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Steam Heat with Subtrative Pressuretrol

JackF Member Posts: 2
I have a Burnham Low Pressure Steam Boiler Model MST396.  The burner has a Subtractive Pressuretrol as seen in Dan's Book on p. 124. The burner technician has set the control for the Differential (left control) at 1. The control for the Main (right control) is set below the marking for 1 and half way to the bottom of the register. I would  comprehend this Main control to be set at 0.5.  I don't understand these settings in order to not exceed more than 1/2 lb of pressure!  Can someone tell me what these settings will do with respect to the operation of the boiler.  Thanks!<a href="http://www.heatinghelp.com/forum-reply/1314107/Subtractive-Pressuretrol-Control">Reply</a>


  • BobC
    BobC Member Posts: 5,470
    it works

    The pressuretrol sets the level at which the boiler will shut off and the pressure it will turn back on - assuming the thermostat is still calling for heat. The newest ones rarely go below 2 PSI while the older ones will go a bit below 1.5 PSI. Usually you set the wheel at one and turn the main down as far as it will go.

    Be warned that the 0-30 gauge on most boilers is useless for reading pressure that low.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • rmoore007ri
    rmoore007ri Member Posts: 45
    Setting your pressuretrol

    I've a new installation with an L4041367 pressuretrol. The main scale's lowest label is "2" but I have it set to where 1.5 would be. The differential is on 0.5

    The system actually cuts out at 1.4 and back in at 0.4 but this was possible only after installing a 0-3 psi gauge. And using that to get the behavior I wanted. You need to keep your 0-30 psi gauge as well (code). And they should go on their own pigtails. Here is one example of what this looks like. There are lots of others here.
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