Using 40gal House Water Heater w/ point of use 12KW for TUB
I am remodeling my bathroom and purchased a 90gal-97gal Jacuzzi air tub.
My 40 gal hot water heater for the house didn't do the job for our old tub (set at 130 degrees) so during remodeling we ran a 6/3 and 240V 50AMP service for a Bosch US12 - 12kW point of use heater to soley be used on the tub.
The idea...was to run the hot to the Bosch and when the hot water ran out or began to drop, the Bosch would take over.
Receiving the heater the instructions state cold water only (max 80 degrees). Speaking to Bosch they say it will work about 2-2.5 gpm if we mix the hot/cold to feed it with 80 degrees, but that will be about 36 minutes to fill.
What I really need is a valve that will divert my hot water from the 40 gal house to the Bosch when the temperature falls. The vendor suggested a manual valve.
This is a luxury spa like bath area and I cannot have some exposed valve like this and I don't want to increase my home unit because 40 gal in FL is fine for the house, the TUB is the problem.
Any suggestions on what to do? Still in rough-in stages, can do ANYTHING!