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Steam Boiler Short Cycling from Pressure

dccoop Member Posts: 1
My 26 unit building has a 2 year old Smith cast iron boiler, Carlin 601 gas burner, and (much older) single pipe steam radiator system. After endless troubleshooting and a series of fixes I am down to two remaining issues: the Carlin burner is unusually noisy and the system short cycles after about 20-30 minutes of operation.

The noise from the burner is present whenever the device is operating: pre-ignition, firing, and post-ignition. I have scheduled a combustion analysis with a trusted boiler contractor but wanted to see if any others had similar experience with this issue.

The short cycling occurs at about 20-30 minutes into the heating cycle and is caused by the system reaching the pressure cutoff set by the vaporstat. I currently have it set at 16 oz/sqin with a subtractive differential of 14oz. Once the boiler comes up to temperature, the pressure (very) steadily rises from 0 to 16 oz and the burner shuts off. The pressure immediately begins to fall and reaches 0 within 30 seconds, at which point the burner cycles back on. This process is repeated 5-10 times (depending upon outside temperature) until the controls stop the heating cycle.

A few questions on the short cycling:

Is this quick buildup and loss of pressure typical? or is it symptomatic of an issue with the boiler/burner/piping?

Is it feasible to reduce the BTUs from the burner so that the pressure being created is able to be used in the system as quickly as it is being created?

Is it recommended to put a Vaporstat that will allow me to increase pressure cutoff beyond 16oz?



  • Fred
    Fred Member Posts: 8,542
    Has it always done this?

    I assume this is a one pipe system? Has the system always done this since the boiler was installed? Did the installer do a survey of the radiators and size the boiler properly? Either the boiler is over sized or possibly the main vents aren't working.   The quick pressure drop is pretty typical once it gets to pressure. Pressure will rise again fairly quickly during those short cycles Seems like your vaporstat should be set more like 12 oz. main with a differential of about 8 oz. Putting a different Vaporstat on and raising the pressure is the wrong thing to do. The lower the pressure the better. If the boiler is over-sized, it may be able to be down fired some but someone has to do the EDR calculations for the radiators and determine if and how much the boiler is over sized.
  • BobC
    BobC Member Posts: 5,473
    Short cycling

    Has the system venting been examined to see if it's adequate. If the venting rate is too low that can cause cycling as well as an oversized boiler. The boiler can be downfired but there are limits to how low you can go.

    Post some pictures so we can see if there are issues with the boiler and near boiler piping that might be causing problems.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Jamie Hall
    Jamie Hall Member Posts: 22,947
    Your short cycling

    is absolutely typical of a situation where the boiler is somewhat oversize for the system.  Don't raise the pressure -- you're OK where you are.  Raising the pressure won't make a bit of difference, except burn a little more fuel.

    You may be able to downfire that system; check with a really good burner and boiler tech. on that.  It's often possible, and you wouldn't need to downfire all that much; I'd try about a 10% reduction first, and see what happened.  You don't want to go too low...

    I don't know Carlin gas burners.  Carlin oil burners are, however, somewhat noisy.  Nature of the beast.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,576
    Short-cycling with noisy Carlin

    Some burners have an optional noise silencing hood which can reduce the blower noise, so you could check to see if one is available for your Carlin.

    The fact that the vaporstat trips 20 minutes into the cycle leads me to believe that the cycling on pressure is an oversize issue, rather than a venting issue. Check and see if the carlin can be down fired, or replaced by perhaps a Riello which would be more adjustable for firing rate, and quieter to boot.

    You could do an EDR survey of your radiators to see how much oversized the boiler is, and fire the boiler at that new rate.--NBC