The Pressure Relief Valve

Pressure relief valve (safety valve)

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Some unusual faults on a few prosumer espresso machines recently, prompted me to write this article, and the entire article should be read VERY carefully. Ensure you understand the operation of this component and how your machine should work, before jumping to any conclusions.

This valve should not be confused with the vacuum breaker valve, I stress this because I have seen articles on the web mistakenly identifying it as the vacuum breaker valve. The purpose of the pressure relief valve is to vent the boiler in the case of a fault situation. There are 3 main faults that could cause this to operate (as it should). All of these faults can potentially raise the boiler pressure to the 4 or 5 bar it takes to trigger one of these.

  1. Failure of the pressurestat causing the boiler to overheat (if an overheat protector or trip switch is fitted, this would have to fail as well)
  2. A fault in the boiler autofill solenoid causing it to continue filling the boiler whenever the pump runs e.g. when pulling a shot, this is often caused by a small particle or lime scale, sometimes a faulty autofill solenoid valve
  3. Scaling of the autofill probe, or a wire on the autofill probe coming loose, causing the boiler to overfill

In the first case the valve would simply vent steam/water vapour (and the boiler would show a much higher than normal pressure reading), in the last 2 cases; water will spray forcefully out of the vent holes and make quite a mess. If the fault was intermittent overfilling of the boiler it would be quite noticeable when steaming (lots of wet steam) and if small amounts of water (1oz) was drawn to heat an espresso cup, even after waiting 30 seconds the boiler autofill certainly wouldn't operate.

So really this valve should never operate and because it never operates….rarely gives trouble. It also has a very simple construction:

The complete component on it's own and in place on the boiler of a typical espresso machine



1 = cap with the vent holes
2 = spring to keep the tension on the seal
3 = sealing plunger
4 = white Teflon (or plastic) disk
5 = base that screws into boiler, disk presses up against flange at top to make seal

So if it almost never fails…why the article?

Well it always helps to understand your machine and knowledge of how this component operates is useful in diagnosing faults with other parts of your machine, such as the autofill etc. but recently a few odd faults resulted in me examining one of these devices.

Here is what happened

A small number of people were reporting these valves continually hissing and spluttering and sometimes a small amount (a few teaspoons) of water was collecting on the same side of the machine as this pressure relief valve. This water was the condensate from the continual steam venting. This collecting of water would not happen suddenly it would be a gradual process. It was certainly a different behaviour to the 3 fault conditions described above. This is NOT the same as the normal hissing and spluttering you will get from the vacuum breaker valve as the machine comes up to pressure from a cold start. The vacuum breaker is sealed by the pressure in the boiler; this pressure relief valve is sealed by a spring. Very often faults in espresso machines are caused by scale, but in the case of this valve it's not likely to be the cause of problems. The reason scale doesn't affect this valve is because it's never open unless you have previously (or still have) one of the 3 faults mentioned earlier, then of course the valve could scale up and not seat properly. Initially the manufacturers assumed it was lime scale jamming the autofill solenoid, but I wasn't convinced as the particular faults I was informed of, did not show the characteristics of the 3 fault types mentioned earlier or lime scale damage as the cause. I asked for a few new ones to be sent to me and got my hands on a failed one, because I wanted to examine them more closely.

This is what I found:


Shows the spring to be bent at an angle, I suspect the spring caught in the vent hole on the top of the fitting during assembly and was bent to this shape, which would also account for the stress fractures. If the spring had been manufactured this way, you would NOT see stress fractures, so it seems reasonable that this could only have happened during assembly of the fitting! Equally, simple aging of the spring would not cause it to bend in this way, it would only case a slight shortening in length.


Shows the two springs together and you can clearly see the difference, what is not obvious from the picture is that the hysteresis of the spring has been affected and that for the first 0.5 mm or so of travel, there is MUCH less spring tension. This means that the spring might appear OK on a simple kg force test after manufacture, BUT that the Kg force required to operate the spring during the first 0.5mm of travel can be much less than for the rest of its travel. I tested both springs using a balance weight system and with approx 5lbs of pressure, the defective spring assembly moved quite easily and enough to allow the seating to fail, whereas the assembly that was not defective, did not!

The good, the bad and the ugly


This defect in all probability occurred during manufactures assembly of the valve (unless the espresso machine manufacturers assemble them). As the spare units I received seem to be OK, I would imagine (fortunately) that only a small number of valves are affected. The brand new units (or at least the spares I had, were remarkably easy to split, whether this is because no thread lock has yet been applied or whether none is used I do not know. Certainly significant force was needed to disassemble the faulty valve (possibly thread lock or scale in the threads?). I personally use PTFE tape when reassembling such fittings or refitting them to the boiler.

The faults came to light on some Izzo machines and the retailer is checking every stock machine with such a valve, and also their entire stock of spares with immediate effect, simply because it's not worth taking the chance. I advised them to contact the machine manufacturer and the pressure relief valve manufacturer, because these valves are used on many different makes of prosumer espresso machine . I first heard of this fault some months ago, so probably valves manufactured before June/July 07 are fine and of course one would hope that by mid Nov 07 information reaches the manufacturers and other people concerned and we don't see the problem again.

How many of these valves are affected?

Who knows, it could be a very small number indeed, possibly no more than the few that have shown faults, possibly the faulty one I disassembled was the only one like that and the other machines had some really odd fault that looked the same?

So are you affected, because this is always going to be the first question? The easy way to tell is:

  • If you have had a gentle hissing and possibly some dripping from the pressure relief valve, NOT the vacuum breaker valve. This has been happening on a newish machine (less than 3 months and has happened from the day you got it).
  • If you have read this article carefully and feel confident you understand the faults being discussed

You probably don't have this specific problem (you have a different fault) if:

  • If you have had the machine a while, never descaled, have problems getting dry steam because the boiler is overfilling
  • Have a spray of water out of the pressure relief valve whenever you operate the pump, or a pump that never stops running until you switch the machine off or it shorts out.
  • Water is vigorously jetting out of this valve

If you are among those few with a defective part, simply ring your retailer, describe the problem and if you feel competent the part is quite easy to fit. It's also a very low cost part (£6 or £7), for anyone who has a machine without a warranty