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Fix a sticky vaporstat?

Motorapido Member Posts: 307
edited November 2019 in Strictly Steam
Single pipe steam. Honeywell vaporstat as primary control, backed up with a Honeywell pressuretrol for safety's sake. Vaporstat operated fine all of last season and half of the prior season (I bought it mid-way through a heating season). I changed nothing on the vaporstat cut in/cut out before turning on the boiler. Pigtails on vaporstat and pressuretrol are clear, boiler water is pretty darn clean. The vaporstat is set up the same as last year with cut out around 15 ounces. I can't remember the cut-in, but it's set relatively low, according to the suggestions posted by various people on this forum. I had made no changes to the vaporstat or pressuretrol settings before starting the boiler for the season. Yesterday, I turned the boiler on for the first time this season and when the stat called for heat, nothing on the boiler happened. The automatic flue damper did not open. Thus, the gas valve did not turn on. I checked for power with a tester and all the control mechanisms were receiving power. I removed the clear plastic sight plate from the vaporstat and tapped the on/off toggle mechanism with a screwdriver to move the on/off pivot, and power was immediately sent to the flue damper, which opened, and then the gas valve turned on and the boiler ran all day (on and off, of course, according to the stat's calls for heat). But in the evening, I noticed that the temperature was dropping in the house. Upon inspecting the boiler, I decided to remove the clear plastic sight plate on the vaporstat, intending to trigger the on/off with a screw driver. But simply the very minor jostling caused by removing the clear plastic plate resulted in the vaporstat triggering the boiler to turn on. I went to bed, and awoke to a cold house. So far today, each time the boiler cycles off on pressure (alas, some fool who owned the house before bought a grossly oversized boiler, so I live the life of so many who deal with cycling on pressure) after the boiler has cycled off on pressure, it will not turn back on without me manually triggering the vaporstat. Today, all I have to do is VERY lightly tap on the vaporstat exterior housing and the boiler turns on. I don't have to move the on/off pivot with a screwdriver. Just minuscule jostling of the vaporstat housing turns it back on. So, long intro over, can I fix this vaporstat? Could dust have made the fulcrum on the on/off mechanism sticky, and could I clean it by hitting very gently with some compressed air? I have read about all the people in the last few years who have had Honeywell vaporstat failures, and I'm wondering if the condition I describe is indicative of the typical failure symptoms of these devices? Please advise. Either way, I'll probably just order a new one from Supplyhouse, since I don't want the dog and cat to freeze while we're away from the house at work.


  • Fred
    Fred Member Posts: 8,542
    Try setting the "Cut-In" a little higher. That will add a little more tension to the spring. It is possible the diaphragm is failing and not popping back into position.
  • Motorapido
    Motorapido Member Posts: 307
    Great idea, Fred. Thanks I'll report my results.
  • Hap_Hazzard
    Hap_Hazzard Member Posts: 2,849
    edited November 2019
    First make sure that your pigtail isn't clogged. You might not think this could cause your symptoms, but remember that the boiler pressure is higher than atmospheric, so there's more pressure being applied to achieve cut-off than to achieve cut-in, so if the pigtail is plugged, the boiler pressure could still force its way through, while the tiny amount of pressure exerted by the diaphragm trying to return to its normal position wouldn't be able to overcome the resistance unless you fiddle with it.

    If the pigtail is totally unobstructed then you have the fun job of inspecting all the moving parts inside the vaporstat to try to find a point of resistance. I find that if something is sticking just a little within its normal range of motion, it will usually hang up completely if you move it a little beyond that range in one direction or another. You can also try the shotgun approach, and just clean and lube everything that turns or slides and isn't an electrical contact.

    Speaking of contacts, if you find that the contacts are closing but not making contact until you wiggle them, try cleaning them up with a burnishing tool or the corner of a crisp dollar bill. Anything more abrasive won't make them happy.
    Just another DIYer | King of Prussia, PA
    1983(?) Peerless G-561-W-S | 3" drop header, CG400-1090, VXT-24