Commercial espresso machine startup, pressure building issues

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#1: Post by number9 »

I have rebuilt a La San Marco 85-16M-2 (you can read about it here La San Marco 85-16m-2, is the rebuild worth the taste?).

The manual notes that from a cold start, a steam wand should be open until it starts hissing, and then closed. The machine also has a vacuum breaker valve on it, and it is brand new.

The issue is that every now and then, I forget to open the steam wand. Something odd happens though, not only does the machine build up false pressure (so when the wand is opened, the pressure goes to zero instantly and it takes another 10 mins to heat up again), but the pressure gauge for the group head pressure goes to about 7 to 9 bar. This is a bit baffling. Looking over the schematic, I do not see how this is possible, or why. If a steam wand is opened after the machine was started up with all wands closed, the group head pressure does not change. The manual fill or brew button must be pressed for the group head pressure to go back down.

Two questions: 1) I was under the impression the vacuum breaker valve would act the same as the open wand when starting up. Is this not the cae? (e.g. why the need to open a steam wand?) 2) Why would the group head pressure gauge rise if the steam wand is not open during startup?

Thank you.

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number9 (original poster)

#2: Post by number9 (original poster) »

Updating this issue, this seems to be a real problem.

Yesterday after turning the machine on, I came in to see the grouphead pressure gauge at 12 bar! The boiler tank pressure gauge was at zero.

Today, I decided to try to isolate the issue. It seems that either the check valve (202201 on the diagram) or the autofill solenoid (108035 on the diagram) is getting stuck, although there is no absolute proof that it is either one of these items. Turning the valves off that go to the autofill and then turning the machine on, the pressure on the grouphead does not rise like it has been. This would seem to indicate the autofill solenoid has a problem (grounding the autofill probe wire makes it click, however, so the circuit seems to be working, perhaps it is getting stuck open when the machine cools?).


Second issue, the front drip tray had been installed previously. During this test, the drain box (106056 on the diagram) was visible, and the OPV (502008 on the diagram) seems to be leaking (out of 607107 on the diagram into the drain box) right when the machine starts to come up to temperature, but nearly stops after it is fully heated. Do OPV valves leak like that normally? Time to rebuild?

I am attaching a picture of the autofill circuit to this post.

Thank you for any assistance.

New information: It seems when the machine brews, the grouphead pressure gauge goes up to 9bar, then falls back down to 4-6bar. Engaging the brew switches does not release the pressure when off. However depressing the manual autofill releases the grouphead pressure and it falls back down to 2.5 to 3 bar. This seems to indicate the system is not releasing this pressure. How would one proceed to figure out what is wrong? Is it the safety check valve? Is it the three way solenoid?

number9 (original poster)

#3: Post by number9 (original poster) »

After looking into this further, perhaps it would be better to start over:

Forgetting what happens when the machine is turned on, the following is happening, and I suspect it is a "bad thing". Does anyone else here have a large HX machine with grouphead pressure gauges and boiler pressure gauges that can comment?

When the machine is sitting at idle after all false pressures are released, the grouphead pressure gauge is right at 2.5bar, which is fine as that is the water line pressure. If either grouphead is activated, the grouphead pressure gauge goes up to 9 bar. When the grouphead stops, the grouphead pressure gauge only falls to about 4.5-6 bar. The only way to get this pressure reading to drop back down to 2.5 bar is to activate the manual fill lever. It then immediately drops to 2.5 bar.

I am under the impression this is not normal. Is that correct? Does anyone have an idea of what could cause this problem on a LSM 85-16M-2?

Thank you.

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#4: Post by cannonfodder »

Couple things. If your machine is cold the brew pressure will increase as the cold water in the hydraulics expands as it heats from radiant heat on the machine. So a brew pressure gauge that goes from 0 to 12 is not unusual. There is a vent valve in the brew path that will open to bleed pressure when it gets excessive. 12 bar is normal, once you activate the brew solenoid and run some water thought it, it will normalize (give it an hour to heat). If you look at your drip catch tray there will be a tube connected to it. That is your vent line. That dripping as it heats is normal. Remember this is a commercial machine. It is designed to be turned on and left, not on/off/on/off etc... I turned my 2 group off about twice a year. Vacuum breaker should prevent most false pressure but if it sticks closed it will obviously not work. I assume that breaker valve is installed into the top of the boiler or inline with the steam wand.

What you do have to worry about on old HX machines is when your boiler pressure shoots up when you run the brew group, or it overfills when you brew. That is a cracked heat exchange of fill solenoid which it sounds like you are not having an issue with.
Dave Stephens

number9 (original poster)

#5: Post by number9 (original poster) »

Cannonfodder, thanks for the reply.

Glad to know the dripping on warmup is normal. Yes, I was using the original, and it needed a replacement. I just received it, so the opening of the steam wand is not an issue as of this writing. I agree, I do not think I have any HX cracks or fill solenoid issues.

One thing of concern is that even if the machine is left on for a several hours the pressure at the grouphead does not equalize. After bewing it will remain high (4.5-6 bar), but pressing the manual fill will knock it back down to water line pressure (just under 2 bar at the time of this writing).

I feel that this is pointing to something amiss. Oddly, it happens no matter which grouphead is used. Does your machine exhibit the same behavior, or does your grouphead pressure equalize totally and it always goes back to water line pressure after brewing?

Thank you.


#6: Post by gor »

Why would it equalise?

Water can't flow backwards past the fill solenoid or the manual fill valve, unless you have a leak past those two points.

Water is just expanding due to it heating up, that is the pressure rise you are seeing.

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#7: Post by cannonfodder »

It will read higher than the line pressure. Not an issue, unless it jumps to 15 bar and something pops.
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number9 (original poster)

#8: Post by number9 (original poster) » replying to cannonfodder »

So this behaviour still makes me wonder: after the machine has been on for an hour, if a shot is brewed, the grouphead pressure goes to 9bar during the shot, but after the gauge still reads over line pressure (for some reason it has been behaving more lately), around 3-3.5bar. Is that normal? Why after it is warm would it not read line pressure at the grouphead after brewing?


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#9: Post by kolu »

because you just pushed cold water in. it will warm up and expand. so your gauge will read a higher pressure, than line pressure.

number9 (original poster)

#10: Post by number9 (original poster) » replying to kolu »

While I agree with this statement from a purely thermodynamics point of view, there are conditions in my situation that would not make the gauge behavior accurate for just thermodynamic expansion.

My observations:

My unit is in the basement, the water coming in from my mixed RO system is just under 60 degrees F.
If both groupheads are activated and left on for say 60 seconds, all of the water in the fresh water line would be evacuated and replaced with new 60 degrees F water.

At that point, we would expect the gauge to fall to say 2 bar and then climb back up, as the cool water expands. But this is not what is observed. If the machine is left on for an hour to come up to temperature, the grouphead pressure gauge will sit at about 3.5 bar. If the grouphead brew are activated (both of them) for 60 seconds, the grouphead pressure gauge goes to 9 bar during this time, but then goes right back to 3.5 bar after the grouphead brew is stopped. We would expect it to go down to 2 bar (line pressure) and then go back up to 3.5 bar as the water expands, correct?

I am just trying to be sure what I am seeing is not say the one way check valve leaking or the autofill valve not closing all the way.