Thermal Expansion Bypass

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by natalie1999, Feb 15, 2009.

  1. natalie1999

    natalie1999 New Member

    Feb 9, 2009
    Can someone explain to me how the thermal expansion bypass on a water pressure regulator works? Like, after the water heater shuts off, is the regulator supposed to readjust the pressure back down or what? I can't tell if we need to get an expansion tank.

    The pressure went up to 95 after the WH shut off. The regulator is set to 45-50 static pressure. The pressure is now 55 after 20 minutes (someone may have used water during this time). I know the bypass only works if the pressure is higher than the mainline -- the mainline is supposed to be around 80 -- I'm confused. Help?
  2. Cass

    Cass Plumber

    Nov 12, 2005
    The PRV by pass will stop the house side pressure from rising when the house side pressure exceeds the street pressure...I am not positive but I believe not all PRVs have the by pass feature.

    Example: your street pressure is 135 and you have a PRV set at 75 and you have a properly set expansion tank...the PRV fails...the house side pressure goes up to 135 and stops because any additional pressure will now bleed over, through the PRV, into the street...because it (the street pressure) is lower than the house pressure...

    Yes you need an expansion tank...

    If you call your water supplier and ask what the approx. water pressure is in your area they should be able to tell can't guess...night time pressures can be much higher...With an inexpensive pressure gauge you can test it your self...
    Last edited: Feb 16, 2009
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  4. wraujr

    wraujr Member

    Sep 24, 2007
    Watts N35B

    The specs on a Watts N35B (a typical albeit obsolete design):
    "This feature prevents downstream pressure from rising to more
    than 10psi above the supply pressure"

    Therefore if you think street is around 80psi, then 95psi is probably about right given that the 10psi is "typical" and you have gauge accuracy and you're not really sure of street pressure...
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