The thermostat reset switch keeps flipping on my FB/70 (aka "a purty Linea Classic"). I figured either my thermostat or pressurestat is malfunctioning so I replaced the cheapest one (thermostat) and it's still tripping.

Watching it today it went up to 2.5bar and then after letting all the steam out returned to about 1.5 bar, where it stayed for a while. It went up to 2, went down to 1.5.... everyone's happy, the coffee tastes good. Then I went out for a few hours and came home to a flipped thermostat

Does it sound like I need a new pressurestat or can this be symptomatic of another problem? Is there any way to test my pressurestat before spending the big bucks?

edit: Sirai pressurestat

You mean you replaced the thermal reset switch not the thermostat right? If your steam boiler went over 2 bar chances are you are overheating it, the safety over pressure valve will open over 2 bar. If the temp went up more than 130C then the thermal reset switch will pop up.
There are a couple things that could cause it, the Sirai Pressure stat is one of them, If your machine even has it. But If you are not familiar I would strongly recommend you get a qualified tech to look at it. These are safety component and you shouldn't skimp on them.

Yeah it's called "bipolar safety thermostat with manual reset 130c". Interestingly it's the exact same part that's bolted to the top of one of the groups.

It's got the sirai pressurestat.

None of the safety valves vented. I suspect the pressure gauge could have been a bit faulty

From what I can deduce:

Pressurestat normal range: up to 1.5 bar
Steam safety valve- 1.8 bar
Pressure required to boil at 130c - 2.7 bar

Does this mean that the safety valve should be operating far before the thermal switch flips, assuming everything is working properly? I might start out by cleaning the contacts, because carbon buildup seems to be a common problem

hugz wrote:Pressure required to boil at 130c - 2.7 bar

See Saturated Steam Table and note it expects absolute pressure, not gauge pressure. When we refer to steam boiler pressure in the forums, it's gauge pressure. You have to add 1.0 to get absolute pressure. So, for example, if your gauge reading is 1.7 bar, that's absolute pressure of 2.7 bar at sea level, or 130°C.

hugz wrote:Watching it today it went up to 2.5bar and then after letting all the steam out returned to about 1.5 bar, where it stayed for a while. It went up to 2, went down to 1.5.... everyone's happy, the coffee tastes good. Then I went out for a few hours and came home to a flipped thermostat

Sounds like the classic description of a failing pressurestat due to a stiffened diaphragm. Search on "sticky pressurestat" for similar reports. Sirai pressurestat rebuild kits are available for around $40, but I recommend full replacement.

Thank you

I'll replace my pressurestat. It's a shame to have unnecessarily replaced the thermostat, but it was in shocking condition anyway

Update for anyone who stumbled on this post by searching for a similar problem of their own:

A new pressurestat fixed it. All good now