Maximum temperature into tankless hot water heater.

alifewithtrees

I live off grid and have a wood stove with a Hilkoil stainless steel coil in it that heats up water in an tank behind the stove. I also have a Navien Tankless (NPE-240A) installed that we use when the tank by the stove isn’t hot or doesn’t have enough hot water. We just close a valve by the tank to switch the using the Navien. When the valve is open, the Navien doesn’t kick on.

Here is a basic schematic of the current configuration.



My question is, could I just plumb the hot from the tank to the cold infeed for the Navien? Or would doing so cause problems for the Navien? I could also just install a mixing valve before the Navien so tank would be a like tempering tank to limit the amount of temperature rise the Navien needs to provide. Is there a maximum recommended temperature into a tankless hot water heater from a tempering tank?

The other idea that I’ve had is to make a separate loop from the Navien to the top of the tank and have a return line from the bottom of the tank feed the Navien into the circulation port. This way, the tank would be heated by the wood stove but the Navien would help keep the entire tank a consistent temperature and the Navien would only run when the temperature in the tank is low. The disadvantage of this is that the circulation pump would need to run which would consume more power than I would like since we are off grid.

Also, if the suggestion is (if it ain’t broke don’t fix it) I’m open to that feedback. Our current system works and we spending less than $300 a year on propane. My goal is to consume as little propane as possible and to make the system as reliable and functional as possible.

Thanks for any suggestions

Ryan

alifewithtrees

I just spotted an error in what I wrote.   When the valve by the tank is closed the Navien is on.  When it is open the water from the tank is what gets used.   

hot_rod

Do you want an automatic switch over?
Two thermostatic valves could do that

This allows you to use solar down to the lowest temperature you adjust, say 110, then flow goes through the tankless. 

A third thermostatic is sometimes use to protect if the solar gets really hot, above 140 for example

Some tankless can be put in series with a 
solar tanks,  solar feeds directly go the tankless.
Takagi used to have a special model for solar applications, check with the manufacturer if yours is solar compatible