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Short Cycling?

Steamhead and other steam gurus:
Was down in the basement and took some time to monitor the cycling of the one pipe steam boiler servicing our first floor. Almost like clockwork, the boiler was cycling 90 seconds on, 90 seconds off until the thermostat was satisfied. I gather "it depends," but is this short cycling, and is there anything that could/should be done?
The details: Mains are well ventilated and insulated. Pressuretrol settings are as low as they'll go. HOWEVER, the boiler is easily 100% oversized for the attached EDR (though Steamhead-San downfired it), and the boiler itself, while in apparently good shape (doesn't lose water, lwc tested regularly, blown down regularly, etc.) is in its golden years: as soon as Burnham puts out a gas Megasteam, we're calling Steamhead, pronto!

Under the circumstances, is this cycling alarming? Anything I might attend to (short of updating the boiler) that wouldn't be a waste, assuming it'll only be another season or two before it's updated?



  • Tom Hopkins
    Tom Hopkins Member Posts: 552
    short cycling

    Patrick, I'm not a contractor (actually I'm a happy Steamhead customer, too!), but I am very familiar with the cycling you describe. After the first long burn, my boiler cycles on and off similarly to what you describe (although mine is two pipe and oil).

    Basically, once the steam in your radiators and pipes condenses, you lose your steam pressure and the pressuretrol will call for more steam. Since everything is hot, it does not take long to build the pressure back up.

    My feeling (based upon some cycle measurements I made while manually increasing the cycle time) is that the system would be more efficient if the off cycles were longer.

    I have pondered various options to reach my goal, such as adding a second thermostat in series and locate it near the coldest radiator so the boiler will not fire until the radiator starts to cool a bit. The other option I have pondered is just an off timer that could be set to delay the boiler start-up for 10 -20 minutes after it cycles off on pressure. When I am really dreaming, I ponder installing a PLC so I can really control the operation the way I wish.

    In actuality, the only time I really get the short cycles now is after a setback. Otherwise, the boiler is cut off by the thermostat before it cuts off on pressure.

    Although the short cycling is easily explained, I believe a lot of the energy consumed in these cycles is just used to repressurize the pipes. The radiators stay extremely hot for 20-30 minutes at our house once the boiler cuts off. I plan to borrow a contact thermocouple in the near future and evaluate the cool down rate with and without cycling.

    Good luck!
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