Strange modulation behaviour on Vaillant ecoTEC plus 418

[Yikes]

I have the aforesaid boiler with a VR65 control centre and VRT 392f wireless thermostat which is set to two-point control.

The boiler's fault memory is full of F.72 (NTC fault) although the boiler appears to be working normally. I asked my heating engineer to change both NTC sensors as a fairly cheap initial attempt to fix it but it didn't.

So I hooked up my ebus monitor and found the modulation was looking very unstable:

I have set d.0 to 10kW which is 55% modulation. There is an initial spike as the flame is established then it drops down to 55%, but after a while it starts to oscillate.

Has anyone else come across this? Is it an indication of a faulty control board?

Mike

 

[Johntheo5]

What is the target flow temperature?

 

[Yikes]

60C.

Mike

 

[Johntheo5]

The flow/return temp are equalizing regularly which normally indicates burner off and anti cycling followed by refiring, the burner appears to trip on the second and third period even though the modulation was steady for some time after at ~ 57%. Did the S codes show snything?.
Not sure what d.09 actually means. d.17 is normally set to 0, flow temperature control.

Maybe worth doing a factory reset, d.96.

 

[Johntheo5]

I was just reading your problems of last April, maybe try with "manual" target (or return) temperature control. If a factory reset does nothing maybe ask Vaillant for a fixed rate fix.

 

[Yikes]

The flow/return temp are equalizing regularly which normally indicates burner off and anti cycling followed by refiring, the burner appears to trip on the second and third period even though the modulation was steady for some time after at ~ 57%. Did the S codes show snything?.
Not sure what d.09 actually means. d.17 is normally set to 0, flow temperature control.

Maybe worth doing a factory reset, d.96.

I monitor the boiler remotely and I wasn't looking at the S codes.

d.09 is the flow target temperature if determined by an external controller. So if I set my thermostat to analogue mode it would adjust the flow temperature down as the room approaches the set temperature. As my thermostat is in on/off mode the target flow temperature is set by d.5 (60C) and d.9 reads either 0 or 90 depending on whether the demand for heat is on or off.

d.17 is 0.

Here are a couple more graphs showing when the burner is on or off and the target flow temperature.

What I don't understand is why the modulation becomes unstable.

Mike

 

[Yikes]

I was just reading your problems of last April, maybe try with "manual" target (or return) temperature control. If a factory reset does nothing maybe ask Vaillant for a fixed rate fix.

Oh yes, this saga is never ending. I have tried just about every setting at one time or another. I do have a different thermostat I could try (a VRC470f with weather controller) but that would introduce even more variables and I don't think that's the problem. I did consider sending my graphs to Vaillant tech support to ask for an opinion but I doubt they'd give me one without an expensive home visit.

Mike

 

[Johntheo5]

I monitor the boiler remotely and I wasn't looking at the S codes.

d.09 is the flow target temperature if determined by an external controller. So if I set my thermostat to analogue mode it would adjust the flow temperature down as the room approaches the set temperature. As my thermostat is in on/off mode the target flow temperature is set by d.5 (60C) and d.9 reads either 0 or 90 depending on whether the demand for heat is on or off.

d.17 is 0.

Here are a couple more graphs showing when the burner is on or off and the target flow temperature.

View attachment 320395

What I don't understand is why the modulation becomes unstable.

Mike

Is the modulation reacting to a apparent rise in flow temperature?, still shouldn't be unstable though.
Does the modulation mean that 0 to 100% modulation = 0 to 18kw, if so and assuming the min output of the boiler is, say 3.5kw then as long as the demand is > 3.5kw then it will read from 19.4% upwards?.

I would definitely give that a hard reboot/factory reset, its little brain might be a bit confused with all the different settings or whatever, you've nothing to lose, you may have already tried it anyway.

 

[Yikes]

The logic behind changing the NTCs was to see if modulation was reacting to flow temperature but as the behaviour hasn't changed it must be the other way round. If I saw that behaviour in an audio circuit I'd suspect a faulty decoupling capacitor.

100% modulation is 18kW but my boiler won't go below 5kW.

I'll try a factory reset this weekend.

Thanks,

Mike