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PRV discharging water

Hi all, just want to make sure I'm not missing something here (this seems to be normal / make sense to me but LMK if you think otherwise).

Due to a washing machine situation (don't want to bog down with details), I have been turning off main shut off valve to whole house before sleep, and before leave for work. This morning, when I went to turn it back on, I noticed a small puddle beneath the PRV discharge pipe on my 2 month old IBC tankless WH. I did a little testing / trouble shooting, and found that pretty consistently, it will spit out a little water when I do the following:

1. turn off water main
2. open one of the hot faucets somewhere in the house to make sure water is in fact off, then close faucet once the little remaining water stops coming out
3. turn the water main back on

once i turn the water main back on, you can hear the water hammer (apologies if my terminology is off) for a split second or 2, as the emptiness in the hot water piping (due to opening a faucet before) fills itself back up. as that happens, the WH spits out a little bit from the PRV, but that's it

to me as an unprofessional, this seems to make sense, just the PRV doing what it's supposed to? When you let the pressure back into the system especially when there is empty air somewhere in the hot water pipes, there is that brief moment of much higher pressure that causes the PRV to open?

just trying to make sure this isn't indicative of something else.....i have never seen any signs of the PRV releasing anything prior to this. thanks!


  • HomerJSmith
    HomerJSmith Member Posts: 2,511
    Where do you shut the main off. With a valve in the house or a valve out in the yard? Is the faucet that you drain the little water out until it stops on the first floor of a 2 story house or the basement of a one story house?
    You have let a lot of air in the hot water distribution pipes and when you turn on the main shut off valve water rushes into the low pressure distribution pipes thru the heater. That water has momentum and when the water stops it slams against the faucets and causes water hammer. Then the momentum opens the PRV and it drips.
    Crack open a faucet before you open the street main valve. You can open the street main valve very slowly to fill the heater sys which will minimize the momentum. OR...you can install a Expansion Tank in the sys.
  • camiarrobino
    camiarrobino Member Posts: 49
    i turn the valve off where it enters the house, in the basement. the tankless WH comes very very shortly after the water main entrance, basically right next to the meter / main shut off. the faucet that i drained was 1st floor, single floor home.

    what you say makes sense and that's what i was hoping, only because of the sudden & extreme momentum into the WH. i don't have to keep turning the water off now but any instances in the future, will do it much more slowly
  • SummitMechanic
    SummitMechanic Member Posts: 25
    I can almost confidently say your assumption is correct. But i would need to know the make and model of your prv to know for sure. Most devices are designed the same way, but they all have small little quirks that can do this under different situations.
    Experienced Boiler Mechanic In Summit County, Colorado.