I have a semi-automatic Livia 90 that I bought used about 3 years ago, in good shape. It had been converted to use for pods, so I bought a new grouphead and converted it back. It worked flawlessly for more than 2 years; I pull about 30 shots a week and never use the steaming function. This past September I had the common problem of a failed pressurestat; when I would turn the machine on it would just heat up until it blew the boiler release valve. I bought a new pressurestat, replaced it, and all was well.

That lasted for about 6 months and then a couple of weeks ago - right after I had descaled the machine - I turned the machine on one morning and it blew the release valve again. But when I turned it off and on it was fine. Watching it carefully it seemed the pressurestat was WORKING in that it would turn the heating element off and on as it's supposed to, but occassionally the machine would heat up to over 1.6 bar of pressure. I thought the problem might be that I needed to adjust the pressurestat, since it seemed to be turning the element on at about 1 bar and switching it off too close to 1.5 bar (the RED on my gauge). So I adjusted it down slightly, and that leads to my question.

When the boiler pressure drops down, the pressurestat consistently turns the heating element on right at .9 bar (where the blue on the gauge meets the green). However, it will often turn the element OFF at 1.2 bar and then sometimes it will let it run up to 1.5 bar.

My questions:

1. Is this just the sign of another failing pressurestat, or could it be some other problem?

2. If it IS a failing pressurestat, should I just replace it again (I have one on order)? Is it common for these to fail this frequently? Could it be some other underlying problem?

3. Have I somehow created a problem in this pressurestat by descaling my machine? And if so, how can I avoid this problem in the future? I've tried to think through how I can descale the boiler but not let the deposits flow out of the boiler and into the pressurestat, and my only idea is to make sure I flush the machine before I let it heat up, but I'm guessing a bit here and definitely out of my element.

I can't figure out what other problem it could be that's making the pressurestat fail. I read a thread where someone had this problem and the Pasquini people "rewired it for them" - and someone in that thread said they probably put in a new relay - but I'm not sure what that means or why it would be necessary. The pressurestat is itself a relay. The heating element seems to be working fine.

Any ideas or helps would be MUCH appreciated. Thanks!

Sounds like a failing pressurestat.
BUT if its a Sirai that has three contacts AND you are using only one contact, then you can switch to another set and be okay -- until that one fails too. Then you can move to the third.

The problem is that the contacts get oxididation layers that increase resistance, which burns the contacts and causes them to stick. The high-but-not-critical temps are probably sticking that resolves under the force of the spring.

If you switch hot and neutral it it may or may not be less likely to stick. At least if one sticks, there is still the one that doesn't stick and it won't be a problem until there is a double failure.

Thanks, allon. The pressurestat is a mater, not a sirai, so I don't think I can switch any contacts.

I'll definitely switch in the new pressurestat when it arrives later this week, but it just seems odd to me that the new pressurestat would fail after only 6 months. Is that common?

.. but it just seems odd to me that the new pressurestat would fail after only 6 months. Is that common?

I wouldn't say its common, but it does happen sometimes.
The Maters are not the best pressurestat made ! ( but fortunately quite cheap !)
There is a thread somewhere about how to "tickle" the mater if it acts up like this...by turning the adjustment up and down a few times....It seems to settle them down again !

Absent scale, Mater pressurestats rarely fail. On the other hand, the microswitch on the side of the Mater pressurestat fails quite often, the internal contacts carbonize and eventually weld together. If you can find a supplier of microswitches with fairly soft springs, a torx screwdriver and 2 minutes is all it takes to replace the switch.

Alan

Reading more on this, I notice that Chris' Coffee sells a Jaeger pressurestat that they say is much more reliable than the mater. Of course, it's also twice as much money. And as beanbag rightly points out, the mater's are very cheap.

Alan raises a really interesting point. I did a quick search and Amazon has a ton of microswitches that look like the one on my mater pressurestat, but does anyone know how to tell which one to get?