La Marzocco GS/3 vacuum breaker stuck closed?

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Peppersass
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#1: Post by Peppersass »

GS/3 owners may recall posts from me and others about the infamous vacuum breaker problem: when the machine is turned off on a daily basis, water on the vacuum breaker valve evaporates and leaves deposits that can prevent the valve from closing completely. When the machine is turned on, you hear a continuous hissing sound, which is caused by steam and water escaping from the partly-open valve.

The solution that seems to be used by most, including me, is to run the machine 24/7. That way, the valve stays closed all the time and deposits on it are minimized.

Lately, I've noticed an odd symptom with steam pressure that I think also may be related to the vacuum breaker. There are some occasions when I must turn off the machine. I unplug the machine when lightning storms pass through (just a precaution) or when we leave the house for more than a day. Twice now after turning on the machine again I've noticed higher than normal steam pressure after warm up. I set my steam pressure to about 1.4 BAR, but after warmup it will be about 1.7-1.8 BAR.

Running the hot water and letting some cold water into the steam boiler doesn't correct the problem -- the pressure goes down when the cold water enters, but soon builds back up to 1.7-1.8 BAR. However, if I open the steam wand valve and let steam escape for a few seconds, the pressure drops (as it normally would) then rises and stabilizes where it's supposed to at 1.4 BAR. After that, the steam pressure always returns to 1.4 BAR after a reheating cycle.

The only cause I can think of is the vacuum breaker being stuck in the closed position (i.e., not performing its function at all.) This is allowing excess pressure to build up in the boiler, which gets released when I open the steam wand valve.

Am I on the right track as to the cause? If so, is this potentially dangerous? The highest pressure is within safe bounds (the machine can go to at least 2.0 BAR or higher), but I'm wondering if the situation could get worse.

If the breaker valve is the cause of my problem, then I'm thinking that there's no way to avoid cleaning it on a regular basis.

The only other solution I've heard of for the stuck-open problem is to use a valve with a hose barb on it (available from a number of vendors) and use a silicone tube to route water splashing out of the valve to the drip tray or a tray below the machine. But it seems to me that this solution only prevents water from splashing inside the machine (which is a good thing) but it won't prevent water from falling back on the valve, evaporating and leaving deposits.

Comments?

mitch236

#2: Post by mitch236 »

I can't see how the vacuum breaker could cause higher than normal pressures since it's not involved in pressure regulation, only overpressure relief. Maybe the pressure you see on the gauge isn't correct until you cycle the wand on and off after the machine has sat cold?

On the valve side, i've had to replace mine several times due to the slight unseating you describe after turning off the machine. I've tried cleaning the valve without success. If you figure out something that works to correct that, let me know because it gets expensive!

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Carneiro

#3: Post by Carneiro »

As GS/3 uses a temperature probe at the steam boiler, could the air locked mess with the temp reading and you get a higher pressure to the same temp SV on the PID?

Márcio.

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HB
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#4: Post by HB »

Peppersass wrote:The only cause I can think of is the vacuum breaker being stuck in the closed position (i.e., not performing its function at all.) This is allowing excess pressure to build up in the boiler, which gets released when I open the steam wand valve. Am I on the right track as to the cause?
False pressure occurs when the steam boiler is filled with a mixture of air and water vapor. When heated to a given temperature, air + water vapor exerts greater pressure than pure water vapor. This happens when a vacuum breaker sticks closed, i.e., the pressurestat is "fooled" into turning off the heating element at a lower temperature. Normally a vacuum breaker is open when the boiler heats, allowing air to escape so the steam boiler re-establishes an environment of pure water vapor.

