Expansion Tank On A Hot Water Heater

Yesterday, I was at a home where the seller was boasting about his hot water tank replacement that he did himself. There were no inspection stickers on the tank, so I gather there was no permit pulled. He installed a expansion tank on the hot water pipe about a foot up from the tank. Now with a hydronic heat conveying medium, the expansion tank will obtain the extra water created when the water is heated and it expands. The diafram divider in the tank can also help increase water pressure in the system which I have read in the HVAC course here on Nachi. But what about the water tank?? Does this expansion tank help, hurt or no change in the system??. Your thoughts, Thanks Rick

The diafram type does not help increase the pressure. The closed steel tank does. I stand corrected.
Closed Steel Tanks
The closed steel tank has no diaphragm and no moving parts. It is simply filled with water (about two-thirds) and air (about one-third). When the boiler heats up the water, the water expands and enters the tank. As the water enters the tank, the air inside the tank gets compressed. The compression of the air inside the tank results in an increase of system pressure. That pressure can be measured at the pressure gauge of the boiler.

He installed it on the wrong side…It should be on the supply side. The purpose is to allow the expanding hot water a place to go. On homes with a check valve or pressure reducing valve at the main, there is nowhere for the expanding water to “back-up” to. So the expansion tank does that for you. It won’t do any good on the hot water side (I don’t think)… Well,maybe it will. …Now that I’ve said that and I think about it, I don’t know why it wouldn’t do the same job on the hot side as it would the cold side. It’s defiantly not the norm though.

One expansion tank manufacturer states:

Mike, thanks for your reply. I am trying to understand your answer. If there is apporx 60 psi of constant cold water coming through the cold water pipe to the tank, how does the expanded water get up to an expansion tank, when the expansion tank is mounted to the cold water pipe?? thanks Rick

If he has it on the hot water side it is wrong,( Potable refers to water in an open domestic hot water heating system. This is water that could potentially be consumed by people and is not recirculated within the system. Non-potable refers to water in a closed hydronic heating, radiant floor heating, or a chilled water system where the water is recirculated and does not leave the system. Each of these hot water heating systems have different thermal expansion requirements explained in more depth in the specification sheets accompanying each thermal expansion solution.) More info here

Wayne, excellent article. It explains everything I need to know and answered the question. Thanks Rick

They are required on closed systems.