Odd Expobar problem - no thermosiphon

[CrackAddict]

My Expobar Office Control worked flawlessly for several months. When I first received it, pressure was at 14 bar, so I cranked that down (turning the white nylon screw in the valve counterclockwise) and also took the opportunity to route the (increased) bypass overflow back to the tank, as I use $$ Volvic water to reduce scaling in this high-scale area. Note the pressure on this machine must be checked using a PF gauge as there is no integral gauge.

As mentioned, all was fine for several months and machine would be up to temp from preheating on a timer in the AM. Then, one day, machine is much cooler. Grouphead only warm. Actuating vibe pump results in no flow for +20 seconds, then a tone change and pumping commences. Machine then warms up normally, but not ready to go for 20 minutes or so.

I rechecked all internal componets for leaks, etc. Vacuum break works correctly. No air or water leaks. Powering up without covers and then powering off after one hour, I noted the top pipe from the HX coil to grouphead is HOT, while the bottom (return) pipe is COLD, so NO thermosiphon effect even though boiler is HOT. Actuating the vibe pump manually seems to initiate thermosiphon, but the "passive" start has stopped working. ??? I disassembled the top of the group head (large brass plug). No scale. Small circular screen and orifice - both clean.

Once warm, this machine works fine - but I cannot figure out what happened to the "preheat" phase. On a practical note, this "costs" me a few minutes each morning as the machine requires intervention to achieve a ready state. I also replaced the Ulka pump - just on a long shot and because it was easy. No effect.

I have discussed this with the dealer - who considers it not a "fault" since the machine still makes great coffee. I have also asked the importer and other experienced people and all seem at a loss to explain the problem. Help!

[HB]

Actuating vibe pump results in no flow for +20 seconds, then a tone change and pumping commences. Machine then warms up normally, but not ready to go for 20 minutes or so.

Do you mean that without coffee or a blind basket, the flow takes +20 seconds and the pump tone changes (indicating it's under load)? If so, then you need to find out why the line from the pump to the group through the heat exchanger is partially blocked. The heat exchanger is part of the thermosyphon loop, so blockage there would explain why the lower return is cold. My first thought is scale build-up. BTW, twenty minutes or more to heat up is "normal" for heavy group machines like the Expobar.

For reference, the picture below is the boiler of an Expobar Lever. I think it looks similar to yours.

[cannonfodder]

Sounds like air-lock. What position do you leave the lever in? Should be down when heating.

[CrackAddict (original poster)]

Wow! Thanks for the replies.

This is the Office Control, so it is solenoid-actuated - not lever. The internals are similar to the posted photo, however. I would imagine the grouphead is similar but with the valve actuated by the solenoid. The delay in vibe pump water delivery is only for the first pump of the day. If the machine remains on, water delivery is normal after this first delay. So if it's a blockage, it's a strange one.

I do not think scale is the problem, as the machine was only used with Volvic water from new, and in addition I have a resin filter in the tank. The machine was brand new when I received it (still had the gummy plastic on the panels) but had been new/old stock (manufactured in 2001). It worked perfectly for about the first month. The machine is on a timer, so it comes on about an hour before I wake up. Normally, that would be plenty of time - I could do a purge and pull a shot immediately.

Now, the grouphead is so cold when I get up I can rest my fingers on it. The top of the machine is warm, though - about 100F - and there is plenty of steam in the boiler. So the boiler is hot, but the HX is not circulating. Operating the vibe pump seems to get this flow going - I assume it is injecting cold water into the HX coil at the bottom, and this either jumpstarts the thermosiphon or physically circulates the water through the grouphead (I am uncertain). As I mentioned, there was no scale in the top of the grouphead when I investigated. Neither was there any scale on the vacuum break.

While I suppose some rogue scale or foreign object could have migrated over a month into a location where it partly plugs the HX loop, why then does the machine not cool off again after use? Once I have "kickstarted" the thermosiphon using the vibe pump, it stays hot all day. It only stops thermosiphoning after an fully-cooled off cycle.

I notice that often when repowered on during the day, the machine will actuate the vibe pump immediately. That is, on power-on, the vibe pump runs immediately for about five seconds. When the machine is well and truly cold (as in morning startup from overnight), the vibe pump does NOT run. I wonder if - rather than a hardware problem - this is a software/sensor control problem in that the "kickstart" for the thermosiphon is the brief vibe pump action upon startup that is not happening?

