Solar preheat for Rinnai Tankless

jnewkirk

Greetings from the Rocky Mountains, where the groundwater is cold and snowfall is above average ;-)

I recently installed a Rinnai tankless water heater for domestic hot water that's been working out very well. This replaced an old Caleffi sidearm tank which had one coil connected to the boiler and another to rooftop solar collectors. The solar collectors did a great job preheating the water for the boiler (and no doubt saved countless BTUs of propane over the years). I'd now like to use those panels to preheat DHW for the Rinnai; a tankful of solar-preheated water standing by would not only save BTUs, but also allow higher DHW gallons per minute.

The attached diagram captures my general idea, but I'm not sure how to plumb the system in light of Rinnai's internal DHW recirculation pump. The building has a DHW recirc loop (see photo), but I could see a scenario where the Rinnai itself might try to preheat the 40-gallon tank. Any suggestions on how to keep the recirc loop running without it passing thru the tank? I assume check valves would be involved at strategic locations.

Any advice appreciated - and thanks in advance for all replies.

hot_rod

I don't see a recirc loop on your drawing?

Here is what a Rinnai rep told me about sending warm water to them.

Larry Weingarten

Hi, You're asking for a difficult combination. Gas tankless heaters don't like warmed incoming water. They just don't have the turn down ratio needed. Electric tankless could work, but that's not the equipment you have. Both solar thermal and tankless work most efficiently with colder feed water. Recirculation is best done through a tank so it doesn't have to overcome the high head losses going through a tankless heater. An equipment intensive fix would be to use the tankless to heat the upper part of a storage tank, and use the solar collectors to heat that tank from the bottom. This would automatically give solar priority and the tankless would fire when the upper part of the tank got too cool.

Yours, Larry

hot_rod

Another option is to use two 3 way thermostatic valves as diverter valves.

So when the solar tank is warm enough, say 120 or hotter flow goes through it.
As that tank temperature drops, flow is diverted to the tankless.

Some versions use a 3rd valve in case the solar tank gets extremely hot. A final mix valve as you show in the drawing.


Caleffi builds a solar assembly in one block, but not available in the U.S. Build your own with two off the shelf valves.

A recirc adds some complexity as Larry mentioned due to the tankless pressure drop.
More of the new tankless haves all buffer tanks built in to eliminate the recirc issues. And cold sandwich effect.