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Thread: Boiler running up to 30 psi

  1. #1
    DIY Junior Member Mjc441's Avatar
    Join Date
    Mar 2013
    New York

    Default Boiler running up to 30 psi

    I just bought this house and am noticing water on the floor by the downspout of the pressure relief valve. While watching a cycle of the boiler i notice that the pressure gets up to about 29 to 30 psi. When i watched one cycle on a semi cool day (45 degrees), the pressure went up to almost 30 but no water out of the PRV. I am figuring that under more of a load the pressure is going higher and releasing onto the floor. Anyhow, I know the previous homeowner installed the boiler himself and I am questioning the location of the expansion tank. The tank is brand new by the way. The boiler is piped on the hot water supply side to a horizontal header. The header has three vertical circulators for the three zones in the house. The expansion tank is after the circulator at the top of the rise on the main zone. Should this be before the circulators off the header? Would the circulator for the main zone hold back enough pressure for the PRV to release? And i also have a hot water coil in the boiler. I am wondering if the boiler fires for hot water and the heating circulators are not running for a heat call if that is why i am getting higher pressure? Also, the expansion tank is horizontal. Not sure if my theory is correct about the pressure building because the expansion tank is after the circulators on the supply side. Any help would be appreciated!!


  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    The expansion tank works best for the circulators if it is before them...this ensures that when they turn on, there's water there that doesn't cavitate (when all's well). But, it should work anywhere on the hot side as long as there's no valve between it and the boiler.

    Three reasons the pressure rises that I know of: the expansion tank (ET) is defective or not properly precharged; the autofill valve or the fill valve are leaking or misadjusted; the indirect water heater's heat exchanger has a leak in it, and potable water is leaking into the boiler system from there. I guess you could add an improperly sized ET, but that isn't too common. The pressure relieve valve (it's not a PRV - pressure regulator valve!) on a normal residential boiler is usually 30psi, so whenever it approaches or tries to exceed 30, it's normal for it to release. Since that is an emergency device, after being forced open multiple times, it's best to consider replacing it. It may very well seal, but the water could end up leaving some mineral deposits and mess up its operation, and possibly once you fix things, locking it in a closed position and then things can get dangerous.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    The pressure rises because the water in the system expands with temperature, which is what the expansion tank is supposed to manage. If the expansion tank is on ZONE plumbing after a ZONE pump it's absolutely not in a position to do it's job, since it's pressure-isolated from the rest of the system by the pumping head of the zone plumbing & radiation. With the expansion tank between pump & radiation is active it's doing practically SQUAT for managing the water expansion of rest of the system when that pump is running, and not a whole lot even when it's idle.

    Pumping away from the expansion tank is normal. DIYers with undersized, incorrectly charged & mis-located tanks is also (unfortunately) pretty normal. Start by calculating the volume of water in the system and verify that the tank is at least the right size for your operating temperature range, and put in a place where it can actually do it's job.

    It's possible in dual-temp systems to calculate too large an expansion tank, and there are even rarer instances where injection pump isolated zones may need their own expansion tanks, but those are exceptions, and probably more complicated than your system is:

    Most boilers are rated up to 50psi, but are usually shipped with 30psi PRVs, since other system components may have the lower limit, and most systems work just fine a much lower pressures. If you have a fairly high water-volume system that would require a gia-normous expansion tank to manage (not likely), it's possible to swap in a higher-pressure PRV if you do your homework on the rest of it. With the tank correctly placed, right-sizing the tank for the volume is always a better solution than higher pressure, if at all possible.

    When you move the tank, install it vertically, otherwise when the bladder inevitably leaks, filling the tank, it may become heavy enough to literally break off.

    This system sounds like a real DIY hack- can you post some pictures? There may be other stuff worth fixing when you bring the system down to deal with this.

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