It happened once, then a few weeks later, then just about every day. My Expobar Office Control's high temp limit switch (the sensor on top of the boiler) keeps tripping. The little red button at the top pops, I have to open up the machine and reset it. I ordered a new replacement and swapped it out this week, but sure enough, the next day, it was tripped again! Each time it happens, the boiler stops heating and I come downstairs to find a cold machine. I lowered the boiler pressure to 1 bar (measured with a gauge on the steam wand) and this did not help either. What else could I try? I've searched the forums here and elsewhere and have come up short.

I previously thought it was happening while the machine was first heating up, but now after observing for a couple of months, it only occurs when the machine is left on for a few hours or more, and pretty randomly. I have sometimes found the machine way over temp but had not tripped yet, it took forever to do the cool down flush from the group head and the steam pressure was double the norm. I figure I caught it just before the overtemp sensor tripped. What tells the boiler to cycle? Could there just be scale buildup on the sensor inside the boiler (if there is one) that is giving false readings and telling the machine a lower temp?

Thanks!

It sounds like the pressurestat is sticking and the high temperature sensor is cutting power. That's a good thing. Search on "sticky pressurestat" for similar reports. Below is my latest reply on the subject:

HB wrote:Pressurestat contacts may overheat and stick together (fail closed). Or the contacts may be coated with carbon (fail open). The diaphragm may harden (fail open). The Sirai pressurestats have rebuild kits; all others I'm familiar with are replace-only. Pressurestats are a wear item and will eventually need replacing.

Pressurestats are not expensive and are easily replaced. How to adjust an espresso machine pressurestat offers hints on how to identify the make/model of your espresso machine's pressurestat.