Problem with combi boiler since service

[peweuk]

We have a Vaillant gas combi boiler which was in the house when we bought it from the developer in 2017.
It worked fine up until late December 2019 when it would intermittently make a noise when the heating was fired up by the stat - the hot water was fine when called for by either water being used or the pre-heat topping up. The noise is similar to trying to start a car with a very flat battery.
As the service was due in March/April, to save a separate call out charge we booked a service in mid January.
When the engineer arrived as the boiler would not play up he spent 20 minutes or so palying with settings etc in an effort to get the boiler to make the noise - but no joy, so he suggested next time the noise started we should record it and call them again.

The day after the service the noise recurred so we recorded it BUT we now experience a different problem which we have never had before.

We noticed that the radiators were getting hot at times when the thermostat was set much lower than the actual house temperature, so the thermostat was not calling for heat.
I turned off the heating at the boiler just leaving the water on - but the radiators still kept heating up intermittently (they could be cool for an hour then suddenly get hot)

My question is, could the engineer have changed a setting in the boiler that is causing this. I know some boilers used to have a facility to work on boiler water temperature rather than be activated by an external thermostat, but this is not mentioned anywhere in the user manual for this boiler.

 

[ianmcd]

Unlikely to be a setting sounds like your diverter valve is sticking

 

[peweuk]

Unlikely to be a setting sounds like your diverter valve is sticking

Thanks for your comment.
Let me see if I correctly understand the consequences of a sticking diverter valve.
The diverter valve I guess is what directs the water heated by the boiler to either the hot water preheat tank or the central heating circuit, and if it was sticking, it could be that the water destined to pre-heat the hot water is able to circulate in the heating circuit - or am I way off in my understanding.

Not sure if this is relevant, but one thing we did notice was that when the heating was called for and the boiler made the noise, if we opened a hot water tap the noise (in most cases) would stop and would not return when the water was turned off and the boiler continued to heat the water for the radiators.

 

[ianmcd]

yes you are pretty much correct, yes operating a HW tap could have an effect on the noise

 

[peweuk]

Thanks.
I will investigate further and then speak to Vaillant.