Vaillant ecoTEC 418 unstable modulation

[Yikes]

Happy New Year everyone and good luck with all your plumbing adventures.

Regular visitors to this forum will know that I have an unhealthy obsession with monitoring my boiler's ebus. I noticed recently that the modulation during the CH on periods has a weird instability that has been getting gradually worse. The boiler is 13 years old and recently had new flow and return NTC sensors fitted. Any guesses as to what may be causing this? Control board? The graph shows HW period followed by CH period.

Mike

 

[Johntheo5]

Is it running close to its range rated output, d.00. if this not set to Auto?, or if on Auto maybe change to a manual setting?.
Have you weather compensation?.

 

[Yikes]

Is it running close to its range rated output, d.00. if this not set to Auto?, or if on Auto maybe change to a manual setting?.
Have you weather compensation?.

d.0 is set to 10kW, hence the 55% modulation for CH (HW is 18kW). The controller is a VRT350f set to 2-point control.

Mike

 

[Johntheo5]

10kw at 55% modulation = 18.2kw max output to CH?, there is no instability until the target temperature is achieved.
Have you tried changing d.0 to Auto?.

What is this 2 point control?.

 

[Yikes]

10kw at 55% modulation = 18.2kw max output to CH?, there is no instability until the target temperature is achieved.
Have you tried changing d.0 to Auto?.

What is this 2 point control?.

There is no Auto setting for d.0, it's just the part load in kW. Factory setting is 18 which is far too high for my system.

2-point control is like a conventional on/off thermostat. Flow temperature is set to 65C. The controller has an analogue mode which reduces the flow temperature as the room temperature approaches the target but I haven't played with that much.

This problem only appeared a few months ago, prior to that the modulation was clean. I wondered if a component was failing on the control board.

Mike

 

[Johntheo5]

A failing component is possible of course.
The instability has the modulating at ~ 38/43%, maybe reduce d.0 while this is happening (system heated up) to say 35% and see does it steady down, obviously the flow temperature will start falling.
In your older posts you said you ran on retrurn temperature regulation?.

 

[Yikes]

OK I'll try reducing d.0 when it's stabilised.

In the past I have tried many configurations, including weather control and return regulation but at the moment I'm keeping it simple until I'm confident it's working properly.

Thanks.

Mike

 

[Yikes]

Shortly after the heating came on this afternoon I reduced the modulation from 55% to 33% but the instability is still there.

My heating engineer is going to replace the control board shortly so I'll leave it until then.

Thanks for your input.

Mike

 

[Yikes]

I was right about the control board. I had it replaced and it fixed the problem. Not surprising really as it was 13 years old.

Mike

 

[Johntheo5]

That's good to hear but why is your room temperature only reaching 18.0C max, with a target room temperature of 20C?.

 

[Yikes]

Sorry that's a typo. The target was set at 18C.

Mike

 

[The831Bunny]

Put the 350F on analogue (room modulation), turn the pump up to 2, change D.1 pump over-run to 10mins, and you‘ll get much better and longer burn times instead of all that cycling. You can also drop the maximum flow temperature to prevent over shoot too in D.71.

 

[Yikes]

Put the 350F on analogue (room modulation), turn the pump up to 2, change D.1 pump over-run to 10mins, and you‘ll get much better and longer burn times instead of all that cycling. You can also drop the maximum flow temperature to prevent over shoot too in D.71.

I've always struggled with the boiler settings so thank you for that advice.

Regarding the pump speed, I reduced it to increase the temperature differential between flow and return. My heating engineer said it was fine on 1 and the house warms up just the same as it did on 2.

I've now switched back to the vrc470f weather controller to see how that works.

Mike

 

[Yikes]

Dear Mr Bunny

Could you advise me on another mystery please, this time with the VRC470F weather controller. It gives longer burn times but the boiler fires up every 17 minutes before the HW cycle comes on at 5am. Any idea why?

Mike