Pressure relief valve to reduce pipe bursting?

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by superdragon, Mar 20, 2013.

  1. superdragon

    superdragon New Member

    Messages:
    7
    Location:
    KS
    Hello. As I understand it, when water freezes in the water pipes, the point of burst is usually downstream from the freeze and due to excessive pressure build up in the pipes. In other words, pipes burst not due to the freezing itself but due to pressure that builds up in the non-frozen section where water is still in fluid state. I was wondering whether the installation of pressure relief valves downstream can help to reduce or eliminate pipe bursts? I have googled but haven't found anything about this which is puzzling since it seems to me that pipes: freezing--> too-much-pressure--> bursting............. well, it seems like a pressure relief valve installed in the insulated space would be the perfect prescription to combat this problem. What am I missing here?
  2. Tom Sawyer

    Tom Sawyer In the Trades

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    3,175
    Location:
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    The short answer is no. You would have to install the PRV in the exact location where the water remained in a liquid state.
  3. hj

    hj Moderator & Master Plumber Staff Member

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    Location:
    Cave Creek, Arizona
    Water expands when it freezes, so it can just as easily burst the line at the point of the freeze, and usually does, which is why the pipe will be "expanded" at the point of the break. IF the unfrozen water were the cause of the break, the entire pipe would be "expanded/enlarged", not just a localized point.
  4. superdragon

    superdragon New Member

    Messages:
    7
    Location:
    KS
    That runs contrary to what this website is saying:
    http://www.weather.com/activities/homeandgarden/home/hometips/severeweather/pipefreeze_prevent.html

    I'm no expert, just trying to educate myself. However, it does make sense seeing that I have heard from many other sources that keeping the water running, even a slow drip, is enough to relieve pressure build up and prevent bursting. If it were the case that the pipe will burst at the point of freeze, then leaving a slow drip 20' away at the faucet will be ineffective. Speaking of, IF a slow drip at a faucet is enough to prevent bursting, then going back to initial question, wouldn't a pressure relief valve, installed in the insulated space (presumably temperature would be higher there and water still in liquid state) be effective in relieving excessive pressure that would cause line breaks?
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
    New England
    Copper pipe has a bursting strength in the 3000# range. Believe me, while a valve may leak or a seal may fail before that pressure is achieved, it is the actual frozen section that bursts since it CAN exert that amount of pressure regardless of what the water pressure is. Water expands when it freezes, which is why ice floats. It is the expansion of the ice plug that splits the pipe. If you had an expansion tank somewhere in the house, it would keep the pressure in the line from the fluid water from rising more than a few pounds...well within normal parameters, but would have no effect on the ice plug expanding.
  6. superdragon

    superdragon New Member

    Messages:
    7
    Location:
    KS
    Initially, I also thought that ice freezing would cause pipes to burst right at the point of freeze. However, after thinking about it some more, it makes sense that the pipe burst would far more likely occur in a fluid section. The reason is this: When the water in the pipe freezes, it is unlikely to freeze in microseconds, nor would the entire length of pipe going to freeze at the exact same moment. The freeze would slowly occur starting at the outside diameter edge where it contacts the pipe (which presumably, is losing heat due to cold air contact). The ice expands and then pushes the still fluid and ever shrinking liquid core section section back upstream (where is no check valve is installed, = no increase in line pressure) or downstream, where if the water valves are all shut, will increase pressure. Pressure increases, until pipe bursts. This is the reasoning behind why leaving water running, even a small drip or trickle is enough to eliminate pipe bursts.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    The reason to have the water running is that you are constantly adding warmer water through that section, and adding heat, preventing it from freezing in the first place It's also why a stream doesn't freeze solid because there's running water underneath. Believe it or not, the ice creates enough force to split the pipe...you'll not get enough pressure to split it via the liquid water.
  8. superdragon

    superdragon New Member

    Messages:
    7
    Location:
    KS
    So.... the website at the link in my previous post speaks rubbish then? :confused:
  9. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    It's a rare valve that will stand up to more pressure than the pipe requires to burst and thus hold the water in to allow the pressure of the stored water high enough to damage the pipe. Ice is 9-10% larger in volume than water in liquid form at the same temperature. While some of the expansion might be along the pipe, a lot of it will be in increased diameter. Expanding the copper pipe by around 10% and it's going to split. If their explanation was accurate, why would running water keep the pipe from freezing in many situations in the first place? In a house with a closed system (one with say a PRV or a check valve on the inlet), one needs an expansion tank to pass code. If you have one of those in the system, that can easily accommodate the water expansion without stressing the pipes. Say it froze near the inlet to the house. If their explanation was valid, every pipe in the house could burst if there was no open valve as the freezing water raised the pressure. Doesn't happen, won't happen. Pex would just expand, then recover when things thawed (except for a fitting if it was frozen, too). CPVC would shatter at the ice block.
  10. superdragon

    superdragon New Member

    Messages:
    7
    Location:
    KS
    After giving it some more thought, I have now changed my mind and believe that in fact, a pressure relief valve will not be effective in reducing pipe burst issues. The reason is that initially, I was thinking that as the water inside the pipe begins freezing along the inside circumfrence, it would expand causing the still fluid areas in the center core to displace and squish either up or downstream. This would prevent the ice clog from expanding radially as it would be far easier for it to squish out of the center core than to expand the pipe diameter. My thought was that the PRV would then prevent excessive pressures that would cause pipe burst.

