Tankless backup with tanked heat pump primary

Jellis

Good afternoon, i am looking to install a tankless Rinnai as a backup to a 50gal heat pump electric. want the Rinnai to activate when tank runs out of hot water.

Rinnai sent me the attached piping diagram as recomended piping, but im not sure how the mixing valve off from the tank is supposed to work, i get removing the check valves but which port goes where.

i asked Rinnai tech support and they said "install it as the drawing shows"

what is the principle of operation here?

WH1-S-2.pdf

psb75

Something doesn't seem right with Mixing valve #1. I've never seen one plumbed backwards before. I don't believe it would work that way. Does Rinnai know something we don't?

hot_rod

It's actually a copy of a product 263, that Caleffi builds, this explains it better. It's usually done with 3 valves to protect a hot solar tank from going to the tankless. Then a final mix/protection valve.

https://www.caleffi.com/sites/default/files/media/external-file/01164_EN.pdf

It can be built with standard off the off the shelf mix valves, like this.

The yellow knob valves are Caleffi solar mix valves, a bit higher temperature rating.

Going through three mix valves expect some pressure drop. Depending on the gpm you want to push.

psb75

Kudos to hot_rod and Caleffi. But Rinnai needs to "up their game" with the plumbing schematic with the Caleffi product numbers. That is quite a "chunk of Caleffi hardware" that is not referenced by Rinnai. The schematic also doesn't allow for the OP's electric heat pump water heater component. The OP makes no mention of a solar component in their query. Interesting ideas. So…OP its up to you to pick and choose. These are the "goods"—you put 'em together. Rinnai tech support may not be "up" for this one.

hot_rod

any 3 way thermostatic valve can be used in these assemblies. We built some prototypes back in the last solar thermal heydays for the tankless manufacturers. The project never went anywhere.

The 282 and 283 as well as several motorized 3 way versions are still a European product offering

Not all tankless would work properly with warm feed water however. I think Takagi finally offered a solar friendly model towards the end of the era. Maybe all of them now are able to ramp properly with warm supply? Check with the manufacturer.

In 2018 a handful of tankless manufacturers developed a ASSE 1082 standard. Basically they were able to prove or convince ASSE that they could control outlet temperature as accurate as a mixing valve. Eliminating the need for a mix valve in the states where they are required on all hot water devices.

So in these mix valve assemblies, it is possible the final mix valve could be eliminated if you can find a tankless that has that ASSE listing attached.

Probably the electronics and control on all tankless now, can regulate accurately enough regardless of that listing. Tankless seem to fail cold moreso than hot as they start to scale. So you could start with just the two valves

So just substitute HPWH for solar in all these examples. The mix valve is gender neutral😉

Kaos

The condensing units I've worked with (Rinnai included) had no issues with hot water at the intake, not sure you need the mix valve from the tank. You can plumb the water from the HPWH straight to the tankless, set the tankless temp a bit bellow what the water heater puts out. The tankless will automatically fire when the HPWH runs out and water temp drops.

If you are only looking for a bit more capacity occasionally, a cheaper/simper option is to put a regular electric tank after the HPWH. This doesn't have to be anything powerful, a 20gal 120V tank or a budget 40gal tank will do as the element there would mostly run to make up for standby losses. A regular tank is simpler install and lower maintaince than a tankless.