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Honeywell L404A pressuretrol on a steam system

I've seen this question asked before, but either I'm missing something or my pressuretrol is broken.

I currently have an L404A 1354 (mercury switch) which is set to 0 Main and 2 Diff (subtractive). This makes no sense to me at all, but it's how the installer left it. Exactly how is the system running with these settings? I can't rely on my gauge to read pressure, because it sits at 1.5 psi all the time, whether the boiler is hot or cold.

I understand the goal is to run the system at 0.5 to 1.5 psi. What I don't understand is whether this is even possible with this control. This pressuretrol is only rated down to 2 psi for the main and the diff, so my first question is how can you even use this for steam? Guess at anything below 2?

Second question, when I try to lower the diff, it just bounces back up to 2. The screw will actually pop out of the top, keeping the level at 2. Is this by design or is my control just broken?



  • Mike Kusiak_2
    Mike Kusiak_2 Member Posts: 604
    No sense

    You are right. The current settings make no sense. The differential should never be set higher than the main as the operation becomes unstable, and may not work at all causing an overpressure condition. The calibrations on the face are notoriously inaccurate and the only way to set them properly is with a low pressure gauge and trial and error. For more accuracy at low pressures you need a vaporstat instead of a pressuretrol

    The fact that your main is set to zero may explain why you can't adjust the differential. Try setting the main to 2 and the diff to an "estimated" 1 and see if the screw still comes out.
  • BrianStMarie
    BrianStMarie Member Posts: 24
    Best I could do

    Setting the main didn't have any effect on the diff. The best I could do is in the attached picture. Should be roughly 2.0 with a diff of 1.5ish? I hope...
  • Jamie Hall
    Jamie Hall Member Posts: 20,500
    That should

    work -- as well as it can be expected to.  As Mike said, a vapourstat is a better idea.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
This discussion has been closed.