Existing Domestic Water Heater as Buffer Tank for new Heat Pump Radiant System

Dipsea

I have an existing small radiant floor heating system covering about 400 square feet. I tested it using 40 gallon natural gas water heater and that worked very well. We're in the process of converting as much as possible to electricity (I installed a solar array a few year ago). I have since replaced the gas water heater with a new Rheem heat pump water heater which does an excellent job efficiently supplying domestic hot water, but struggles to run the radiant heat.

My question is can I attach the radiant system to the existing water heater per the diagram above in order to use the water heater as a buffer tank for the radiant heat?

Diagram above based on a drawing from blog.heatspring.com.

hot_rod

what will heat the buffer tank. I doubt the HPWH can handle dhw and radiant?

WMno57

You should not use the same water heater for DHW and hydronic heating. It is a cross connection between two different systems. One intended for human consumption, the other not. Code violation and risk to your health.
Also, water heaters do not make good boilers. Yes, they both heat water, but DHW appliances are made for high temp rise (Delta T), low volume. Boilers are are made for low temp rise (Delta T), high volume.

Hot_water_fan

I don’t see a compelling reason to connect DHW and central heating here. As you’ve seen, a HPWH can’t run a radiant floor well because it takes heat from inside the room to heat water.

if you go the air to water heat pump route, I’d install a separate buffer tank.

I’d also consider not installing the heat pump at all for a room that size. You could use an electric boiler and pay more for energy but a whole lot less for install. 

Dipsea

Thank you all for the input. It makes sense not to mix the systems for the reasons explained - the DHW system works well on its own anyway, (BTW it draws and exhausts air from and to outside). I was thinking of incorporating it just to avoid adding a buffer tank.

What if I were to use the old 40 gallon natural gas water heater as a buffer tank plumbed similarly to the way the HPWH is plumbed in the diagram? (Configuration explained in this article: https://blog.heatspring.com/2-pipe-versus-4-pipe-buffer-tank-configurations/). Alternatively, if its size is not right I could purchase a buffer tank.

I realize the radiant load is small, but I hope to expand it via a heat exchanger to also heat an existing 300 gallon hot tub. Cost is a consideration, (partially mitigated by federal tax credits), but my primary goal is to move away from natural gas and to try to keep annual electricity consumption under the solar array’s annual output.

hot_rod

You can find plate heat exchangers online fairly inexpensively. So a HX, pump and expansion tank could give you separation between potable and hydronics. Maybe 300- 400 bucks worth of parts.

Leave the gas tank connected in case you need the extra output.