My steam boiler, which supplies both heat and hot water, turned off a day or two ago, resulting in cold water. After some debugging, I found that the problem was with the pressuretrol, the device that turns the burner off when the boiler reaches operating pressure. It is a very simple device, as shown here:


The brown box is a microswitch. Below it is a lever. The lever is pushed from below on the right side by a piston, which in turn is pushed by the steam. On the left, the lever is anchored to an adjustable spring. When the steam pressure is great enough to pull the spring, the switch opens and turns off the burner.

The problem was that either the lever or the piston was stuck. A very gentle touch with the screwdriver was enough to loosen it, and bliss and hot water was restored.

My question is, do I have to worry about the pressuretrol getting stuck in the closed (“on”) position, causing the pressure to rise too much and blow a pipe or (much more likely) an old, rusty radiator? Should I have the pressuretrol replaced or inspected?

1 Answer 1


If your pressuretrol is truly acting up, I would replace it. I'm sure you've already ensured the pigtail is free of sediment, etc. In my experience as an HVAC controls tech, once mechanical controls start to act up, it's usually more cost effective to replace the item in question than continuing to rely on a compromised pressuretrol. (Murphy's Law is in full effect here).

Nothing like waking up mid-winter for work with zero hot water and a replacement pressuretrol is backordered (no, this has never happened to me before...)

You have a high pressure safety relief for the boiler if in fact the pressuretrol did stick on, so you're covered from catastrophic events - however ensuring your BMS is working properly is more important. Plus if that safety lifts, if the overpressure in the system hasn't damaged any control valve(s) - you're still gonna have a huge mess on your hands.

Basically - don't rely on built in safeties to protect you from out of bounds conditions you can control :)

  • +1 for proper double application of murphy's law (fixing easily replacable parts is not worth the hassle, and the store will be out when you need it the most)
    – longneck
    Aug 26, 2010 at 16:39
  • Response from OP: Eventually it did get stuck in the "on" position, and the safety valve opened at ~15 psi. Accept for mentioning sediment in pigtail. I had checked the part of the pigtail close to the pressuretrol. After replacing the pressuretrol didn't fix the problem, I took the rest of the pigtail apart, including the pressure gauge and the pipe to the low-water shutoff switch. It was completely clogged by gunk. Clearing it (with much effort, especially in the loop that I think is called a siphon pipe) fixed the problem. Dec 10, 2010 at 19:36
  • Thanks for the feedback - nice to know what the root cause really was
    – kkeilman
    Dec 11, 2010 at 0:17

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