Answer: |
It is okay to do so, however, there is a question whether it is worth efforts and money? Installing your new tankless hot water heater to the hot water line is enough to guarantee a good supply of hot water. Additional efforts might prove unnecessary. |
Why? There is a coil heater
Why? There is a coil heater you can get for most all boilers that is cheaper than a tankless and usual produces just a bit more.
But you have to fire the boiler year round.
As to preheating water for a tankless some of them DO NOT like hot inlet water above 65-70F. I know of some older units when fed hot water and can't temper their output or get their burner below 29,000 BTU they scald everyone.
If you're on a Water Tower or a well that gets cold in Winter then keeping the inlet temp at 55-65F rather than 38-45F will produce a marked benefit. There are graywater heat recovery systems with NSF and U/L listings that can do this. There are also ways of piping a unit that can recycle some hot water back into the unit with cold inlet but make sure you have a backflow preventer out there to prevent any hot water entering the house cold water under low pressure situations. The recycle loops are a bit self defeating too so look at the Graywater.
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