Good point. Here are my guesses on why I don't want only an hpwh:
1. The tank has to be insulated well, especially if the temperature is set high. But I can set the temp lower if I have the tankless ready to do the heating for a shower or other long high temp draws. (Some technical issues here in terms of how the units work together--might kill the plan.)
2. I don't want to invoke the inefficient electric backup should the heat pump not be able to keep up to demand for a lot of (very) hot water. I want to shut that capability off entirely. My backup will be the tankless water heater.
3. There are times during the winter months when I would use the hpwh in my basement at very low temps, if at all. I would rely on the tankless during those months because I don't want to take heat out of my basement during those cold months.
Thanks for any help on this.
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