Dual Water Heaters

I came across this configuration at an inspection that I have not seen before. Can anyone shed light on its purpose. See all 3 pics below.

Water heater on left serves an in-floor heat system, water heater on right serves the home.

The inlet of the house water heater (green arrow) is branched, goes through a backflow preventer, and what I believe is a pressure regulator (red arrows). The backflow preventer is vented to the floor (blue arrow).

After the pressure regulator, the branch is connected to the TPR valve on the water heater serving the in-floor heat (red arrow). What is the idea behind this configuration?

Can you be more specific with your question. You have already identified that the water heater on the left is for a radiant heat system.

I guess I really wasn’t clear. Why is the supply of the main water heater tied to the TPR valve of the 2nd water heater?

Looks like they are using it as thermal expansion device. I do not think anything can be added to the TPR or its discharge.

Because I see a shut off valve along with a pressure regulator I think that pipe may be the make up water for the system. Is that water heater manufactured and approved to be used as a component in a hydronic heating system? It may be best to just ask a local plumber to confirm the installation methods are acceptable. It’s difficult to see what’s going on with the limited photographs.


So if the secondary tank overheats, the pressure/steam/hot water will travel to the pressure regulator, then to the backflow preventer. Not really sure why that would be desirable.

That is probably correct now that you mention it. I was thinking because the secondary water heater had a pipe connected to the inlet, it wouldn’t need make up water. But that pipe connected to the inlet isn’t actually supply water, it is return water. Now it makes sense. Thanks!

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Sorry for the piss-poor pictures. The furnace and both water heaters were in a closet.

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From the manufacturer…
I see where installation steps were missed now.

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Both directions and same distance! I have failed two water heater replacement with a low pressure. nice photos btw

A little late to the game, but I found this.

“Serve a single relief device and shall not connect to piping serving any other relief device or equipment”