1. WV Hillbilly

    WV Hillbilly New Member

    Messages:
    178
    Location:
    WV
    I have recently read several threads about expansion tanks being needed if a prv is used . I am confused about this . I own a home I once lived in & now use as a rental . It has public water & the incoming pressure is 110 #'s . I installed a prv at the water meter which is about 200 feet from the house . I adjusted the pressure to 50 # 's . This was several years ago . I have had no known problems . What causes the high pressure in the water heater ? The only thing I can think of is that water can't go backwards through the prv . If that is the case why wouldn't a check valve cause the same problem ?
  2. kingsotall

    kingsotall Plunger/TurdPuncher

    High pressure in the hot water is thermal expansion. Why would a check valve be plumbed in¿ (in regards to your inquiry)
  3. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    The issue of a bypass vs. non-bypass PRV is actually a red herring in this discussion. Even a bypass PRV will only limit the house side pressure to equal street side, and this is often high enough to cause weeping of the water heater T/P valve.

    The pressure build up is just that....expansion of the water as it is heated. A problem sometimes exists where it didn't used to manifest itsefl, because newer WH may have higher BTU and higher efficiency than older ones.
  4. WV Hillbilly

    WV Hillbilly New Member

    Messages:
    178
    Location:
    WV
    I understand thermal expansion in the water heater , however it seems to me that if the pressure raised in the water heater it would raise in both hot & cold pipes . It also seems to me that you would get an intial burst of pressure when a faucet is first opened . Most public water systems require a check valve so no water from the house can siphon back into the public water system . Maybe I shouldn't have said most but the one I'm hooked up to does . Should every house on that water system have an expansion tank ? Am I right in assuming a prv acts as a check valve & creates the need for an expansion tank ? Thanks
  5. Redwood

    Redwood Master Plumber

    Messages:
    7,453
    Location:
    Connecticut
    yes you are correct that the PRV acts as a check valve.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,802
    Location:
    New England
    If you have a typical PRV or a check valve in the system, and you don't get the T&P valve leaking, it's because you have some hoses that can expand and take the increased volume, a valve is leaking and yo udon't know it (maybe a toilet) or the check valve is leaking itself. It doesn't take much to raise the pressure with a liquid since it doesn't compress under pressure. Water is one of the weirder compounds as it both expands when it gets hotter, and again as it freezes. If it didn't, we'd end up living on a solid block since the ice would sink to the bottom and build up since the heat from the sun doesn't penetrate that much.
  7. WV Hillbilly

    WV Hillbilly New Member

    Messages:
    178
    Location:
    WV
    Not trying to be difficult , just learning something I hadn't thought about before & have another question . The home I live in now is on a well system . The submersible pump of course has a check valve . When my water heater kicks on & heats the water the pressure gauge at my pressure tank should rise if I have no leaks anywhere , correct ?
  8. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,802
    Location:
    New England
    Assuming the check valve is before the pressure tank, no...the bladder tank is functionally equivalent to an expansion tank. Since it has a built-in air cushion, it will absorb the volume increase. If you had a really sensitive pressure gauge, you might see the pressure move up a very small amount. Air compresses easily, water does not.
  9. valveman

    valveman Moderator Staff Member

    Messages:
    4,369
    Location:
    Lubbock, Texas
    Thermal expansion from a water heater may only raise the volume of water by a teaspoon or so. With a pressure tank for a submersible, you would not see a pressure increase for adding a teaspoon of water. However, without and expansion tank, expandable pipe, or a leak somewhere, a teaspoon of water increase can make the pressure go really high.
  10. WV Hillbilly

    WV Hillbilly New Member

    Messages:
    178
    Location:
    WV
    Thanks , Back to the house I own that's on public water . Is a small bladder tank normally used or something else ?
  11. valveman

    valveman Moderator Staff Member

    Messages:
    4,369
    Location:
    Lubbock, Texas
    A small bladder tank will work. You only need a place for that teaspoon of expanded water to go.
  12. jecole13

    jecole13 Junior Member

    Messages:
    5
    Location:
    Pinehurst, NC
    Where I live all the water meters have an internal check valve which creates the need for an expansion tank. The expansion tank is needed independantly of the PRV. PRV's are only required when the water supply exceeds 80 psi Although they are not a bad idea to prevent damage from ever changing conditions in the city or counties water system.
  13. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    21,802
    Location:
    New England
    Two general rules: if you have a closed system (check valve or prv) and a WH, you need someplace for the water to go - an expansion tank is the normal solution (although if you are on a well, you likely have a place for it to expand - your storage tank; note, someone had said code still requires one, but I'm not up on that); and, two, if your incoming water pressure exceeds 80 pounds, prudence calls for a PRV (and may be required).

    Where I grew up, for the first few years, if you weren't careful filling up a glass with water from the tap, the pressure would force it right out of your hand! We got a PRV eventually, but not before a few glasses got shattered. We lived a 1/4-mile from the pumping station. Boy, that was good water from the springs.

    Regarding water pressure...if you measure it during the day, you may not get a true indication of the maximum that may be in your area. With all of the uses during the day with people showering, doing laundry, flushing toilets, doing dishes, the pressure is likely lower than it will be late at night when the load is lower and they may be trying to pump the water into the towers. the difference between typical daytime and nighttime pressures can be significant.
  14. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,233
    Location:
    Cave Creek, Arizona
    espansion

    There IS an initial burst when the faucet is opened, but since it only takes about a cup, or less, or water to eliminate the expanded water, it occurs gradually as the faucet is opening.
Similar Threads: PRV's
Forum Title Date
Plumbing Forum, Professional & DIY Advice, Tips & Sizing Residential PRV's May 11, 2009

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