Vaillant combi DHW needing tweaking

[Jethro_uk]

So finally changed the aqua sensor on the boiler. As described the old one was clearly damaged - with a bit of plastic (presumably) stopping the impeller spinning correctly.

With new sensor in place, DHW fires up every time. Perfectly. And all but one tap pumps out hot water as long as you like.

The exception is the bath tap. Which not coincidentally (I believe) is the one which calls for most water.

Last few baths SWMBO has had (I shower :) ) she's noted that it starts hot, then goes cold, then hot then cold.

Watching the boiler display I can see the boiler firing, stopping, firing, stopping ....

Today, as an experiment, she opened the tap about 3/4 of the way. This resulted in a constant flow of too-hot-to-touch water for well over 5 minutes.

My hunch is there's some setting in the boiler which when the water flow is at max is managing to just overheat something, causing a cut out/cut back in cycle.

Is there a calibration step I may have needed with a new sensor ? Will restricting the flow of DHW a tad solve the issue ?

As always TIA :)

[SH]

Combi boilers are rated to provide so many litres of hot water per minute with a given temperature rise. so one thing to check is the flow rate of the bath tap and compare that with the other taps

One other possibility is that the DHW heat exchanger within the boiler is scaled up with limestone affecting heat transfer efficiency.

[Jethro_uk]

It's definitely got a higher flow rate ... 22mm pipe direct into the mixer. All others are 15mm ...

With the caveat this is the softest water area in the universe (our 15 year kettle still hasn't a fleck of scale) is that fixable, or needing a new device.

My scepticism is slightly raised because everything worked fine until November last year.

[Tim Lamb]

Could it be a higher draw off leading to a higher flame setting leading to an over temperature due to blocked/sooty heat exchanger fins?

[John Rumm]

Has your boiler got a separate DHW temperature (limit) knob on it? If so what is that set to?

It's a slightly odd sounding problem - since the behaviour you are seeing sounds more like the behaviour you get on some boilers when the flow is too *low* rather than too high. Typically in those cases the water gets heated to above the temp set on the DHW setting even with the boiler modulated to minimum output, and hence it has to cycle the burner off to stay under the limit. In effect giving a rather crude pulse width modulated approximation of the desired temperature.

Normally with combis (or at least those without a flow regulation valve) the symptom of drawing water too fast is that the boiler does not have the power available to meet the demand, and the water temperature then falls to lower than required even with the boiler running flat out.

It could be that the aqua sensor is giving a erroneous output at high flow rates. (IIRC there is a D code register that you can check on the boiler where it reports what flow rate it thinks it is seeing).

IIUC the aqua sensor is usually a 3 wire device, with a ground and permanent live (at ~4.6V DC) supplied via the main PCB, and then a pulse train returned from the turbine. So with the tap off you should see either 0V or supply V on the green return wire. With water flowing you get a pulse stream where the frequency is proportional to the flow rate.

You would see this as a DC voltage with a normal DMM typically around an average DC level of ~2V.

However if you have a DMM that can measure frequency (or a scope!) you can look at the actual pulse rate which should rise with flow rate. ISTR the docs suggest ~120Hz being indicative of 14 lmp.

[Andrew]

Your boiler has the menopause ?

[Andrew]

Does this boiler modulate ?. If so, is the circuitry that does the modulation at fault, and how could this be proved anyway ?.