cannonfodder wrote:Neither Dan nor I have had a similar issue, that is not to say it could not happen to someone else but we did not experience it and I tried to replicate it by short flushing my machine long ago just to see if it was a one off instance or an inherent issue.

I have seen that once or twice- Since I am new to HX machines it is difficult for me to give any scientific details, but it seemed to happen when the machine was left on for an extended period of time and not used at all-- just idling for four to six hours (since I didn't know it was possible I wasn't timing it). The brewhead temp as reported by Eric's adapter dropped to around 125 or so, and it took a lot of flush- wait- flush to get it back up to temp again. At first I thought the thermometer was bad, but after reading the thermosyphon stall discussion, what happened to me it made more sense.

I've not seen this, but I didn't try hard to reproduce it. More to the point, how does a thermosyphon "stall"? Introducing air into the line?

I was thinking something like vapor lock, or airlock. A bubble in the flow restrictor?

More to the point, how does a thermosyphon "stall"?

Gosh, I have no idea! I didn't mean to hijack this evaluation, or rehash an old post, I just thought that if it were a design issue (as experienced by another user of the machine in question), then there might be a known, identified fix (for my machine). Clearly it's not a design issue (with the VBM anyway), so I'll drop it. Sorry for the diversion.

HB wrote:I've not seen this, but I didn't try hard to reproduce it. More to the point, how does a thermosyphon "stall"? Introducing air into the line?

A thermosyphon stalls by setting up thermal conditions that prevent it from operating. In normal use the water at the top of the hx is hotter than the water at the bottom of the hx. Water in the group cools and sinks down, being replaced by the less dense, hot water from the top of the hx. It's possible to have a situation in which the water in the hx is cooler than the water in the group. This can occur if the hx volume is small, as in small home machines and if the hx has been overly flushed, with the metal of the group heating the water within the group to a high temperature relative to the hx. Another possible cause is related to the geometry of the hx water inlet. E-61 systems usually have the water returning from the group re-entering the hx at the same point as water from the pump. The plumbing layout is such that flow to the group during brewing gets split between water that never actually enters the hx and water that flows from the top of the hx. Good brew temperature control requires extablishing the correct proportion of water from each source. It's possible to screw this up so that you stall the thermosyphon.

Good e-61 machine setup establishes enough thermosyphon flow to get the group temperature to idle at the desired brew temperature. The proportion of cold and hot hx water mixing must also be adjusted so that the right brew temperature is achieved. If done right you can set up a system that requires little or no flushing at all over a wide variety of duty cycles.

-Greg

I wrote a couple of pieces on this subject a couple of years ago when i was learning how to tune my Astra. They are on Alt.coffee and might be worth a look.

kanoyu wrote:
Gosh, I have no idea! I didn't mean to hijack this evaluation, or rehash an old post, I just thought that if it were a design issue (as experienced by another user of the machine in question), then there might be a known, identified fix (for my machine). Clearly it's not a design issue (with the VBM anyway), so I'll drop it. Sorry for the diversion.

That is not slight at your question, but rather another question, which Greg answered quite nicely. Don"t take it as an insult.

Thank you for that info Greg.

Taken on their own, my previous comments about the Domobar might be interpreted as my not liking the machine. That is not the case. A couple of days ago I did the electrical tape "fix" for the front edge of the cup-warming tray and that really quieted that down a lot. I also remounted the brew pressure gauge to eliminate that rattle as well. While I had the case apart I dropped the brew pressure by adjusting the OPV about .2 bar, down to an indicated 9.6 or so.

This morning my wife walked by as I was pulling a double and she actually stopped and said, "That sure sounds different this morning!" I explained why, and she stated that it really was a lot quieter.

While we were sipping our morning cappas she talked about how much smoother the coffee had been since the VBM arrived, and that the morning drinks were so much more consistent now. "Except for that one morning, they have been really good every time." 'That one morning...' I'll never hear the end of that one....

So, yes, as stated by others, many of the things I was commenting on seemed to be anomalies on my machine for whatever reason, and Jim was really great about the spring replacement for the OPV [and what an amazing difference that made in the machine's performance!]. But on its own merits (since I do not have experience with other HX machine to which I could compare) the performance of the machine is exceptional.

cannonfodder wrote:Neither Dan nor I have had a similar issue, that is not to say it could not happen to someone else but we did not experience it and I tried to replicate it by short flushing my machine long ago just to see if it was a one off instance or an inherent issue.

Well, I have to take that back. I drained the boiler Friday so I could take the machine back home for a final set of test and measuring but I left it in my office. So today I powered it back on and let the pump prime up the boiler. It has been on for 6 hours and I am just getting a chance to pull a shot. Luke warm group. I can put my hand on the group for 3 seconds before it get pull your hand off hot. It appears that I have experienced a thermosyphon stall, or failure to prime after dumping the boiler.

I am going to run some back flushes to see if I can prime it and get it hot before I shut down for the work day.

cannonfodder wrote:Well, I have to take that back...... It appears that I have experienced a thermosyphon stall, or failure to prime after dumping the boiler.

Happened to me again last night. I turned the VBM on at about 5:00pm or so, anticipating dinner guests. When I went to make coffee at 7:30 PM it had stalled. I flushed and waited, and flushed and waited, and it took a long time just to get the brewhead temp back to the high 180's F. using Eric's adapter. The boiler pressure was fine.

I don't know what the solution might be,. but a preventative measure might be to flush a bit of water through the group every hour or so when idling the machine-- sort of defeats the purpose of having a machine that can be left on if you desire, but there it is.....

Randi -

Certainly this whole business of a thermosyphon stall should be considered a true phenomena and I believe the culprit responsible for same is the thermosyphon restrictor at the hx outlet. Or, better said, the culprit is the combination of TRUE pstat setting, boiler water level, and thermosyphon restrictor sizing.

A quick fix might be to simply bump the pstat up a touch, say .05 bar. Think of the thermosyphon "system" as a see-saw - the event you don't want is to be balanced.