We are starting to roll out heat pump water heaters in the Pacific Northwest (Oregon, Washington, Idaho, Montana, and Wyoming). One of the concerns we have in colder climates is the recovery time when the ambient air is cold. The lower the ambient temperature the less effective the heat pump water heater will be in generating hot water causing issues in households with more people, more bathrooms, and in colder months if the heat pump water heater is installed in garages or basements.
Thinking about this specific issues and discussing it with home performance contractors we have decided to explore the possibility to employ a two tank heat pump water heater system:
The first water heater in the two tank system acts as a pre-heater (this would be a solar thermal heater or in our application a heat pump water heater).
The second water heater collects the water from the first tank and boosts it if necessary to the 120-135 degrees Fahrenheit. Most of the time the booster tank is a standard electric water heater. The two tank system adds a back-up, a booster, and more thermal capacity opening up hybrid electric water heaters to larger households.
We have not seen any issues so far with this solution. Any input, thoughts, experiences that you have with two tank systems? Please share.