Parents vaillant 831 ecotec plus grumbling noise on startup

[scoobydo123]

Hi Parents have a 831 with VRC 470 and are getting a grumbling noise occasionly on CH from start up. Boiler is a couple of years old. The noise will be there for around 10 seconds and then disappear and doesn't happen all the time. Once it has got over its moment it will run and modulate perfectly in silence.

They have said it doesn't happen on DHW.

Have asked them to check the TF1 that was moved from the old boiler to the 831 and they said the water came out clean.

Cheers

 

[Steelmasons]

Possibly a combustion issue.

 

[scoobydo123]

ok time to get someone out I guess. Think it is 2years old in April but dont think they had the 1st year service it was running fine and saving gas and keeping the house at nice even temp so got forgot about.