Hi all,
It's my understanding that HX siphon pipes are designed to a certain length so that from cold, water flowing continuously through the heat exchanger will reach optimum temperature by the time it reaches the group. However, this is the second time I have encountered a two-group machine with each of the two HX siphon pipes of different length. Is this intentional, or was one pipe possibly replaced with an inferior aftermarket pipe?
The heating element is long enough to reach both outer tubes comfortably. The only difference between the two on the VFA-Expres I am refurbishing is that the shorter siphon pipe feeds the left group and the hot water spigot while the longer feeds the right group only. However, the Simonelli Program V I dismantled earlier fed the water spigot directly from the boiler and both pipes fed only the groups.
If these HX siphon pipes are supposed to be of equal length, how can I determine which is the correct one? On a used machine or even one I've been called out to repair, with so many variables affecting the output temperature, how does one get this variable right?
Thanks for any input or advice.

Just to acknowledge the "use the search" reply, I am aware this is called thermosyphon loop tuning, and I have searched, but so far have only found mention that this concept exists, but not much detail about how to do it. I am a perfectionist by nature, and have come a long way in the time I've been working on these machines, but would really appreciate the extra push by someone who might want to help me deliver a better product for the benefit of espresso aficionados in my area. I really want to be the one who can do it. There isn't even a local service tech in my city, so I want to be able to step up for the cafe owners and baristas and give them a product that will let them be their best. If you aren't comfortable publicly replying, by all means PM me.
Thank you

skydragondave wrote:It's my understanding that HX siphon pipes are designed to a certain length so that from cold, water flowing continuously through the heat exchanger will reach optimum temperature by the time it reaches the group.

I've seen restrictors used on the thermosyphon loop to control heating of the grouphead (e.g., E61 Faema Legend). I've seen different length HX injectors to control mixing. But changing the length of the legs of the HX syphon loop to control temperature? New to me. Would you post a photo?

Thanks very much Mr. Kehn, here's a photo of two VFA-Expres Elite syphon tubes, and the two tubes I found in the Simonelli Program V to the right. I just pulled the VFA tubes out of the vinegar for this, but I would guess the NS tubes were doctored as I found a lot of monkey business when disassembling that unit. Best regards

As regards the VFA hydraulics, I would term those tubes as receptors for the stream of cold water "injected" through the lower heat exchanger connection. This is possibly best shown by an illustration of a Quickmill boiler/hx arrangement:


Cold water enters the hx via the short lower tube and pressurizes the delivery system. As the average velocity of this water is painfully slow, the actual water delivered to the group is primarily that which is contained within the tubing connecting the hx to the group. On most (?) hx machines, the lower delivery tube is termed the "injector tube" and its length is usually fixed. When its outlet is much closer to either of the hx outlets or when restriction orifices are placed in the tube(s), the flow mix is altered. This would be an exercise in CFD (computational fluid dynamics) which is beyond me at this stage.

The shorter length of the hot water tube is perhaps used to tune the temp of the tea water delivery which is, as you might know, unique to VFA machines. As an FYI, the WEGA Mini Nova uses a separate hx system for hot water delivery that is unique for semi-pro machines.

Hi Eric,
Thanks very much for the detailed response and sorry for the lateness of my reply. It's been a very exciting weekend.
I would imagine this is much more complicated than simply plugging a thermofilter into the group and experimenting with different lengths, or you'd have worked this out long ago. The amount of detail on your website is humbling. Do you think the 3/4" difference here is going to have much effect in the temperature between the two groups, or am I fretting over nothing?
As Dan Kehn correctly points out, I do have thermosyphon loop restrictors pressed into the ends of the upper tubes going into the groups, and they appear to be visually identical in diameter, though I do not have any way of gauging size here. Perhaps one is marginally larger than the other, which might allow both groups to be the same yet allow the hot water valve to draw from the HX at a suitable temperature. Fascinating.

Thanks for the interest and your willingness to help.

I would say that the 3/4" differential would have minimal effect in the grand scheme of things. What would NOT have a minimal effect is drawing off a typical(?) quantity of HW from that particular group and then pulling a shot from that group soon thereafter.

Amoungst a dozen or so speciality tools, you really need a basic Scace thermofilter basket, a means to read it, and a set of numbered drills (1-60) if not beyond.

Hi Eric,
If you don't mind, please list them. The thermofilter and digital thermometer is on order. The numbered drill set is a point well taken. Any other imparted wisdom will be very much appreciated. I cannot afford to fail here. I am trying my best to glean as much as I can on the subject but am simply coming up short. I welcome any and all messages, private or otherwise. Best regards