Click here to Find a Contractor in your area.
Welcome! Here are the website rules, as well as some tips for using this forum.

If you've found help here, check back in to let us know how everything worked out.
It's a great way to thank those who helped you.
Need to contact us? Visit

two pressuretrols

Gary SmithGary Smith Posts: 272Member
My 150,000 btu/hr (gross) boiler has two pressuretrols, one originally set at 3 psig cut-in and 4.5 psi cut-out, the second set at 9 psi cut-in and 10.5 psi cut-out.

Following advice in Dan's book I lowered the "lower" pressuretrol to 0.5 psi cut-int and 1.5 psi cut-out, and thte system seemed to work fine.

My question is: what is the second pressuretrol for? And how should I adjust this second pressuretrol?

They are both wired to a Honeywell VS820A regulator which is supplied by a millivolt generator. Thanks in advance


  • Big Ed_4Big Ed_4 Posts: 1,126Member
    Second Pressuretrol

    The second one is a safety if the first one fails. This may be a code in your town. The second saftey usually in a manual reset model that when the pressure reached the higher set point , it will trip and shut down the unit until system is serviced.
    I have enough experience to know , that I dont know it all
  • Supply House RickSupply House Rick Posts: 1,404Member
    2 p-trols

    sometimes these setups are used in conjunction with a 2 stage gas valve for a hi-lo-hi burner. therefore the burner starts on a call for heat using the "hi" gas valve, , and when the pressure rises to a certain setpoint, the 2nd p-trol cuts the "hi" to "lo", maintaining a low pressure sweet spot without cycling the burner.

    this would work best on steam systems with a "lo" of from 2-6 oz. and a "hi" of 6-12 oz. naturally you would need vaporstats to be effective. systems withis this feature have less cycling on and off to maintain the low pressures best for steam. the "lo" is about 75% of "hi" and just has enough gas to maintain the pressure.

    the 2nd one may also just be a redundant safety to cut every thing off is the 1st one fails, but usually the 2nd one has a reset button, and no cut-in and cut-out. does the 2nd one interupt firing if you disconnect the wires?--nbc
  • The second pressuretrol

    is an auxillary control tied into the 750 millivolt pilot generator as a safety that if you exceed the pressure setting of the primary it will open and break the millvolt feed to the VS820 gas valve. This will cause the pilot to go out and totally shut down the system.

    This was done in lieu of dual seated gas valves which were required back in 1979 and up to the present on all boilers and furnaces. The backup pressuretrol allowed powerpile systems to still be used on steam (hot water boilers had a back up limit) boilers even though the powerpile valve is a single seated valve.

    Honeywell and others attempted to make powerpile valves dual seated but the 750 millivolt generator could not support two valve coils.

    The setting on the second is okay as it is.
  • Gary SmithGary Smith Posts: 272Member

    Thanks to all for the explanation, I kind of thought it was a backup, but couldn't easily find a wiring diagram. It's nice to know how it works.
  • Big Ed_4Big Ed_4 Posts: 1,126Member
    Thanks Tim

    Thanks For the lesson..
    I have enough experience to know , that I dont know it all
  • Gary, if you want

    a diagram I can send you one just contact me by e-mail at [email protected]
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!