La Marzocco GS3 MP brew boiler pressure shows 9 bar. Why?

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#1: Post by koonyue »

Hi all,

I just upgraded to gs3 from HX machine so very excited! One question is why the brew boiler will show 9 bars? The temperature of the brew boiler is lower than the steam boiler but steam boiler only have 2 bar.

Is it because of the smaller size of brew boiler?


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#2: Post by HB »

The steam boiler pressure gauge is lower because it's reflecting the steam pressure, not the pump or water expansion pressure of the brew pressure gauge. The maximum pressure shown for the brew boiler will be the set point of the expansion valve, typically 12 bar (NB: the expansion valve pressure setting must be higher than the pump pressure, otherwise water would escape from the expansion valve every time the pump was engaged).

At idle, the brew pressure gauge will typically not stay at zero. That's due to the water expanding when heated in a closed system. The longer answer from What is normal pump pressure at idle? in the FAQ:
HB wrote:The pressure reading when the machine is idle isn't important.

To elaborate, pulling a shot introduces cooler water into a closed system, but then you suddenly close off the main exit at the end of the extraction. As the water in the boiler [or heat exchanger] warms up, it expands and the pressure rises. The expansion valve is typically set to open around 12 bar for rotary pump espresso machines since they have their own bypass valve to regulate brew pressure. For vibratory pump espresso machines, the expansion valve (traditionally called an over-pressure valve in this context) is set to open at the desired maximum brew pressure, usually around 9 bar. This valve vents water into the driptray to avoid subjecting the hydraulic system to excessive pressure. It only needs to let a few drops of water escape to reduce the pressure sufficiently.
Dan Kehn

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#3: Post by Peppersass »

Dan's answer is correct, but I'd add a few things.

The 9 BAR you see on the coffee boiler pressure gauge is primarily a function of the water temperature and the pressure from the last time you ran the pump. When you stop the pump, the closed 3-way valve on the output side and a series of check valves on the input side stop water from escaping the boiler and lowering the pressure. The heater comes on to compensate for the cold water that's been introduced, causing the pressure to rise, at which point the expansion valve, when properly adjusted, limits the pressure to 12 BAR. When the heater goes off, the pressure quickly falls to about 9 BAR. As the machine sits idle with the heater off, the pressure will gradually drop to about 6 BAR, at which time the boiler temperature has fallen below the set point, the heater will come on again and pressure will rise to about 9 BAR. I'm not sure if it's a happy coincidence that the pressure tops out at about 9 BAR when the boiler hots the set temperature while the machine is idle, or if it's designed that way.

When you turn off the machine and the water cools, the residual pressure from the pump and water temperature are long gone and the pressure will drop to your line pressure, if plumbed in, which is typically around 3 BAR. This happens because the 3 BAR line pressure is enough to open the check valves to let water flow into the boiler. It has no place to go, so the pressure remains at 3 BAR. The pressure will drop to about 0 BAR if you use the reservoir because the check valves will remain closed (I think there can be a bit of residual pressure in the boiler when using the reservoir, but it's been years since I ran my GS/3 off the reservoir and I don't remember.)

The steam boiler, in contrast, never gets to the 9 BAR pump pressure because it never gets completely filled with water. Even if it did, the Over Pressure Valve (OPV) will open at 3 BAR and vent the boiler into the drain box as a safety measure. Although the target water temperature for the steam boiler is much higher than the coffee boiler, the pressure increase is limited because the steam compresses, whereas water in the full coffee boiler does not compress. If the steam boiler temperature is set high enough, the pressure will continue to rise as the steam compresses more and more, but at 3 BAR the OPV will open and vent the excess pressure.

koonyue (original poster)

#4: Post by koonyue (original poster) »

Wow! Thanks so much for the detailed reply !

So I understand now, the tricky point is steam boiler is never fully fill, while brew boiler is always fully filled in both the boiler and the group head, so result in a close loop environment and being fully fill with water, the 9 or 12 bar brewer boiler pressure is caused by the steam and most importantly, the expansion of water while being heated up.