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Rick, Construction Supervisor
Category: Plumbing
Satisfied Customers: 21815
Experience:  Licensed Master plumber with 40+ yrs. experience.
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I am a geothermal heat pump contractor. Most of our

Customer Question

I am a geothermal heat pump contractor. Most of our installations, as you are probably aware, include a device called a desuperheater or hot water generator that allows the geo heat pump to provide free hot water but this process requires a storage tank.
I have a customer that wants the benefits of both the desuperheater and Rinnai tankless hot water heater. To do this I'm thinking cold water enters the tank where it's heated by the desuperheater then the hot water from the tank goes to the Rinnai tankless where it serves as a backup in case the hot water in the tank is depleted. I'm asking if this is safe? Will the tankless sense hot water and not come on or will it ADD to the entering 130 degree hot water creating a dangerous situation? Thanks
Submitted: 1 year ago.
Category: Plumbing
Expert:  Rick replied 1 year ago.

Welcome to Just Answer, my name is ***** ***** I will do my best to help you with your issue. If my initial response doesn’t answer your question then let me know and we can continue our conversation.

130 degree water is the max inlet water temp for a Rinnai so you risk damaging the Rinnai with water that hot. The water won't get hotter since the Rinnai burners won't fire when the water temp is above the set point. Off hand this is the case (inlet water temp etc) with all tankless heaters. There is a workaround if you're not turned off by the complexity. You could put a strap on aquastat on the line out of your tank and have 2 solenoids down stream. If water temp drops below say 120 the aquastat would open the valve to the Rinnai. Above 120 and it sends it directly to the house.