Water Heater Expansion Tank Question

[atxjmy]

I have two water heaters installed in parallel (both roughly 2 years old), one serves the first floor, the other serves the second floor. The house has one cut off obviously, but at the cut off valve there are two cold water supply lines, I assume one for each circuit (i may be using this term incorrectly). Both had expansion tanks installed at where the the supply line met the tank inputs. Long story short on those, tank fittings developed some leaks (likely because galvanized pipe were used and they had begun to corrode). I wasn't super thrilled with the previous plumber for other reasons, so I found a highly rated, licensed master plumber to come out and perform the fix.

While performing an otherwise excellent fix - the plumber mentioned that I only needed one expansion tank per my city's (major city) code. And i questioned this, and pointed out my units are in parallel not series - twice! I even asked how a water heater own its own water circuit will take advantage of the expansion capacity of a water heater on another circuit. He told me that was a great question, but that's what the city inspectors have repeatedly told him and he didn't want to charge me for something that isn't required.

Now here's my question - city codes aside - why wouldn't a water heater on installed in parallel require its own tank. The expansion tank does not appears to be installed on a common supply line, so i just don't see how my non-tanked water heater is going to use that expansion capacity.

 

[dave8647]

I am not a plumber but I think I understand the concepts well enough to help. Your system appears to be independent heaters, not parallel, since each heater serves a separate zone. But it doesn't really matter if your configuration is independent, parallel or series. As long as the heaters are piped such that pressure buildup anywhere will cause pressure buildup everywhere, then you are free to place a single expansion tank anywhere in the cold water piping that serves your configuration.

So if heater A is off but heater B is heating, and thus causing thermal expansion, pressure will build up everywhere. So even if a thermal expansion tank is physically near heater A, it will still absorb this expansion and keep the pressure from building up throughout the system.

Of course, the single expansion tank needs to be sized based on the total capacity of the heaters in case they are both heating at the same time.