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Thermostat difference setting

I have a large older home converted to apartments. It is a two pipe steam system.  I have a new Weil McClain boiler.(400,000 input, 325,000 output) I installed new air vents at the end of the steam mains. I am installing a low pressure vaporstat with a pressuretroll back up. Can anyone help me with an ideal differential on the thermostat? It currently turns on when the setting and the actual temperature are both the same. (digital display)  Thanks in advance.

Joe Jesse


  • Jamie Hall
    Jamie Hall Member Posts: 19,954
    Most thermostats

    at least the digital sort don't have a differential setting, as such.  The older mechanical ones didn't, either -- they used an anticpator (simply a little electric resistance heater) which had to be set for your particular installation.  Typically, though, they will look for half a degree rise or so between on and off, and also typically they will turn on when they reach the set temperature.

    You do need to set the cycles per hour, though.  Steam or gravity hot water is typically one cycle per hour.  Other systems are different.

    There is a differential on the vapourstat, though, usually subtractive.  If you set the cut out at, say 10 oz. per square inch and the differential at say 6 to 8, you should be fine, at least for starters...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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