    However......... since the pressure within the pipe is equal throughout, there is no reason for the water inside the freezing core to move anywhere along the length. It would be content to just sit within the core of the freezing section since the pressure is the same no matter where within the pipe.

    So, if the water line was dripping or trickling, then the movement of the water, even if ever so slightly gives the water within the freezing core space to shift and squish out of the core as it's displaced by the expanding ice.

    The whole point of this exercise in considering the use of a PRV to mitigate bursting pipes was to provide cushion for the expansion of the freezing water without the waste of resources that occur with allowing water to run. Unfortunately, since PRV's don't allow the movement of water until a preset pressure is reached, it would not be effective since stagnant water will as you all say, freeze in place. I wonder then if an expansion chamber of sufficient size would be more effective since unlike a PRV, it CAN accomodate the movement of water along the length of the fluid section. Maybe I'll try an experiment and freeze a water filled pipe inside a freezer.... one with and one without an expansion chamber.
    Last edited: Mar 22, 2013
  11. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
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    Location:
    Cave Creek, Arizona
    quote; keeping the water running, even a slow drip, is enough to relieve pressure build up and prevent bursting

    You COMPLETELY misunderstand WHY the drip is effective. As long as "fresh" water is flowing through the pipe, it is "washing" the "chilled" water away from the freezing area, thus the temperature of the water is constantly being elevated so it should not freeze, UNLESS the chill is so severe that it can freeze "running" water,which can happen if the drip is too slow and/or the temperature is too cold. It has NOTHING to do with "relieving pressure". IF that were the case, you could shut off the water to the house, not drain the water out, but open a faucet to "relieve the pressure", and the pipes would NOT burst, but as a practical matter, i would NOT advise that you try it because you would have MULTIPLE breaks which would have to be repaired.
    Last edited: Mar 22, 2013
  12. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    Every Winter I go out repairing burst pipes from freezing. We plumbers "know" why pipes burst from freezing and "where" they will burst.
    Most of those will be at an outside hosbib, where a rubber hose has been connected. If expansion was an issue, they wouldn't be splitting where they do. It's pretty much like the ice cubes in the freezer. You put water in, and it expands in the tray. That's why ice floats on lakes. It gains volume.
    Like hj mentions, leaving a facuet running isn't about pressure, it's about keeping the water temperature above the freezing level.
  13. superdragon

    superdragon New Member

    Messages:
    7
    Location:
    KS
    OK, thanks.

    Like I said, I'm certainly not an expert. I'm actually moving from a moderate weather area (CA after almost 50 years) to the colder climates of Kansas. Obviously, there will be a lot of changes and things I need to be aware of that I previously took for granted before and so, I wanted to research best I could about how to prevent burst pipes since I might be traveling back/forth for the first couple years and wanted to avoid this problem if possible. I wasn't trying to be argumentative, but understand that every single website including those of weather.com, redcross, insurance companies........... they all said to allow a small drip which THEY claim is enough usually to prevent pipe bursting. Those all seem like they would be reputative organizations and they almost all stated that the purpose of the drip/trickle was not to prevent freezing but to prevent the buildup of pressure that would occur between the freeze/blockage and the faucets. In other words, the pipe may freeze but the drip prevents bursting. The number of such informational sites is hard to ignore so I guess my point is..... are they all wrong? Also, if we keep the faucets running, it seems to me they have to go almost full blast in order to prevent freezing since introducing 34 degree water at a drip or trickle is in no way effective in keeping a 10 degree hosebib from freezing. Anyone that's ever turned on the cold water faucet during cold weather knows how cold that water is... if it's not in the 30's, it's probably not much higher in temperature than that. The skeptic in me can't see how a drip, or even a trickle would introduce enough heat to offset the ultra cold 20, or 10, or 0 degre pipe.

    I'm more confused than ever now but I guess I'll just have to shut the water off at the meter and drain the lines when I leave for extended periods. Thanks for all the comments and contributions.
    Last edited: Mar 22, 2013
  14. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    Ice will form on a lake, and it's expanded water that floats on top.

