Why does the idle temperature drop during the day?

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#1: Post by klino »

This thing has started recently. I have my timer turn on the machine at 6am, get up at 8am and the idle temp showing on eric's device is around 208-209 degrees, like it has always been. I make the habitual 2 shots, clean the machine and leave it on usually for the rest of the day (10-12 hours). The temperature then slowly goes back to around 209 degrees.

Now recently I have noticed that around 12 pm, the temperature starts dropping very slowly during the day - today at around 2pm it had reached 194. I can get it back to the usual idle temp pretty fast (about 15 minutes), just by flushing a few ounces through the grouphead (btw no water-dance, so it's not the device being out of order), but it's still quite annoying if you're in a hurry. I haven't tried to see how far down it would drop, as I usually will make a new couple of shots at the same time of the day when the temp is around 194.

Has someone had similar problems (if this is a problem...) with their Andreja Premium or other machines, and what might be the answer?


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#2: Post by Beezer »

I had a similar problem with my Anita, which is almost the same as your Andreja. It turned out to be a leaky seal in the OPV that was apparently allowing water to leak out of the HX circuit and preventing the usual thermal convection from heating the group. Replacing the OPV seal fixed the problem, but I also replaced all of the seals in the group because I wasn't sure which one was leaking. They were all starting to fail, so it was probably just as well that I replaced them.

Here's the thread where I report the problem, but I guess I never posted the solution.

Help diagnosing false pressure (?) / temperature issue
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#3: Post by HB »

That was my thinking too; Randy has a great writeup that explains: Understanding and Preventing Thermosyphon Stall.
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#4: Post by decaf_Ed »

I've enjoyed some self-inflicted wounds on this topic, in the form of adding local high points to my thermosyphon loop. I've gone so far as to add bleeder valves to get an instant-gratification remedy. Having had so much interaction with thermosyphon stalling, I feel compelled to write something, if just for my own therapy.
The article and graphics by Randy are great. I'll offer an alternate phrasing of the phenomena.

There is one common cause of thermosyphon stalling: loss of pressure in the thermosyphon loop.
The loss of pressure can have many possible causes: leak here, leak there, etc.
The gas bubble, initially at least, is not air (nitrogen, oxygen, etc.), it is pure water vapor.
Many HX machines will re-pressurize the HX thermosyphon loop when the pump kicks in to top off the boiler. This isn't the 9 bar pressure of a pull, this is the 1 bar or so of pressure that's in the boiler. Re-establishing pressure will cause the water vapor to condense and the gas bubble will disappear (if it can fill in before the pump turns off), but only if the temperature around the gas bubble has not dropped too much. Usually by the time the effects of the stall are noticeable, the temperature around the bubble has already dropped too much for water to condensate at just 1 bar, and topping off the boiler does not remedy the problem. At this point you resort to flushing, or more extreme measures.

In my (extreme) case, flushing didn't always flush the bubble, hence my resorting to bleeder valves. I only open the valve(s) while the pump is on, to not risk getting air (which will never condensate back to liquid) into the thermosyphon loop. So the hot-water dispensing valve on my machine now has a purpose: to incite the boiler-fill exercise... either to bleed, or to prevent it (the fill operation) from happening during a shot.

I feel better already.

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#5: Post by BradyButler »

Thanks to all for an educational post on thermosyphon stalling due to flash-boil.

I'm curious about whether this would occur on hard-plumbed small commercial-style machines as well?

I ask because it's my understanding that the water within the heat exchange loop on these machines is at incoming line pressure - 2-5 bars ish... so well above the 1-1.2 bar pressure within the boiler. Wouldn't the boiling point at these elevated pressures be higher than the temperature achievable within the boiler?

I'm actually quite certain that the water within the hX system on these type of machines is at line pressure, since there are no valves upstream. Its easy to tell, because when a machine like this is connected to water for the first time the heat exchanger system fills before power is connected.

This seems relevant to this discussion because if one was experiencing this problem on a hard-plumbed commercial-style machine the cause might be different - perhaps an obstruction in one of the orifices within the system.

Like I said, great discussion... just trying to get a little more info for those running slightly different machines.
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Carolina Espresso Services - "Quality Service for Espresso Equipment"

klino (original poster)
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#6: Post by klino (original poster) »

Thanks a lot guys,

The illustration from Randy gives me a clear idea of what's going on, and it makes sense, since I haven't yet replaced any seals on my now two-years old machine. So that's my next move.

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#7: Post by cannonfodder »

I dont think you would get thermosiphon stall on a plumbed in machine. The HX loop is always under pressure from the mains. If you had a small leak somewhere the HX loop would not develop a vaporlock because of the positive pressure from the mains. Water would flow back into the loop from the mains as it leaked out. On a reservoir machine there is no positive pressure and you are relying on the thermodynamics of the loop to keep the water circulating. I have had my tank machine at work stall before but my plumbed in machine at home has never had that issue and it has quite a few years on it.
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#8: Post by Richard »

cannonfodder wrote:I dont think you would get thermosiphon stall on a plumbed in machine. The HX loop is always under pressure from the mains.
With a caveat: There are machines "out there" which have a plumbing connection and a vibe pump, e.g., an Andrea Premium with the direct-connect kit available from the vendor. Those machines do not have an HX loop under pressure from the mains.
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#9: Post by erics »

Those machines do not have an HX loop under pressure from the mains.
Because those machines have a solenoid valve (as part of the "kit") that operates in conjunction with the pump, AFAIK.

Eric S.
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#10: Post by cannonfodder »

Vibe pump machines that are plumbed in are typically an after market config. The water line gets a solenoid that shuts off the mains pressure when the machine is not actively brewing or filling the boiler. That would obviously remove the positive pressure on the machine. Rotary pump machines will take the line pressure without issue.

A funny story. I was working on my very first plumbed in rotary machine. I wanted to take the group bolt out to check for scale and clean the filter in the group. I turned off the machine and let it cool down for an hour. It was still a little warm but not hot. I started to unscrew the group bolt and a little water started trickling out. Figured it was just residual pressure so it let it dribble. A moment later it was still dribbling. So I thought that turning the bolt a little more to open the threads up would let the pressure vent faster. It kept dribbling. That's odd, I thought. So I unscrewed it a bit more and took the bolt out. Then I had a fountain of water spraying everywhere in the kitchen. After a moment of what the #$%^$ is going on?!? It dawned on me. This is plumbed in, you did not turn off the water. That is the mains pressure doing a garden hose impression. Last time I ever did that.
Dave Stephens