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Pressuretrol setting drift?

Charles_8 Member Posts: 74
Last year the PA404 pressuretrol on my boiler kept "drifting" to the point where the burner would not light even at zero pressure and with the thermostat or aquastat calling for heat. About every month I had to give the adjustment screw another 1/4 turn, and then it would work for a while.

(I thought there might even be some kind of one-way blockage in the pigtail but it is wide open).

Finally I replaced the pressuretrol, although I inadvertently ordered the wrong one (3-15 psi cutout instead of the 0.5-9 psi cutin). It was working properly when new at 3 psi cutout, 1.5 differential (the minimum recommended cutin setting is 1.5 psi), and my system doesn't usually build pressure above .5 psi anyway, so I installed it and forgot about it. Until last night!

The house got chilly (it's still mid-30's overnight here in northern Maine) and there was no hot water... so I went to the basement and tweaked the adjustment screw. There was obviously zero psi (verified on both the useless 0-30 required gauge and my 0-10 added gauge that does indicate low pressures correctly). When I turned the screw so the pointer said 3.5 psi the boiler kicked on, but quit again when unscrewing it to 3 psi again. It properly shut down when the aquastat hit 180.

So why do these units keep drifting? It's not that the screw is turning itself since the pointer doesn't move unless the screw is turned. Does the spring stretch, is there an internal mount that comes loose, or what? I'm reluctant to spend the $$ on the proper vaportrol if it's going to behave the same way!

Is there a solid-state replacement that eliminates the mechanical differential and moving parts?



  • Big Ed
    Big Ed Member Posts: 1,117

    Not normal to find bad pressuretrols .I change very few..Replace the pigtail and all connected fitting leading to the boiler.Which you did not mention. Make sure mud is not plugging the boiler tap..Replace the high end pressuretrol the supply house off loaded on you for the correct model . The three pound kick in will hold shut your steam vents. A vaporstat would be a better choice..

    Electronic unit ?
  • Charles_8
    Charles_8 Member Posts: 74
    Pigtail is open

    Thanks for the tip. But it's definitely not plugged. I checked that when I had the old one off and it was wide open...

    I agree that the 0.5-9 psi one would be a better choice, but even the 0.5 psi cut-in can't be set to cut-out much below 1.5 psi. A vaporstat is the preferred control since a few ounces of pressure should be plenty in my small 2-story house. They're expensive though.

    But I don't understand how a plugged pigtail could change the adjustment of the pressuretrol in the *high* direction, anyway.

    Tell me if this makes sense: If it were plugged up, the boiler pressure would rise but the diaphragm wouldn't see it, so the adjustment would need to be lowered, not raised, to make it cut out at the desired pressure.

    Also wouldn't the boiler run too *long* (pressuretrol not seeing the pressure and shutting the burner off) rather than being "locked out" even with 0 psi in the cold system after an hour or two?

    So again, what would make the adjustment keep changing and always in the same direction? That has to be something bending or coming loose in the differential assembly. I can't think of anything else...

  • Larry C_3
    Larry C_3 Member Posts: 4
    Furnace vibration?

    Home owner here. Is the pressure switch subject to vibration? Could it be that the adjustment is backing off due to the shakes?

    When the furnace is operating, does it vibrate, shake, rattle, buzz, hum, beat on the pigtail? Is the fuel line attached to the pigtail? Is the pigtail mounted vertically?

    Can you you post pictures of the miscreant?
  • Charles_8
    Charles_8 Member Posts: 74
    Not a vibration...

    The adjustment screw is not turning on its own, as you might expect if vibration were the problem. The pointer on the front of the pressuretrol stays set to the same line.

    The actual setpoints appear to be drifting upwards. That's why I think something inside the pressuretrol is changing. You'd have to really vibrate the snot out of this mechanism to get it to change settings, and it doesn't feel that way to me...

    It's a pretty standard PA404/pigtail/boiler but I'll see if I can post a pic later tonight.

This discussion has been closed.