Why not just use a natural gas or electric heating element to heat up the air coming into the heat exchanger when it's below the operating window? In most of the US below zero is a <10 day per year issue. Sure it's terribly inefficient on those days, but I'd bet it's cheaper than maintaining an entire secondary heating system.
The US grid isn't capable of handling everyone running on resistive heating because traditionally many Americans have used fossil fuels or wood for heating. The US grid needs to be improved for this reason and also for EVs.
