Tandem water heaters in series - 1 or 2 expansion tanks?

[Lars Callary]

Hi-

I'm a homeowner who helped a neighbor who had their 'second leg' (closest to house hot inlet) expansion tank fail last night and I went over to help pull baseboards and mitigate damage.

I then inspected my own today (we moved a few months ago - house in built in 2017) only to find a bulging pinhole leak! I have the hot off and was on my way to replace *both* tanks, but upon speaking to other neighbors have heard these fail frequently - and it's always on the side where the 'cold' inlet bridged over from the first water heater in series is - leading me to some googling on here and many of you saying do NOT install expansion tanks on the hot side.

Please see attached diagram- am I correct in thinking perhaps I should - out of an abundance of caution replace the right (true city cold water) tank so we know this is good- and then plug the left heater tank so that there isn't an expansion tank on the 'hot' (cold) side?

I'm a homeowner DIYer but with a dozen houses on the street and people saying this side has failed frequently for them I'm wondering if the original plumbing wasn't ideal in setup.

Thank you!

 

[Reach4]

One bigger tanks or two smaller tanks are both options. Having two is not causing failures.

Bulging or leaking expansion tanks sounds like a really bad batch of expansion tanks. That can't be a common failure mode. I think the common mode of failure is the diaphragm, and that doesn't cause an external leak.

Out of curiosity, check the date codes on the failed tanks. They might have been built on the same shift.

 

[Sponsor]

 

[John Gayewski]

Why do you have an expansion tank? I don't see a prv or backflow preventer.

 

[Reach4]

Went do tot have an expansion tank? I don't see a prv or backflow preventer.

Presumption is that the city puts in a check valve at the meter.

 

[John Gayewski]

Presumption is that the city puts in a check valve at the meter.

Meter buried somewhere?

 

[John Gayewski]

Either way the expansion tank/tanks should be before the hesters. The expansion tank needs to be sized to handle thr volume of water in the system in gallons x max temp rise × .00023

That'll give you the volume of expanded water.

 

[WorthFlorida]

Expansion tanks need to have its pressure checked every six months or so. Water does not touch the steel tank, only air. The water is inside the bladder. When air leaks from the tank the bladder can expand to a point of bursting. Then water is against the steel walls and its expansion properties go south on you and the steel starts to corrode. It is possible that when cold water enters the bladder, inside the tank moisture from the compressed air may condense into enough water to start corrosion. Inside the steel tank should have some kind of coating to minimize rust.

Standard electric water heaters are almost maintenance free, gas units have maintenance, all water heaters with expansion tanks is a maintenance item.

 

[Jeff H Young]

personal opinion. The quick easy fix is a good one in my opinion. Simply replace both tanks nothing wrong with that and its easy , If you want to rework your entire exposed piping we can do that as well at more work and cost . I'd check water pressure and if your pressure is good forget it after dumping your junky tanks and replacing

 

[Lars Callary]

Thank you all - I thought that expansion tanks really wanted cold water in them. In order to try and prolong lifespan this was my fix - replace (2) 2 gallon with (1) 4 gallon, and plus the hot 'cold' side tank. Point re: bad batch is well taken- this has a 5 year warranty on it - I wrote with a sharpie on it to remind me to change it in (3) and I'll continue to throw out in 2-3 year intervals since it's such a simple fix. Ran this by my plumber and his comment was either (2) 2's or (1) 4 gallon would work the same. (Hopefully, y'all agree?)

 

[Reach4]

Agree.
I might put a block under the elbow to add a little extra support.

 

[Lars Callary]

Agree.
I might put a block under the elbow to add a little extra support.

Great point I’ll figure out support there. Thank you!

 

[John Gayewski]

Thank you all - I thought that expansion tanks really wanted cold water in them. In order to try and prolong lifespan this was my fix - replace (2) 2 gallon with (1) 4 gallon, and plus the hot 'cold' side tank. Point re: bad batch is well taken- this has a 5 year warranty on it - I wrote with a sharpie on it to remind me to change it in (3) and I'll continue to throw out in 2-3 year intervals since it's such a simple fix. Ran this by my plumber and his comment was either (2) 2's or (1) 4 gallon would work the same. (Hopefully, y'all agree?)
View attachment 78702

That position will have hot water also. But it should be fine. If it goes bad early move it farther away from the heater.

 

[Jeff H Young]

Great point I’ll figure out support there. Thank you!

If tank is sized big enough only need one , I just figured easy to screw on 2