Hello, I've owned my Alex Duetto II for about 10 years, except for one time I sent it to Chris' Coffee I've made all my repairs myself over the years so I'm pretty familiar with troubleshooting and fixing this machine. Except for this time I am at a complete loss. Started out when the machine was having fill problems with a full reservoir the alarm would go off. Verified the reservoir sensor was working properly and believed the fill solenoid was faulty, so I ordered it and installed the new one but it was still happening intermittently where the alarm would go off. I did find some scale debris in the new fill valve so I decided to do a complete and thorough descaling of the machine as it hadn't been done in 2 years. I even removed most of the parts including the boilers and descaled thoroughly. The fill problem seemed to be solved, I was actually using the machine again making espresso and steaming milk for a few days...

Symptoms 2 hours ago: The machine powered on and came up to the proper temperature for both the brew and steam boilers. The steam pressure was sitting right in the middle of the dial at 1.5 bar. Open the steam knob and I got a little wet steam at first but it always did that and then a full strong blast of steam. So far so good. Pull the lever to draw water through the brew head, the pressure only got to about 4 bar (used to always be 9), and for several seconds no water came out at all. Also, water started pouring out of the steam wand even though it was closed. After several attempts to draw water again, to see if I could repeat the results, same thing happened only now the steam pressure gauge went off the charts up to 2.6 bar, I've never seen it get this high. I cannot understand how drawing from the brew boiler is causing my steam boiler pressure to go so high, eventually it blew the safety pressure release valve on the steam boiler.

Other things I've done is disassembled the expansion valve and the one-way inlet valve, cleaned both thoroughly. Tested the pump independently from the machine. Replaced the invlet valve and solenoid and descaled thoroughly. Does anyone have any idea what is going on with my machine? Faulty expansion valve, faulty pressure sensors, I'm just guessing at this point.

One other thing that I've noticed, when the machine was working fine all these years I'd often turn it on in the morning and the first thing I'd hear is the pump coming on to fill the boilers before the PID display would light and the heating would start. Now when I power the machine on I never hear the pump come on, even if I run the hot water knob and try to drain it. Seems I have to pull the group head lever to get the pump to come on to draw water from the reservoir. Not sure if that is relevant to my problem just something different I noticed.

I just tried to force the pump on by loosening the water level probe and pulling it several inches out of the steam boiler, I would expect that the controller would sense the probe was out of the water and turn on the pump but it did not. Could a faulty water level probe be causing all these issues? Overfilling the steam boiler with water...

g12196 wrote:Could a faulty water level probe be causing all these issues? Overfilling the steam boiler with water...

Welcome to HB Paul

Yes, first step is to extract the steam boiler fill probe and thoroughly clean it with the rough side of a kitchen sponge but photograph the before and afters.

Probe should be 'stainless steel' shiny - no residue!

Your reservoir issue might be a separate issue unless you have it resolved.

I posted a thread about same issue same machine, Duetto III but can't find it right now.

You might have to do a complete citric acid descaling.

You also need to be using non-scaling water.

Post back, pictures and all.

What you describe very much sounds like your pump is trying to push the full 9 bar into the steam boiler.

The first thing that happens is you start filling the steam boiler at 9 bar either from the fill tank or the brew boiler (more on that later) so there's a bit of a pause. Then your steam valve gives (it probably gets a lot more use than the hot water valve so goes first) because it can't take 2 or 3 bar. Then the pressure builds in the brew boiler up to 4 bar and then the steam boiler pressure starts rising to where it blows the overpressure valve.

Looking here https://www.kaffee-netz.de/attachments/ ... pdf.79931/ the only way that could happen is if there's a burst in the pre-heat exchanger for the brew boiler fill which passes through the steam boiler OR somehow an incorrectly functioning solenoid valve is sending the full pump pressure into the steam boiler. The diagram is incomplete as I don't think it shows the fill line for the steam tank.

My guess though is that as you're seeing 4 bar into the brew boiler there's a decent chance the pre-heat exchanger in the steam tank has ruptured as a result of the descale. Now that is not news I'd wanna hear so I'd spend my time on the other hypothesis that the solenoid valve which fills the steam tank is getting stuck feeding 9 bar into it when you brew as that will be a far cheaper fix.

EDIT I'd definitely work on the easier to swallow other hypotheses first. But I think it's unlikely your problem arises out of a fill sensor because the trigger is you raising the brew lever. My Duetto IV fills itself at completely random times so I wouldn't read much into the fill timings, especially given you've descaled and that could have affected sensitivities 100 different ways. And if you're cleaning the fill sensor be sure to unscrew it rather than pull it out - if it's really scaled (seems unlikely given you've just descaled) pulling it out will damage the Teflon sleeve and you'll need to replace it anyway.

Good luck and do let us know how you get on.

I agree strongly with the "It's your boiler fill solenoid valve stuck open" theory. Remove, inspect, clean and either reinstall or replace the 2-way solenoid for filling the boiler.

Thanks for your responses. I'm at work now so I'll check the solenoid valve later today. Unfortunately for me, I don't think that's the issue, because, before all the extensive descaling I did where I was having the fill problem and the alarm, I replaced the solenoid and the mechanical valve, so they are both brand new. Unless I was shipped a bad part, it can happen. Anyhow I've tested this valve in the past, to test the solenoid I check for magnetism being activated to pull the plunger, and to test the entire assembly, I disconnect the outlet port to see if I can get water to flow and to not flow depending on the commanded state of the solenoid. I will do those tests today and report back. Another possibility is more of the descaling particles collected in there to stick it wide open, although usually, it seems to obstruct the flow it's still worth a second look as your other hypothesis would really suck in terms of cost and time.

