Tankless backup for a heat-pump water heater
I currently have a GE Geospring 50 gal heat pump water heater (now discontinued) that has worked pretty well. However, my toddler will be using more hot water as she ages and we’re finishing the basement to rent out, most likely to one person. Lately, the water heater has been maxing out after two showers (2.5 GPM x 10 min x 2 people=50 gal). I’ll be retrofitting a 1.5 gpm adapter which will help, but I’m still nervous about having a tenant and running out of hot water.
To add capacity, I’m considering both a solar water heater and a tankless unit. My main question is: Would it work to put the tankless electric heater in a series in front of the heat pump tank so that the tankless kicks on only when the tank starts dropping in temp? I want to use the heat pump tank most of the time since it’s efficient. I’m not interested in replacing it with a larger unit since the cost would be very high and I can’t get another rebate.
Are there tankless units designed for this or that may be better for this use? Any tips on sizing the tankless unit on a backup basis for 4 users?
I’m in Seattle, zone 4a, ~52F incoming water temp.