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I am getting a PRV installed due to high pressure coming into my house. I have two 50 gallon hot water heaters that are right next to each other and connected. The plumbing supply store has a 4 gallon, or a 20 gallon expansion tank, and they told me the 4 gallon expansion tank is for an 80 gallon hot water heater, but they think that it will be just fine for my situation. The difference in price is astronomical between those two tanks, and the local Lowes and Home Depot carry a 4.4 gallon tank. My choices are limited, and I'm sure any hired plumber in this area would slap a 4 gallon tank on it and be done with it, but I'd like to know if this is going to be alright for my home. Any information or help is greatly appreciated.
 

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Why in the world do you have 2, tanks connected in the first place?
 

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I don't know, it was like this in the home when we moved in. I assume they just wanted to have plenty of hot water for their home. I have seen it in quite a few homes in this area.
 

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Unless this is a 10 bedroom house or a hotel it makes no since to me.
The few that I have seen were dumping hot water into the other heater which does nothing but double your electric bill.
 

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Two water heaters are quite common around here. They can piped in series or with a manifold but thats another argument altogether.
One expansion tank should do ya, if your still concerned- pop in two :laughing:
 

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Given that a 4 gallon expansion tank is suitable for an 80 gallon water heater ...

My gut feeling is that, at household hot water temperatures, one 4 gallon expansion tank would be fine for the two 50 gallon heaters.

(For commercial, about 180 degree, hot water usage, two 4 gallon expansion tanks may be appropriate, if that would cost less than one 20 gallon expansion tank.)

(Did not get that from the Amtrol link, there got as far as "Recommended tank ST-8" and got lost. Or rather came to a fork with too many branches and gave up and posted my gut feelings instead.)

An expansion tank should be supported by more than the pipe to which it is attached. In normal operation, some water will enter the tank and the weight would put stress on the pipe.
 
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