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[solved] No hot water from Vaillant combi boiler (but heating works); no fault code either.. simply does not sense tap water flow (lemmy.dbzer0.com)

submitted 2 months ago* (last edited 2 months ago) by [email protected] to c/[email protected]

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I have a Vaillant ecoTech Plus combi boiler. The central heating works fine. But the hot tap water became intermittent. If I got lucky and hot water would come from the tap, it never went cold as long as it was running. So I think it’s likely a flow switch because there were times when it simply did not seem to detect that hot water was on. I heard no boiler fan and the faucet icon did not appear on the display on the times it failed to produce hot water. The moments where it would work are now history. Now it’s cold water every time. So it was a gradual manifestation.

I popped the cover off and there’s some kind of scaling or buildup of something on the pump. The clip that holds something in place above the pump is corroded. So while the system behaves as if it does not detect water use, the pump does not look good. Would a faulty pump cause my issue?

About two years ago I had an intermittent hot water problem and it turned out the secondary heat exchanger was blocked by limescale. In that case the water would go cold mid-shower. The current intermittency problem is just with the initial switching on of hot water - no interruption once it was going. So I kind of doubt the heat exchanger is my issue.

Any theories?

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[–] Death_Equity 2 points 2 months ago (1 children)

Sensor is more likely than the pump, based on the described failure.

You can test the sensor with a multimeter and somebody to turn the hot water on.

[–] [email protected] 1 points 2 months ago (1 children)

Thanks for the feedback. Indeed that seemed to be the best theory. So I found these quite useful videos:

My voltmeter detected 4.68 V on the red input wire showing that the sensor is well fed. The green “signal” wire is supposed to be 0 V at rest and 2 V with water running. In my case the green wire is ~1.33 V at rest and ~0.66 V when water is running. That’s bizarre, no? As I understand it, a magnetic fan spins when water is in motion which generates a current in magnets around the fan each time the fan blade (impeller) passes a magnet. I can imagine that a faulty impeller or sensor would reduce the voltage when it spins, but bizarre that the voltage would be 1.33 V when the water is still.

I suppose either way the sensor and impeller will need to be removed together since they are part of the same fitting. From there I’m not sure which one to replace. Hopefully it’ll be visible.

Anyway, thanks for the reply!

[–] Death_Equity 2 points 2 months ago (1 children)

Are you certain you were looking at information for your specific unit and how did you configure your test probes? When test results are counterintuitive I always assume the test was done incorrectly, there is a misunderstanding of the mechanics involved, or incorrect information was referenced. I have definitely done all 3, sometimes all at once, and that has lead me to scratch my head more than a few times.

It sounds like the flow sensor, according to the information you provided, is suppose to be on/off signal(like a blade attached to a switch). The results of your test suggest a flow rate sensor that measures how much flow and not if there is flow.

There are different schemes to determine flow rate like comparative pressure sensors, turbine(like you mention with a "fan"), thermal mass, rheostatic, and electromagnetic. Without knowing the specific scheme your unit uses, it is hard to determine what readings you should expect to see. I didn't watch the videos you linked, so I don't know if they explain the method the unit uses.

Assuming the information you found is relevant to your unit the sensor is not working according to your test, which makes sense given the issues you are experiencing.

[–] [email protected] 1 points 2 months ago* (last edited 2 months ago) (1 children)

Are you certain you were looking at information for your specific unit

No, I don’t have the tech details for my specific model. Both videos cover different models of the same brand. The first video (d8ucufoyUlQ) model in the video looks newer judging from the control panel but interior looks the same. The 2nd video (G4bO8MYqQKk) has a different looking interior with a black label on the pump, so I think that’s an older one. I was figuring if a newer and older model both have the same voltage specs then mine is probably the same.

The voltmeter was set to DC voltage, 20 V (which should measure up to 20 V). My meter does not have a frequency function (Hz), so I don’t have the benefit of checking to see what flow rate is measured.

I should also mention that my cold water flows at ~1 liter/5 seconds and the hot water circuit flows at ~1 liter/7 seconds.

(update)
I found the service manual.. did not realise I had one. But it’s not too useful. It does not clearly describe the relationship between voltage and state. There seems to only be a diagram which I posted here:

https://lemmy.dbzer0.com/post/26703241

[–] [email protected] 2 points 2 months ago (1 children)

can you also post the full manual or model number/name? it might be useful

[–] [email protected] 1 points 2 months ago

I forgot to mention, this problem is solved. It mysteriously fixed itself. After unplugging a connector and reconnecting it, the sensor started working again.