Turbomax Starts Up When Cold Tap Turned Off

[plumbandplumber]

I have a Vaillant Turbomax Plus combi of 2004 vintage. When a cold tap is used nothing happens to the boiler, but as soon as the cold tap is turned off, the boiler starts up for a short time. The hot water demand lights up and the fan (maybe also pump??) runs, but the boiler doesn't get as far as lighting (no burner lit light appears).

Do any of the experts know what causes this, and what the solution is? Thanks for any help.

 

[ShaunCorbs]

Sounds like you need a shock arrestor fitting on the cold inlet to the boiler

Little test (on the cold tap that fires the boiler up)

Open a cold tap and let it run for a second or two and slowly close the tap does it fire the boiler up

 

[Last Plumber]

You have a dead leg somewhere, most likely where the cylinder used to be.
You can bleed it, if you can find it but the best bet is to remove it

 

[plumbandplumber]

Thanks to both of you for your replies. Will try turning the cold tap off slowly when I get home.

Last Plumber>>> There was a cylinder in the cupboard upstairs years ago. Can't see any capped pipes in there, but I suppose it could be under the floor which will be a bit of a mare to get to. If there is possibly a dead leg that I can't find, would Shaun's suggestion of a shock arrestor on the cold inlet solve the problem?

 

[Last Plumber]

Thanks to both of you for your replies. Will try turning the cold tap off slowly when I get home.

Last Plumber>>> There was a cylinder in the cupboard upstairs years ago. Can't see any capped pipes in there, but I suppose it could be under the floor which will be a bit of a mare to get to. If there is possibly a dead leg that I can't find, would Shaun's suggestion of a shock arrestor on the cold inlet solve the problem?

I would guess it's possible if the charge pressure was set lower than the compression you get in the dead leg but that is nigh on impossible to judge, apart from through trial and error. You could try it, you never know !
The dead leg would be on the hot side, probably on the pipe that used to come off the cylinder. I bet it's stop ended under the floor where the cylinder was.
If you do try Shaun's idea, let us know if it worked will you? Like Shaun said, if you do try that, fit it on the cold supply.