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Why is My Pressure so High?

I replaced my old boiler at the beginning of last season, when I saw the tell-tale steam leaving my chimney.
Ever since then with the new boiler I have had a problem with high pressure. Single pipe, long loop, only the last 4 feet back to the boiler would be considered wet return.
The system does not run high immediately, but after it has run about an hour (which equates to three 20 minute cycles on the Hydro-level) it produces higher pressures, such that it runs maybe 5 minutes before the pressuretrol cuts it off. This is set a tad around 1.7 psi, and I have a second smaller pressure gauge on the pigtail which confirms this pressure reading, as the 30 psi pressure gauge directly into the boiler reads about a pound over this. There is then about 3 minutes for the pressure to drop to allow the restart, and this continues.
This does not seem to be best. The system seems clean (yes, I could get it cleaner), and is properly piped and vented, many new vents. House is a 3 story twin, 18 radiators (although only 2 on the third floor), sized somewhere in the low 500's. ALl the radiators get hot fine (the large one that the pipe goes outside the house through a crawl space takes a long time, but this is the same as before the new boiler.)
It did not do this prior to the boiler replacement, and since I found out that the boiler was leaking (i.e., self-venting!) I thought maybe I was undervented on the mains as when we bought the house 3 years ago there was only one Hoffman 75 about 2/3 the way along the loop, so I had a Gorton 2 added just prior to the last takeoff, which is actually almost back to the boiler (for arrangement reasons, I could not put this past this spot).

Anyone have any ideas why this wants to run high after an hour and what I could try to improve this?
Thanks in advance.


  • Brad White_202
    Brad White_202 Member Posts: 105
    Is 1.7 PSI

    your high mark? I am thinking on a couple of lines.

    One is that your pressuretrol is only so accurate, does what it can. You are probably setting it in the lower 20 percent of the scale. Have you considered a vaporstat to get you into a more accurate range? Even lower?

    Another thought is that the vents are working well. Once the vents close, pressure will build until cut-out. There is some elasticity in any gas. Naturally this occurs when the pipes are fully hot, thus not contributing as much to condensing and losing pressure.

    A third thought is, can you clean out and check your pigtails and instrumentation piping?

    Overall to me it sounds like it is working well but that you could drop the pressure even more, down to a pound or less, with the right control resolution.
  • 1.7??

    Brad--I am not quite sure I follow about lowering the pressure control more. Wouldn't this cause the boiler to turn off even quicker, and shorten the cycles further?
    I agree that the pressuretrols are hard to read and set accurately, which is why it has a companion dial pressure guage (0-10 psi) t'eed on the pigtail. It's on this gauge that I see the 1.7 when it cuts out--which is about where the pointer is on the pressuretrol, so I am assuming that it's doing what it's supposed to. I was actually running at 2 last season and have dropped it for this season after adding the extra main vent.
    I've seen the reverse of the dirty pigtail and controls. When we first moved in a knucklehead fouled my system, to the point that there was no pressure control whatsoever, only the relief valve opening up, all while the boiler merrily chugged along (and that series of events shortly thereafter led me to this board).
    Maybe I am misunderstanding the term "short-cycling". Are you all going to say that 5 minutes or so on, 3 off (once it's been up for an hour, mind you, as it takes about an hour to produce this condition) is acceptable? If that's the case, I'll happily stop worrying about this!
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    I'm sure Brad

    will come back on this one, but I happened to see it. First, lowering the pressure as much as possible will get you heat faster -- and will make very little difference on the cycling after running a long time. Increasing the differential would make a small difference to the cycling at the end of the run time. That said, sounds to me as though you are about as low as you can go with a Pressuretrol.

    The cycling near the end of the run is almost inevitable, particularly on colder days or recovering from a setback. The long first part of the run is spent getting all the pipes and radiators really full of steam. At some point, everything is full of steam -- and all the traps close. At that point, there is nowhere for the steam made by the boiler to go except condensing in the radiators -- and the condensation in the radiators is not as fast as the capacity of the boiler to make steam (really can't get around that without a modulating boiler), as the boiler has to be big enough to fill everything in the first place. Anyway, when the traps all close, the pressure will rise remarkably fast -- literally less than a minute, usually, from practically none to shutoff. Then, after a few minutes, enough steam has condensed to lower the pressure to your cutin, and off you go again. Quite normal.

    Don't worry -- be happy! That help?
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Yes!

    This helps ease my mind a lot, and is consistent with the fact that there was that pressure relief point (i.e., the undiscovered samall leak) the first two years I was here. Also, given the choices available in sizing the replacement boiler, it was either larger or smaller than the square feet of radiation we added up, so we went over for safety. BTW, I'd have asked for the Mega-Steam in a flash, but we are natural gas here, so it was Crown, which is local here (to the extent that it still can be.)

    But I am curious--if this is not the "short cycling" that is always a concern, then what is?
  • Fred Harwood_2
    Fred Harwood_2 Member Posts: 195
    High Pressure

    How may cycles per hour are you running? What's your thermostat?
  • Cycles

    Thermostat is an older Hunter programmable 44402. Other than the Hydrolevel check every 20 minutes this would otherwise run uninterrupted until temperature call is reached, or otherwise cancel it. We do not heat at night, nor during the day when we are out, and until our lows get to the low 20s we will not call for morning heat before we leave. This has been our usage since we moved in (along with the 'stat which was here). The pressure issue only arose with the new boiler.
    And yes I know from reading this board that there will be some serious disagreement over this style but we believe in the energy efficiency of this. We also saw this work effectively with our hot water system for the prior 20 years, and in our smaller steamer before then (although I will admit as to not having had a clue about that old system back in the early 80s when I was a 1st time home-owner.)
This discussion has been closed.