Water heater expansion tank question

I did an inspection on a new home that had two water heaters. One water heater had an expansion tank and one did not. They were not run in tandem. Am I missing something or should both have expansion tanks?

Is this single or multi-family dwelling? If they are connected to the same DHW system, then a single, correctly sized, expansion tank is all that is needed. Most likely they are connected to the same piping system behind the wall, where you cannot see the pipe.


Exactly…Simon has it! ^^^^^

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It is a single family residence. I’ll often see the water heater lines connected together at the water heaters so that was the reason for the question. Thanks a lot.

The way this was explained to me was that the need (or not) for these tanks depends on how the municipality sets up the water meter. They used to be set to allow expansion back into the city supply system but for sanitation reasons they no longer are (thus the need for the tanks). With that in mind it makes sense one tank anywhere on system is sufficient since it used to be all the way back at the meter.

Good question though… got me me thinking.


That’s pretty much the way I understand it as well. With the addition of check valves in the system to prevent back contamination, the expansion tanks are now installed to relieve excessive pressure from the supply pipes.

One tank per system is the norm in my area.


Prior to my recent water heater replacement (last year or year before) my water heater had no expansion tank and many I see locally do not. Any new tank will now have an expansion tank because the code changed. The same could be true in your situation (although they both look about the same - and the @srechkin explanation is probably correct) installation date could explain different code use.


My concerns have to do with the CPVC plastic lines for the OTP safety valve. CPVC ONLY if approved by the manufacturer AND there are too many direction changes in the lines. Plus, it does not appear that the ends of the line are open so it can be observed if the OTP is venting.