Vaillant 428 ecotec plus microfiring

[SidDIY]

Got a Vaillant 428 ecotec plus vented condenser boiler. It's coming upto 10 years old and a few months back my other half decided to play around with the controls. She said she only adjusted the flow temperature to the radiators. I've noticed recently (though could be completely unrelated) that even when there's no CH or HW requirement so effectively the boiler should be off, it's microfiring i.e. firing up, blowing some steam out the flue then shutting down again. It's does this quite regularly (multiple times an hour) in all temperatures, warm, mild and colder.

Having researched it seems the 400 series shows these traits but I'm unsure what is necessary to stop it, and if it's a fault or setting issue? Any pointers would be appreciated

 

[DP]

If there is no demand, boiler will not be on demand hence off line
I would be looking at the controls which might be the cause of boiler cycling.

Does the display show a radiator symbol indicating call for heat?

 

[jj4091]

May help https://www.theheatinghub.co.uk/articles/Vaillant-hot-water-preheat It sounds like that is how yours is operating.

 

[dilalio]

May help https://www.theheatinghub.co.uk/articles/Vaillant-hot-water-preheat It sounds like that is how yours is operating.

428 is a regular boiler not a combi.

 

[Johntheo5]

Got a Vaillant 428 ecotec plus vented condenser boiler. It's coming upto 10 years old and a few months back my other half decided to play around with the controls. She said she only adjusted the flow temperature to the radiators. I've noticed recently (though could be completely unrelated) that even when there's no CH or HW requirement so effectively the boiler should be off, it's microfiring i.e. firing up, blowing some steam out the flue then shutting down again. It's does this quite regularly (multiple times an hour) in all temperatures, warm, mild and colder.

Having researched it seems the 400 series shows these traits but I'm unsure what is necessary to stop it, and if it's a fault or setting issue? Any pointers would be appreciated

It could be a problem with one of the motorised valve's end switch sticking on even when both HW and CH programmed off as these are fed from a permanent (grey wire) supply, these then supply 230V to fire the boiler re their respective orange wires.

 

[SidDIY]

It could be a problem with one of the motorised valve's end switch sticking on even when both HW and CH programmed off as these are fed from a permanent (grey wire) supply, these then supply 230V to fire the boiler re their respective orange wires.

Is there any way I can check for this? Having done some research I was thinking the PCB has failed making it think it's colder than it is so frost protection keeps kicking in.

 

[Johntheo5]

Does the 438 have frost protection built in?.

If you have a S plan, like below, you can see where all the orange (switched) wires are marshalled together, shown here in terminal 10 but may not be numbered in yours. Just follow any one orange wire from any valve back to where they are marshalled, isloate the power,remove them all, switch the power back on and see does the boiler stay off, if you can use a m.meter or even a phase tester, you can do the initial testing by checking for 230V at where they are marshalled (with power on but HW&CH programmed off), if you still have power there you can remove them one by one to check individually, orange to N (or earth).

 

[SidDIY]

Does the 438 have frost protection built in?.

If you have a S plan, like below, you can see where all the orange (switched) wires are marshalled together, shown here in terminal 10 but may not be numbered in yours. Just follow any one orange wire from any valve back to where they are marshalled, isloate the power,remove them all, switch the power back on and see does the boiler stay off, if you can use a m.meter or even a phase tester, you can do the initial testing by checking for 230V at where they are marshalled (with power on but HW&CH programmed off), if you still have power there you can remove them one by one to check individually, orange to N (or earth).

Good point actually, I'd made an assumption it had frost protection.

 

[Johntheo5]

As mentioned in post #2, "Does the display show a radiator symbol indicating call for heat?", if it normally displayes this and is also displaying it with HW&CH programmed off then quite likely a end switch fault, if not, then unlikely a switch problem.

 

[SidDIY]

As mentioned in post #2, "Does the display show a radiator symbol indicating call for heat?", if it normally displayes this and is also displaying it with HW&CH programmed off then quite likely a end switch fault, if not, then unlikely a switch problem.

Just checked and radiator symbol is flashing when it microfires.

 

[ianmcd]

Just checked and radiator symbol is flashing when it microfires.

Then you do indeed have an end switch problem