Vaillent comby boiler starting when the cold is turned on

[DIYspanner]

Should the comby (Turbo max I believe) start up when only the cold is turned on?

It starts up for about 10 seconds then stops but why does the boiler need to start if only the cold is running?

The pressure is zero in the photo but I topped it up to 1 bar since then.

 

[picasso]

It normally caused by a dead leg on the pipework, have a look at your pipework and see if there are any runs of capped pipes.

 

[CBW]

Yep, sounds like a dead leg as per @picasso. Try looking in what would have been the hot water cylinder cupboard.

 

[ianmcd]

Agree with the others , you have a dead leg somewhere

 

[DIYspanner]

The house doesn't have an airing cupboard. Its had a comby since built around 2004 as far as I'm aware.

I will have caused the dead leg when I fitted x2 push fit end stops to a hot and cold feed for an Agualisa smart shower I decommissioned. The water was not being shut off properly so had a constant drip at the shower head. When time permits, I intend to fit a Mira mixer shower instead. The shower cubicle is fitted in a small ensuite with w/c and sink. The house was plumed with plastic although the comby and main incoming is copper pipework.

Why does a dead leg cause a comby boiler to start up?

 

[CBW]

Why does a dead leg cause a comby boiler to start up?

Usually trapped air, on the hot water side, only takes a small amount, and this sends a false signal to the boiler, especially when 1/4 turn taps are used.