I realize the vibe pump actuation is also filling the boiler and this is not always called for. But I wonder if it should *always* kick in the vibe pump at startup, and for some reason it has stopped doing this? perhaps a bad thermostat?

Sometimes it's easier when things are just broken!

Finally - the Expobar in the photo has the insulation nicely wrapped around the boiler. Is that standard issue? Mine came with just small sheet of insulation between the boiler and tank. I would like to add some as in the photo but have yet to find a suitable sheet to use.

Many thanks!

[HB]

I think Dave is on to something. I remember a number of reports of Expobar pumps "losing prime", although that shouldn't affect on the thermosyphon. My quick read of the search results on CG weren't very illuminating. To make certain we're all on the same page, below is a schematic excerpted from How I Stopped Worring and Learned to Love HXs:


HX thermosyphon

I suggested scale buildup as a possibility because the HX is a big section of the thermosyphon loop. Now that you mentioned that resin water softener, I suggest removing it at least temporarily as they are well known for clogging.

Finally - the Expobar in the photo has the insulation nicely wrapped around the boiler. Is that standard issue? Mine came with just small sheet of insulation between the boiler and tank. I would like to add some as in the photo but have yet to find a suitable sheet to use.

Abe's Suggested Upgrades in the Brewtus Buyer's Guide Epilogue recommends foil-backed insulation (the Expobar Brewtus has the same stock insulation as the Lever pictured earlier).

[CrackAddict (original poster)]

More thanks. I have checked the resin filter - no clogging. I use two and recharge them alternatively.

I have never seen a cut-open HX boiler (might be nice to) but I expect the innards contain a large coil of tubing that forms the actual heat exchanger. Since I have hot water in the exit (top) pipe, any blockage is likely to be in the grouphead itself or in the bottom of the boiler.

I guess my options are to first try descaling (easy, even though I doubt that is it) or disassemble the entire system to find the problem. I wonder if it is possible for the three-way valve that injects water into the HX to be faulty? This might explain the lack of pumping on a cold start (valve sticking). The EX-5 pump itself is fine (I swapped it for a new one). If the valve was sticking in a part-way position, I am assuming it would pump water into the boiler at the same time as the HX, correct. That is, this valve (at the base of the boiler/HX and in line from the pump) supplies water *either* to the boiler (as demanded by a level sensor or pressurestat of some sort?) OR to the HX. So if it was leaking or had some debris in it, it might try and pump water to the boiler when it should be pumping to the HX. Worse, if it remains "open", it might allow the thermosiphon to "short-circuit"?

The simplified schematics, while very helpful, don't quite get to the level of detail needed to sort a problem like this. Do anyone have a detailed diagram of the valve in question?

The other possible clog point is in the base of the grouphead. I see the lever machines have a "mushroom" that is mechanically controlled, whereas this machine has a solenoid, but I suspect the operation is similar, and that it is relatively straightforward to disassemble the water flow controls in the grouphead?

Thanks very much for the link to the Brewtus group - many resources there!

[cannonfodder]

Over on CG EN had a similar problem with his Millennium. After 6 pages of ideas and trials it turned out to be an air lock in the pump. Here is the thread, you may want to give it a look and see if anything works for you. I believe his GH was heating though; it was just the pump that had a problem (brew and boiler fill).

[lennoncs]

Check the solenoid valve that diverts water to the boiler.
if that valve is leaky the HX water can settle back into the boiler when the machine cools.
a faulty vacuum break will suck that water out of the group if the valve is leaky also.

the night before,

Get everything running well, get a measuring cup and measure how much water you get from the water wand before the re-fill kicks in.


the next morning,

when the machine is hot but the group is still cold(malfunctioning) do the water draw and see if you get more water before the refill kicks in, that will tell you if it is backing into the steam boiler.

also;
do a level check on the reservior to see if there is any extra water in there also.


if the group is cold...there is no water in it.

if there is no water in the group...it went somewhere.


we need to find that somewhere.


Cheers
Sean

[barry]

is there a check valve on the cold water in line, between the pump and the water distribution block? it sounds like it might be stuck open.

[lennoncs]

The OPV that expobar uses on the machine includes a check valve on the input (pump), it is screwed into the outlet of the pump.

Here is a document on the OPV.


Sean