    According to your experts, that can't happen because there is room for expansion.
    Water when it freezes will expand.

    I can't tell you how many times I've been interviewed for a qucky story, and sometimes they even quote correctly. Other times they disregard what I say assuming that nearly 40 years of expierence isn't quite up to their ponderings.

    But remember, it's your home.

    If it's my home, and I'm leaving for a few weeks, I'm turing the water off. I know too many people that were out of the state when their pipes broke in mutilple places. I had one home with nine breaks, none in the crawl space.
    They spent months fixing all the damaged floors and walls.
    The first burst pipe would have relieved pressure, but there was still eight more breaks.

    I thinks those that write about things, and those that do things are different animals.
    They say that those that can, do.
    Those that can't, write.

    So when a lake has floating ice? That's because someone forgot to leave a faucet open?

    And all homes have a pressure relief valve at the water heater. Doesn't help a bit. In fact, the hot pipes will freeze and split quicker. At least the last forty years I've been looking, they have.
    Last edited: Mar 22, 2013
  15. superdragon

    superdragon New Member

    Messages:
    7
    Location:
    KS
    Thanks Terry (and others). Guess that's what I'll do.
    Although..... I don't understand the part about the hot water pipes splitting faster than the cold ones.

    Speaking of which.............
    When we shut off the water, does that mean draining the hot water heater completely too? Or is it safe to leave it on vacation mode (lower temp).
    Last edited: Mar 22, 2013
  16. Terry

    Terry Administrator Staff Member

    Messages:
    15,005
    Location:
    Bothell, Washington
    What I actually do when I'm gone for two weeks, is leave some heat on, and the water.

    I suppose a smarter thing to do would be to turn off the water too, and the gas to the water heater. I can alway relight when I get back.
    Normally though, unless there is a power outage, you're going to be okay.

    I have seen burst washer hoses that overnight put three inches of water in a basement.
    I've seen a neighbors water heater go bad to the point that it ran down the hill to the home below them, and the river of water went into that home and caused damage.
    I've seen the home with nine pipe splits. They were in California for the Winter and turned off all of the heat.
    And tons of frozen pipe splits during our power outages around here in Winter.

    On the hot water side freezing quicker. There is science behind that one. Though I find that observation will tell you it's happening on the hot side more, and if you ask around, there are text book types that will know the reason why. For the plumbers, it just gets drilled into you, as often you are repairing the hot side, and the cold side is okay. Two pipes side by side, one is split.

    For years we knew a day was 24 hours by observing it. Once day we realized that the earth was spinning. That was science. Sometimes it takes a while to catch up to each other.
    There was a time when science thought the world was flat.
  17. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
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    Location:
    Cave Creek, Arizona
    quote: .. are they all wrong? Also, if we keep the faucets running, it seems to me they have to go almost full blast in order to prevent freezing since introducing 34 degree water at a drip or trickle is in no way effective in keeping a 10 degree hosebib from freezing. Anyone that's ever turned on the cold water faucet during cold weather knows how cold that water is... if it's not in the 30's,

    1. YES! Unless you are misinterpreting what they state.
    2. It only takes a small amount of flow to dissipate the chilled water.
    3. When you turn on a "cold faucet" in the wintertime, it may be cold, but it is NOT 32 degrees. And since cold water has entrained air and other "minerals", it freezes below 32 degrees, but hot water has that "stuff" precipitated out during the heating process so it WILL freeze at close to 32 degrees, which is why it WILL freeze sooner than cold water. If the heater is on "vacation" or any other operating temperature, it will NOT be able to cool down to the freezing point so you do not have to drain it, but if it is completely shut off, then it should be drained or it WILL freeze and deform, if not rupture.
  18. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,019
    Location:
    New England
    Hot water tends to get any entrapped air stripped out of it, and sometimes minerals, so the water is purer. Just like salt lowers the freezing temp, making the water purer can get it closer to 32. There are two ways to ensure you don't get frozen pipes: keep the heat on, or drain the water which may mean using an air compressor to purge the pipes. If you are living there, and you have pipes running in unconditioned space, that's a big problem. Insulation does not generate heat, it slows the movement of it. IF there's none there in the first place, it can't do much. All it takes is one little air leak on a windy day, no water flow, and a spot can freeze since the moving air moves away any heat that's there more quickly. Where it gets below freezing on a regular basis (and not a bad idea elsewhere, too), it's bad practice to run water pipes in an outside wall if they could have been run in an inside wall. It's a good idea to check any new house. People like to put sinks by a window. In that case, it can work better (from a freezing viewpoint) to bring them up into the cabinet from below, rather than out from the wall. Another tip - if you do have any on an outside wall, overnight, you may want to open the cabinet doors which will allow some of the room heat to keep it warmer there.
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