Recommendation for small expansion tank for potable water recirculation loop

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PeteD

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I am looking for a small expansion tank that can relieve the pressure in a potable water recirculation loop for an on demand gas water heater located farther than desirable from the faucets/showers. The total volume in the piping is not very much (figure a max of around 5 gallons or less), so it seems that a smaller than 2 gallon expansion tank could be used.

Anyone know of smaller potable water expansion tanks than the ~2 gallon size?

I believe the hot water (up to about 120 deg F) in the recirculation loop can harm the membrane in potable water expansion tanks. So in this case, the tank should be installed right after the check valve/BFP on the cold supply, but before the recirculation line ties back into the cold feed to the on demand heater, correct?

In other words:
cold feed>check valve or BPF>expansion tank>recirculation loop hot into cold feed>on demand heater

Will potential back flow of heated water into the expansion tank be a concern? What is the max temperature ratings for these potable tanks?

Thanks in advance.

ETA: Street pressure is under 75 pounds
 
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Jadnashua

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If you have an expansion tank on the supply side to the WH, adding one on the hot side is not going to buy you anything since they are essentially interconnected, and any pressure increase on one side would be dealt with by the ET on the other.
 

PeteD

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If you have an expansion tank on the supply side to the WH, adding one on the hot side is not going to buy you anything since they are essentially interconnected, and any pressure increase on one side would be dealt with by the ET on the other.

I don't have any expansion tank yet, still in the design stages. Limited room for a tank, thus the search for a small tank. Actually, from here, the allowable temperature on the AET-2 potable tank is 140 deg F.
http://www.americanwaterheater.com/media/22209/exptank.pdf

Still looking for smaller options. Does Watts want cold water only in their potable expansion tanks?
 

PeteD

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Actually Watts specs say good to 200 deg F, even though they say to put on the cold supply. I thought I saw in another thread to avoid hot water in the potable ETs, but that does not appear to be an issue.
http://media.wattswater.com/ES-PLT.pdf

Now, if someone made a 1 gallon potable ET that would be great.
 

PeteD

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The other question I guess is does the potable hot water loop need to be closed by the check valve/BPF, or can the heated potable loop be allowed to expand back into the city supply line?

ETA: there will definitely be a check valve in the loop itself to keep cold water from by passing the heater and getting into the hot pipes via the return lines. I am referring to a check on the main cold feed to the heater in the paragraph above.
 

hj

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What is your definition of an "on demand water heater", since a TRUE odwh does not need an expansion tank because it only heats when water is flowing.
 

Jadnashua

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The life of any ET will decrease if it is installed on the hot side. Also, keep in mind that if the WH had a problem, the outlet temperature could exceed the limit on the ET before the safety valve opened.
 
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