Combi kicking in when cold tap turned off and washing machine turned on

[scarface93]

Hi folks, currently have a combi boiler that was fitted for about 2.5 years ago, was previously a boiler and tank so a changed system but there was no issues until recently. This week I've had a bathroom refit but since the change the combi appears to turn on when the cold water is turned off for a few seconds as if it's being called into action. The bathroom refit hasn't caused any extra dead legs in the water system which I've read could be the issue. There appears to be a dead leg in the system upstairs where the cylinder on the old system used to be and also for the old hot water feed for a washing machine which is also in the bathroom.

Is it likely that a dead leg that didn't previously affect the plumbing could be causing this issue now?

 

[Madrab]

It can also be a pull on the hot/cold water pipework by the sudden opening/closing of a new 1/4 turn tap sending a pulse down the pipework that tricks the flow switch in the boiler into thinking there's a demand and then the pump/fan kicks in but the boiler itself doesn't actually fire up.

Sometime a check valve on the boiler cold feed can correct that.

 

[Jurassicspark]

Diverter valve also may need inspecting.