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expansion tank location with tankless heater

I'm installing a noritz tankless dv hot water heater with noritz's hot and cold isolation/pruge valves. Since the valves attach to the tankless with half unions there is no place between the cold valve set-up and the tankless to cut in an expansion tank.

Should I then install the expansion tank between the prv and the iso/purge valve set-up. I have always been under the belief that the expansion tank should be installed before the cold water shut-off incase the valve is closed it would still absorb any expansion without activating the prv.


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    edited January 2012
    I wouldn't put one in....

    It's a tankless, which means it won't be firing unless a tap is open. Expansion tanks are there to allow the expansion to be absorbed. You won't have any expansion to speak of, because if the burner is on, a tap is open. When the tap is closed, the burner is off.

    Save your money for something else.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
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