs $1600 repair. The house is centrally ducted and the conditioned space (including the attic where the HVAC is) is approx. 1380 sq. ft. on two floors.
He would like to replace the AC unit with a heat pump so he can heat mostly with electric. For now if possible, he’d prefer to keep the gas furnace for emergency backup as opposed to an electric coil (proposals have ranged from 7 – 10 kW), as the electric requires up to a 50 amp circuit. The house has 18 PV panels on the roof. They are grid-tied. Its electric service is 150 A.
A Bosch-based system has been proposed by several contractors. Two versions, usually. One where the gas furnace remains, and the other where the gas comes out, and electric backup is installed.
As shown in the attached NEEP charts, the performance of the system with gas backup doesn’t compete with the system using electric backup. The only difference is the evaporator coil in the first, and the air handler in the second.
My uneducated guess (I’m a musician, not an energy/HVAC specialist) is that since the furnace would have its own blower, it could possibly interfere with the operation of the air handler/heat pump on those very cold (emergency) days. If that is the problem, are there ways to have an air handler work in tandem (simultaneously) with both the heat pump and the furnace?
I would appreciate any thoughts, counsel, and input anyone cares to offer.
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