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Pressuretrol set at 2psi, gauge says 15!

I'm spending my first season with steam heat, and am slowly getting the system trouble-shot.  I'm about 1/2 way through The Lost Art, so I'm getting an idea of how things should be...

I have my pressuretrol set at 2, and the differential set at something around 1 (the scale doesn't go down that far).  However, the other day I was in the basement and noticed that my 0-30 gauge was up around 15psi.  I've read in other posts that these gauges are unreliable, but that's really pretty far off.  I went back and checked when the boiler was off, and it returned properly to 0.  Suggestions on what to do here?  Is my pressuretrol shot, or is the line to it clogged?  Is this why I've suddenly got water squirting out of my air vents?

I've also noticed that the boiler only has about a 30 second off cycle while the thermostat is calling for heat.  Once it is heated up and delivering steam the fire will run for 3 to 5 minutes, then shut off for 30 seconds, then start back up again and run for another 3-5.  I'm not sure if this is connected to the pressure issue, but I could make up some hand-waving stories where it is. 


  • alexholman
    alexholman Member Posts: 5
    ... to clarify

    the 1st picture is an action shot with my cell phone.  the 2nd and 3rd are glamor shots showing the pressuretrol settings and where the gauge sits on a cold system
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,846
    Either your gauge is reading a little high

    or your relief valve is bad. The safety valves on domestic steam boilers are supposed to open at 15 psi.

    This is something you never, ever want to let happen, by the way. If you've ever seen a car radiator blow, that's kid stuff.

    Boiling is what happens when water is heated to the point at which the vapor pressure equals the atmospheric pressure. In an open container at sea level, that happens at about 212° F. But a boiler is not an open container. It's connected to pipes and radiators, and there are air vents that close as soon as the steam reaches them. When that happens, the pressure can begin to rise. As the pressure rises, the temperature required to boil the water increases, until you have a large volume of water and steam, under pressure, heated to well above the normal boiling point. When the pressure rises to 15psi, the safety relief valve blows, allowing steam to escape and the pressure to drop, now the remaining water can suddenly flash to steam, because it is hot enough to boil under high pressure, but now the pressure has dropped.

    If you're worried about this happening while no one is home, that's about the best outcome. The worst is just as you've gone downstairs to check on the boiler because the pipes are banging and the vents are hissing. You don't want to be in the basement when the relief valve blows and the whole basement fills with steam. Just ask a lobster.

    It sounds like the steam trap, or pigtail, under your pressuretrol is plugged. It isn't allowing the pressure to reach the control until it reaches dangerous levels. And it is probably getting worse every day.

    Additionally, your venting is clearly not adequate, and your firing rate might be too high. A properly configured, well-balanced system should be able to heat your house without cycling on pressure. I put a lot of work into my system over the summer, and set the pressuretrol as low as it would go, and it never cut out once all winter. The 3 psi gauge I put on it never reached 1 psi. And I've been very comfortable.

    So your first step is to get a new pigtail on there pronto. If you're not confident about doing it yourself, use the Find a Contractor menu to see if you have a qualified steam professional in your area. Either way, this is urgent. You're flirting with disaster.

    Next step, take some more pictures of the rest of your system--pipes and radiators--and post them here and we'll let you know if there are any other things you should address.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24
  • alexholman
    alexholman Member Posts: 5
    edited March 2012
    new pigtail AND new pressuretrol?

    I was wondering about the safety release valve, so with the system COLD I went down and moved the manual release lever back and forth.  At least it appears that the valve innards are moving freely.  I suspect that I have a slightly inaccurate gauge, and I may have been a couple psi under the true 15 required to trip the safety release. 

    As long as I'm messing with replacing the pigtail, is there a good reason not to throw in a new pressuretrol with a much lower max cutout scale (would that be a vaporstat?).  There seems no good reason for my system to ever be running over 2 psi and it seems strange to have to crank the dials all the way down below the lowest gradations to get near that point.  
  • BobC
    BobC Member Posts: 5,478
    Pressuretrol seems to be working?

    The fact your boiler seems to be cycling on and off before the thermostat is satisfied seems to indicate the pressuretrol is working but we don't know if it is working correctly or if that gauge is any good. Do the on cycles seem to be longer than they were before you started to get these high pressure readings? Is the system making any strange noises like excessive hissing or banging? If it was really working near 15PSI I should think it would make it's over pressure pretty evident, that old 0-30 psi gauge may well be flaky.

    If you want to try and test this yourself, kill the circuit breaker that feeds the boiler and unwire the pressuretrol so you can twist it off. When you have the pressuretrol off see if you can blow into the end of the pigtail, if you can blow air fairly easy the pigtail is probably ok. If you can't blow into that open pigtail it is blocked and has to be cleaned or replaced - use a red brass pigtail. There could also be some crap inside the boiler forming a plug that closes off the pigtail as pressure builds and then gives way when the pressure builds enough. Remove the pigtail completely and use some wire to fish around the boilers pigtal tapping to make sure there is no crud near that opening.

    Many of us have added an auxiliary 0-3PSI gauge because the factory 0-30 gauge is useless at the 2 PSI (max) level that the boiler should be operating at. You still need a 0-30 gauge to satisfy regulations.

    If that old pressuretrol is working correctly don't part with it, the mercury ones are a lot better than anything you can buy these days. You could install a low pressure vaporstat but the money might be better spent on new main vents and a set of new radiator vents (if this is single pipe steam.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Greg S
    Greg S Posts: 10
    Pressure Gauge

    I had this same problem. After removing the gauge, there was scale in the gap between the probe and threads. I could see a chunk (possibly rust) wedged in which I assume expanded as the furnace heated up. Once cleaned, the gauge worked fine.

    Greg S (amateur who finds steam entertaining)
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