HPWH as hydronic supplement for A2AHP

A homeowner in a cold climate (zone 7b) posted a question on a building science forum about a homemade fix for a common complaint: their air-to-air heat pump (a system that pulls heat from outside air and blows it into the house) struggles on the coldest nights, forcing backup electric heat strips to run for hours, which drives up bills.

The idea: buy a used heat pump water heater (a water heater that extracts heat from the air around it, rather than using an electric element) for under $500, and put it in a carport shed that gets strong sun exposure. During the day, the shed's warm air would help the water heater heat water efficiently. That hot water would be stored in spare tanks, then pumped through a fan coil (a small radiator-and-fan unit) at night or in early morning to add extra warmth when the main heat pump can't keep up. The poster estimated the shed roof could collect more than 40 kWh of solar heat per day in winter, potentially banking a meaningful amount of heat this way, though they noted the water heater's compressor size might limit how much heat it can actually store each day.

This is a homeowner's personal DIY concept posted for feedback, not an established product or program. No pricing, rebate, or installation guidance came with it. If your own heat pump also struggles on very cold days, this illustrates one way people are experimenting with combining a water heater and stored hot water as backup heat, though it remains an untested, individual project rather than a proven or supported approach.