Thread: Parallel water heaters question.

The Rinnai tankless gas water heater is hooked up in parallel with a conventional electric tank heater. You can see a tee above the tank in the third pic where the hot line from the tank can be shut off.
There is a pump on the hot line from the tankless heater.

I gawked at this for several mins, then moved on. So here was my client's question: "When we run the hot water, which heater is the water coming from?" "Both," I said???.

Can anyone explain why this was installed this way?

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With a tankless system that also has a recirculation pump you need to have a conventional water heater tank in the line. If you do not have it this way then you will not have the instant hot water at the fixtures.

I had a little trouble seeing what you are talking about in the picture.

kind of defeats the purpose of a tankless water heater. Unless it cannot keep up with demand, which I would assume in this case.

Originally Posted by Scott Patterson

With a tankless system that also has a recirculation pump you need to have a conventional water heater tank in the line. If you do not have it this way then you will not have the instant hot water at the fixtures.

I had a little trouble seeing what you are talking about in the picture.

Thanks, Scott. I've seen the tankless used alone, but don't recall if they used the recirc pumps in those cases.

Originally Posted by Wayne Soper

kind of defeats the purpose of a tankless water heater. Unless it cannot keep up with demand, which I would assume in this case.

Two big soaker tubs and a mega shower, maybe that's a good ass-umption.
I think there are enough valves so one heater can be turned off when demand is low.
Looking at the pics now, I see one valve on the tankless was turned off.

Originally Posted by wayne soper

kind of defeats the purpose of a tankless water heater. Unless it cannot keep up with demand, which I would assume in this case.

It's not that they can't keep up with the demand but more along the lines of having instant hot water on demand at each fixture in the home.

The tankless units heat only when the water flows through them. With a recirculating pump it does not produce enough flow on it's own to activate the heating unit in the tankless units. This is why you need to have a small 2-5 gallon electric water heater in the loop with the recirculating pump, it is only heating the water that is in the pipe. The WH tank in the picture is over kill.

Some pumps have enough flow to activate tankless heaters.

Here is a circulator pump made for tankless water heaters:

Tankless Water Heater Pump-Tankless Water Heater Pump

The cheap circulator pump at lowes worked on a 240V 80amp tankless I helped a guy install. Maybe the gas ones are more sensitive and would need the one at the link above.