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System Pressure During Boiler Cycling

BKJohnnyBKJohnny Member Posts: 4
Trying to get the controls working as efficiently as possible for the single pipe steam system on our 16 unit apartment building. The boiler is oversized, so cycling is inevitable (1,875 sq. ft. EDR for the convectors, 2,700 sq. ft. net EDR for the boiler). The pressure doesn't reach the high pressure limit until the system is uniformly hot, so I think everything is working correctly, but the oversized boiler is producing more steam than the convectors can condense.

I have the Pressuretrol set to cut in at 0.5 PSI and out at 1.5 PSI. At those settings, the system pressure drops all the way to 0 and the vents open each time the boiler cycles. The system calls for the burner to come on at 0.5 PSI as it should, but there's a delay between the call and the burner firing as the burner goes through its startup and purge. Should I adjust the cut in/cut out pressure up so that pressure is maintained throughout the cycle? I remember reading in Dan's book that allowing the vents to open during cycling might be a good thing, as it would allow any air trapped behind a vent that closed on pressure rather than temperature to escape, but I don't think that's an issue in this case.

I'd appreicate your thoughts!


  • Jamie HallJamie Hall Member Posts: 12,863
    On a big boiler like that, it's hard to avoid having the pressure drop to 0 during the post purge/pre purge cycle. You can try setting the cutin pressure higher (without setting the cutout pressure higher -- depending on the type of pressuretrol you have there are various ways to do this).

    On the other hand, on a one pipe system such as yours, I think Dan's got a very good point in letting the pressure drop to reopen the vents -- if they need to. On the other hand, I wouldn't set the delay too long -- there's a lot to be said in terms of response for the fact that the boiler won't have gone much off the boil and all the piping is still hot.
    Br. Jamie, osb

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-Mclain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • JohnNYJohnNY Member Posts: 2,443
    I think you're assuming the pressure drops to zero during that downtime part of the cycle. And if the vents are hot, they will be closed. Either way, as you're describing it, none of this sounds terrible especially since you're talking about a boiler oversized by about 50%. My approach to this would be to add a 0-5 psi gauge or smaller and watch the pressure then. I might also be tempted to downfire the boiler by 10%.
    For troubleshooting and private consulting services, find John "JohnNY" Cataneo here at :
    "72°F Mechanical, LLC"
    Or email John at [email protected]
    John is the Boilers and Hydronic Heating Systems Course Instructor at NYC's Mechanics Institute, a professional Master Plumber, licensed by The Department of Buildings of The City of New York, and works extensively in NYC while consulting for clients in and out of state.
    John also oversees mechanical installations and maintenance for metro-area clients with his family's company, Gateway Plumbing and Heating along with his brother/business partner.
  • exqheatexqheat Member Posts: 53
    I use a delay after fire time to thermostat satisfaction for the pressure to drop for vent opening, and to allow the heat to fill the room. This makes for more even comfort and savings on fuel use.
    John Cockerill
  • EBEBRATT-EdEBEBRATT-Ed Member Posts: 7,057

    I would leave it as is. Raising the pressure will do no good. The fact that you said everything gets heat before it builds pressure is a condition everyone should strive for
  • ScottSecorScottSecor Member Posts: 384
    I did not notice whether you have a atmospheric burner or a power burner but I suspect the latter. If I'm correct, I too would recommend you reduce the firing rate and thereby get slightly longer run times and perhaps provide slightly more even heat to the building. Keep in mind that the burner will have a minimum input rating.

    We work on a lot of power burners and have similar issues often, especially with a long pre-purge and post purge. Lately it seems that every commercial steam boiler we service is over-sized for the load and we struggle with the cycle times of the burner (especially with a mild winter here in NJ).
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