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Thread: VBM thermosiphon problem?

  1. #1
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    VBM thermosiphon problem?

    Gene Cafe Coffee Roaster $850 - Free Beans Free Freight
    Im not too sure what is going on, but again today the group of my VBM started to cool off after being idle for about an hour at 101, then stopped at about 70 degrees. The boiler is up to pressure, everything else seems fine etc.. so I cant seem to work out whats going on.
    It takes quite a bit of flushing to get it back up to temp, its not all that much of a problem (unless I dont have the TC connected, and brew a cold shot, but its obvious its cold if there is no water flashing when you do the cooling flush after it has idled a while)

    It seems you can flush it for a second or two and it will start to heat up as normal to 101 over the next few minuites
    This has happened 3 times now and I cant understand what is doing this or why. Is it something I am doing wrong, or is it the machine?
    Or is this a common thing with HX machines.

    Any suggestions?

  2. #2
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    Re: VBM thermosiphon problem?

    No responses? Ill have a go.

    I dont really know but I read some things in the past that might be related to your problem. From reading parts of the following two URLs, and from what you said, to me it sounds like the boiler is slowly losing water and it is not being replaced. When the boiler water level drops enough, the thermosyphon stops working(this is my understanding on how it works - could be wrong). When you "flush it for a second or two" the boiler fills up enough for the thermosyphon it start working again. The drop in water level could be from some slow leak in one of the valves - or maybe it is just a normal drop in water level and the problem is with the boiler refill not functioning adequately.

    URLs:
    http://www.home-barista.com/forums/odd-expobar-problem-no-thermosiphon-t538.html
    http://coffeetime.wikidot.com/local--files/opv-over-pressure-valve/OPV_Valve.pdf

    Note that the above are from different machines to yours but the operating principals should be similar.

    So what to do about this? Probably just call your supplier and ask them for advice on what to test and what they recommend you do. I am just guessing and could have this all wrong. A supplier/repairer will probably be able to diagnose this very easily. On another thread I think you mentioned that you got the machine from Chris a short while ago so I am sure he would be happy to help you. Then tell us what you did to fix it.

    Other than that, maybe someone else can suggest what to test and look for. If you want to try searching for more info on this problem, also try this spelling: thermosyphon

    fabcat

  3. #3
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    Re: VBM thermosiphon problem?

    Apols,

    I must have missed the initial post John,

    If you think that all is not well, bring it in and Ill use it for a day and see what I can find....

    Chris

  4. #4
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    Re: VBM thermosiphon problem?

    Thanks for the help everyone.

    Sorry its taken so long to get back, just got the internet connected here a few days ago.

    It has not happened again since moving house, so I wont take any "drastic" measures yet (meaning anything that involves me not having the machine for any amount of time)

    I had a quick read through the links and it sounds similar to a sticky boiler fill valve and/or vacuum breaker valve, as each time it did happen, it took a while for the HX to prime/fill (with only steam coming out the group) before the pump made the proper (loud) noise.

    If it happens again, Ill have to work out where the water that was in the HX on power off went. But ill just hope it dosent, and that it was just a small piece of dirt or something in the valve that flushed itself out.

    John.

  5. #5
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    Re: VBM thermosiphon problem?

    Suggest neither of the 2 valves mentioned has anything to do with it.

    Take the machine in to the service provider to be checked over as this is not normal behaviour.

  6. #6
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    Re: VBM thermosiphon problem?


    The thermosyphon/HX part of the machine should be a closed system. The thermosyphon will only work if it is fully charged with water. Any air or steam in the system and it will stop. Operating the group should expel any air/steam and re-prime the group. If it happens again then there is probably a slight leak somewhere that is allowing some water in the HX/thermosyphon circuit to escape probably as steam. So youll need to track down the leak.

    Thats my best guess.

    Cheers,

    Mark.

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    Re: VBM thermosiphon problem?

