Vaillant - Ecotec DHW cycles hot/cold/hot

[JayUK74]

Hi all,

I have an old Ecotec 837 combi and experiencing the usual hot/cold/hot cycling issue with my DHW (S14/S10/S14 - Ignoring the other steps).

There are no faults codes on the boiler.

If the flow is around 5L/min it goes hot/cold/hot/etc. If I increase the flow slightly it stays hot.

I cleaned the plate heat exchanger with brick acid, but it was still slightly blocked (I could hear trapped water inside sloshing about). I then managed to get hold of a used plate heat exchanger. I also cleaned that with brick acid (I think it had a slight blockage too), but still experiencing the same hot/cold cycling issue (I don't think that was fully unblocked).

The CH pipe from the diverter is getting warm with the DHW on (The diverter was replaced a few years ago).

The temp sensors seem to be reporting the correct temps and are changing according to when the pipes heat up/cool down.

The cartridge filter has also been replaced. The pump seems to be running ok.

Before I spend on a brand new plate heat exchanger (not a reconditioned one), does anyone have any suggestions/advice as to what else it could be?

Thanks,
J.

 

[Johntheo5]

You may have a flow switch problem, S.10 (and then S.14) coming on/off seems to indicate that its operating at ~ 5.0LPM, should be ~ 2.5/3.0LPM I think, so the burner will just keep firing on/off, its not a alarm condition, as such. Could also be a mixer X over problem between the cold and hot.
S.10 Hot water demand via impeller sensor
S.14 DHW mode, burner on.

 

[JayUK74]

Thanks for the reply.

I'm trying to understand why you think it might be the flow sensor? The boiler is reporting a varying flow rate when I open the tap further. It is detecting constant demand when the tap is opened (tap icon on the display). Yes, I believe the minimum flow is 2.5/3lpm which I am above.

I'm not sure what you mean by mixer crossover? What mixer?

Thanks for your help.

 

[Johntheo5]

If, as you say, that S10 is being flagged with the boiler cycling then the actual (DHW) flow through the boiler is < 2.5/3LPM, or the flow meter is faulty.
Mixer crossover can be caused by a faulty mixer, the shower will generally have a thermostatic mixing valve (TMV), any basin set that have hot and cold taps will have a mixer and the cold water can leak into the hot side and reduce the flow through the boiler to < 2.5/3LPM, ideally, there should be check (non return) valves installed on the hot and cold supplies to every mixer to prevent this but isn't allways the case.
That boiler has a minimum output of 12kw and the HW control can be set to between 35C and 65C on the boiler front, suggest setting it to 50C, open a hot tap only fully, measure the flowrate and measure (or estimate the temperature) and see does the boiler run continuously.

 

[JayUK74]

Your understanding/interpretation of S10 is different to mine. I thought S10 simply meant that DHW demand had been detected at that time but nothing else was happening (eg: no ignition, no over-run, etc).

If the flow rate is 5 or less then the boiler cycles, 6 or above and it's fine.

I've checked what the flow sensor is reporting in relation to flow rate with what is actually coming out of a tap and they match.

The boiler is set to 50C.

Thanks,
J.

 

[Johntheo5]

Your understanding/interpretation of S10 is different to mine. I thought S10 simply meant that DHW demand had been detected at that time but nothing else was happening (eg: no ignition, no over-run, etc).

If the flow rate is 5 or less then the boiler cycles, 6 or above and it's fine.

I've checked what the flow sensor is reporting in relation to flow rate with what is actually coming out of a tap and they match.

The boiler is set to 50C.

Thanks,
J.

The S codes IMO are just status codes, nothing else, when the boiler does run continuously then I would expect S.10 & S.14 indications on when the boiler fires up and would expect them to go off when the HW demand finishes, the alarm codes are F prefixed.

All I can say with certainty is that the boiler demand with a 5LPM DHW flowrate at 50C (and 19C mains) is 10.81KW and at 6LPM is 12.98KW,
if the boiler's minimum output is 12.0kw then it means that the DHW flow temperature at 5LPM will be 53.4C which should not be a problem even with a 50C DHW setpoint, you could just set the DHW setpoint to say 60C to prove that this is not a problem (at 5LPM).

 

[DP]

Check the primary temperature ( temperature of water out of the main heat exchanger). At stipulated flow rate, hw stat at max, I would expect a temperature reading around 70 degrees. When this starts to creep up, overheat stat cuts in and shuts the burner till the sensor is happy again so boiler fires up and cycle begins