Let's talk about your second hypothesis, something you said that I did not understand,
"...you start filling the steam boiler at 9 bar either from the fill tank or the brew boiler"
I thought the brew boiler water fill line only passes through the steam boiler for pre-heating, and that it did not source water to the steam boiler, and that seems to be confirmed in the (incomplete) diagram. In my actual machine I see that the two boilers are only connected via the expansion valve, (which I'm not 100% sure what function it performs), could the brew boiler be back-feeding water into the steam boiler via a faulty expansion valve device?

Last 2 questions for now,
1) is there a way to test the ruptured heat exchanger hypothesis as I'm sure they are not cheap?
2) does your hypothesis still hold true when you consider that if I only run the steam circuit, eg. power up both boilers, wait till they both come up to temp, and operate the steam valve to steam milk that works fine? It is only when I try to activate the group head and draw water from the brew boiler that all the problems I previously described occurred.
Thank you

It might be the solenoid valve not seating and not fully closing due to very small scale particle interference. Check the steam boiler fill probe first.

g12196 wrote:...could the brew boiler be back-feeding water into the steam boiler via a faulty expansion valve device?

My understanding is that the expansion device just drains into the drip tray - it just happens to hang off the interconnector which takes the pre-heated inlet water into the brew boiler, but it has atmospheric pressure at the drip-tray end and brew boiler pressure at the other end. If the two boilers are separate then there is no way this can cause a rise in pressure in the steam boiler.

g12196 wrote:Last 2 questions for now,
1) is there a way to test the ruptured heat exchanger hypothesis as I'm sure they are not cheap?

Great question. I'm just thinking things through in my head here - so this could be completely wrong.

I'm not sure if the brew pressure manometer is connected on the pump side of the brew boiler solenoid fill valve or the boiler side (maybe you can see from the top?). If it's the boiler side and there is cross pressurisation taking place in the steam boiler pre-heat exchanger, then potentially you should see the brew boiler pressure gauge show the same pressure as the steam boiler. So an easy experiment is to see if the brew-boiler pressure is 1.5bar when the two boilers heat up. But if it's not, then it could be that the manometer is sat pump side of a properly working solenoid fill valve so this won't be a conclusive experiment.

I guess another experiment you could do is to disconnect the brew lever switch so that it doesn't trigger the pump when you raise the lever. Then if you raise the lever with both boilers heated up to the right temperature, if there is cross pressurising going on you should see water coming out of the group head as a result of the steam boiler pressure. I guess there's a small chance 1.5 bar isn't enough to push the water through the gicleur, but I think you ought to be able to see it. If it does this and you open the steam wand and that stops the water coming out of the group head, that will be pretty conclusive as you've shown that dropping the pressure in the steam boiler reduces pressure in the brew boiler.

g12196 wrote:2) does your hypothesis still hold true when you consider that if I only run the steam circuit, eg. power up both boilers, wait till they both come up to temp, and operate the steam valve to steam milk that works fine? It is only when I try to activate the group head and draw water from the brew boiler that all the problems I previously described occurred.

Yeah - it does because if the pre-heat exchanger is ruptured the steam boiler stays at 1.5 bar (enough to steam normally) but tries to go up to 9 bar when the brew pump is on.


I do absolutely think you should check the solenoid fill valves first as others are suggesting.

I'm struggling to see how the fill probe could be causing what you're seeing because it only seems to happen when you raise the lever, but no harm in checking that either. If it is indeed the boilers coupling, it's possible you've overfilled the steam boiler when trying to put 9 bar into it which is why raising the fill probe didn't do anything. Does the Duetto II have boiler drain valves? Maybe draining the steam boiler (when it's cold!!) and repeating your fill probe experiment will add a useful data point.

Really hope it's not the boilers coupling for you!

I doubt the problem is a ruptured heat exchanger in the steam boiler.
Attached are pics of our before and after cleaning of our defective steam boiler fill probe and tiny specks of scale blocking the closure of the fill solenoid plunger that is enough to cause serious flooding when direct plumbed - in seconds !

The solenoid is easy to disassemble as it does not have to be removed from the machine.

You must use non-scaling water; if direct plumbed a Leak Detector with automatic water cut-off is essential to avert kitchen disasters!

Baristaboy, completely agree either the fill probe or the fill valve having a problem as you describe could create the symptoms seen. However, if the fill probe were faulty wouldn't it have to be permanently sending a "steam boiler empty" signal in which case the pump would be permanently on which isn't being described? So that seems unlikely to me?

LittleCoffee wrote:However, if the fill probe were faulty wouldn't it have to be permanently sending a "steam boiler empty" signal in which case the pump would be permanently on which isn't being described? So that seems unlikely to me?

Not exactly or necessarily so. It might depend on how the boiler fill probe scaled up over time with regard to its resistance to ground at any point along its total length. The scaling, thus its resistance, might not be linear. Under those conditions the problem might be first exhibited as 'intermittent' processing to worse and more frequent over time.

... And again, should the fill solenoid be prevented from complete closure by a failure to properly seat when direct plumbed - you have a real problem demanding immediate intervention either manually if you're present - or automatically if you're not!