    Quote Originally Posted by Sparky link=1190785412/0#5 date=1192272931
    The thermosyphon/HX part of the machine should be a closed system. The thermosyphon will only work if it is fully charged with water. Any air or steam in the system and it will stop.
    Are you sure? My brain agrees my machine does not...
    I have noticed that with the HX not quite full (ie. when it cuts out due to low water when flushing, if you leave the lever up for longer than a second or two without the motor running, or put the lever in the brew position just before it activates the motor and let a bit of water out the group) the group heats up very quickly to 105+ when it normally sits at 98 idle and 92-96 when in use.
    Remembering this machine is running off a tank it has no mains pressure to keep the HX/thermosyphon path primed in these situations. Normal or is my machine doing something it shouldnt?

    Anyway, back to the cooling down problem, still has not happened, even when I was playing with the timer and left it on for 16 or so hours accidently. So It has only happened about 4 times, over the space of 3 days. Then nothing for the next 2+ weeks still fine.

    Also, is it normal for the boiler to fill while brewing a shot? I didnt think this should happen as it would effect the quality of the extraction stopping it half way through for a second or so.

    Thanks again for any help.
    John.

  8. #8
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    Re: VBM thermosiphon problem?

    John,

    Nothing sounds abnormal to me.

    A low water level signal will overide everything else- to preserve your element. Its irritating but thats how it goes with non-commercial sized boilers....

    Im happy to have a look at the machine if you wish.

    Chris

  9. #9
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    Re: VBM thermosiphon problem?

    Quote Originally Posted by Sparky link=1190785412/0#5 date=1192272931
    The thermosyphon/HX part of the machine should be a closed system. The thermosyphon will only work if it is fully charged with water. Any air or steam in the system and it will stop. Operating the group should expel any air/steam and re-prime the group. If it happens again then there is probably a slight leak somewhere that is allowing some water in the HX/thermosyphon circuit to escape probably as steam. So youll need to track down the leak.
    Ah, I assumed the thermosyphon used water from the boiler water rather than from the HX. My bad. Thanks from the clarification on how it works Mark.

    fabcat

  10. #10
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    Re: VBM thermosiphon problem?

    Quote Originally Posted by itsme5k link=1190785412/0#6 date=1192346453
    Quote Originally Posted by Sparky link=1190785412/0#5 date=1192272931
    The thermosyphon/HX part of the machine should be a closed system. The thermosyphon will only work if it is fully charged with water. Any air or steam in the system and it will stop.
    Are you sure? My brain agrees my machine does not...
    I have noticed that with the HX not quite full (ie. when it cuts out due to low water when flushing, if you leave the lever up for longer than a second or two without the motor running, or put the lever in the brew position just before it activates the motor and let a bit of water out the group) the group heats up very quickly to 105+ when it normally sits at 98 idle and 92-96 when in use.
    Remembering this machine is running off a tank it has no mains pressure to keep the HX/thermosyphon path primed in these situations. Normal or is my machine doing something it shouldnt?
    It sounds like you are describing a transient reponse from opening the brew valve without the pump. Under those circumstances the water entering the group will be superheated... For the group to cool youll need to wait a while (if theres air in the thermosyphon) for it to cool down. If you had your machine plumbed in then this wouldnt happen at all as the HX is usually maintained at mains pressure, so any leaks shouldnt result in a gas pocket forming... but as you are using a tank, then theres no room for leaks.

    Cheers,

    Mark.


    Anyway, it sounds like its working now.

  11. #11
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    Re: VBM thermosiphon problem?

    I read on the buyers guide on home barista about thermosyphon stalling - sounds exactly like what has happened to you. The thermosyphon works by relying on a temperature differential between the thermosyphon line in the group head and the HX. I think under some certain circumstances this differential can be the wrong way ( think maybe if the water in the line in the group manages to get hotter than usual or something) and stop the flow. (Im not techie but thats what I gleaned from the page or so of posts I read). Apparently increasing the boiler pressure about 0.5bar gives good results - not completely eradicating the problem but certainly making it better.



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