This is explains further in Can someone please explain false pressure?
another_jim wrote:"False pressure" is air pressure rather than steam pressure. Air expands and hits a level that will fool the pstat before the water even reaches boiling. A temperature PID would not be fooled and would not need a vacuum breaker (although the initial steam output would be thin). I have no clue where the air comes from if there is no vacuum breaker and the system stays sealed; but if the pressure isn't from water, it has to be from air.
Stadler wrote:Valves like the steam valve and hot water valves are not designed for negative pressure diff. This means that air will be sucked into the boiler as a result of the vacuum build up inside the boiler during cool down. The air is then vented by the vacuum breaker during heating of the boiler. The vacuum breaker will be open until the boiler pressure is slightly over 1 bar (total pressure).
Returning a follow-up post...
mitch236 wrote:I can't see how the vacuum breaker could cause higher than normal pressures since it's not involved in pressure regulation, only overpressure relief. Maybe the pressure you see on the gauge isn't correct until you cycle the wand on and off after the machine has sat cold?
There are two valves involved in steam boiler operation; the vacuum breaker allows air to enter when the boiler cools and air to escape when it heats up again. The safety release valve allows steam to escape if the pressurestat fails to turn off the heating element.
Dan Kehn

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allon

#5: Post by allon »

Peppersass wrote: when the machine is turned off on a daily basis, water on the vacuum breaker valve evaporates and leaves deposits that can prevent the valve from closing completely.
Where do these deposits come from? Condensation should be pretty mineral-free.
LMWDP #331

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erics
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#6: Post by erics »

Dick -

Your GS3 sets the pressure in the steam boiler by measuring the temperature (PT1000 RTD) of the gaseous mixture in the upper 1/3 (?) of the boiler. If this gaseous mixture contains any air, then the indicated pressure on the gage will be the summation of the partial pressures of the components (steam, air).

Opening the steam valve expels the mixture and recreates a pure steam environment.
Skål,

Eric S.
http://users.rcn.com/erics/
E-mail: erics at rcn dot com

mitch236

#7: Post by mitch236 »

Thanks for the education about the two valves but I still don't see why the pressure would be higher unless the GS3 uses temps to control steam pressure (my machine uses a pressure stat). If the has air pressure causing the pressure to read higher than water then his measured pressure should stay "normal" while the actual steaming ability would be reduced.

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JonR10

#8: Post by JonR10 »

allon wrote:Where do these deposits come from? Condensation should be pretty mineral-free.
Evaporation effectively bakes out any dissolved solids leaving deposits behind.
Condensation has nothing to do with it.
mitch236 wrote:Thanks for the education about the two valves but I still don't see why the pressure would be higher unless the GS3 uses temps to control steam pressure (my machine uses a pressure stat).
The GS/3 uses a PID to control both boilers.....strictly on temperature measurement.
Jon Rosenthal
Houston, Texas

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allon

#9: Post by allon »

JonR10 wrote:Evaporation effectively bakes out any dissolved solids leaving deposits behind.
Condensation has nothing to do with it.
But where does the water come from? If the water in the breaker is condensed steam, there should be nothing left behind. If it is boiler water, then you've got bigger problems.

The only other explanation I can think of for mineralized water to be there is water dripping from newly washed cups which are stacked onto the machine.
LMWDP #331

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JonR10

#10: Post by JonR10 »

allon wrote:If it is boiler water, then you've got bigger problems.
I'm at a loss. Perhaps you don't understand the function of the vacuum breaker....

This valve is mounted to the boiler, and communicates a pathway from inside to outside the boiler. When the valve is OPEN, then there is no steam because the boiler is cold, or just starting to heat up. The valve is normally open, with a small piston held open by a spring.

As the boiler heats, the valve allows air+water to escape until the boiler gets hot enough to pressure the valve and seal it shut by forcing the piston against the seal. At this point, there is some water (droplets) trapped above the seal, and so as the boiler heats further then that water is evaporated.

This cycle is repeated every time the machine is started up, so what happens is small amounts of water get trapped and evaporated repeatedly, leaving behind deposits. Even very clean water has some disolved solids, so unless one fills the steam boiler with distilled water then the breaker will eventually need cleaning.
Jon Rosenthal
Houston, Texas