combi boiler fan coming on when use cold water

[emlo]

Recently had a new bathroom fitted and a new kitchen. The main stop cock in the kitchen has been moved and some new piping installed for radiators and fridge. The problem is that everytime we use and switch off the cold water (including flushing the toilet) the combi boiler fan is coming on and sometimes the hot water heating light comes on briefly. Its almost as if the boiler thinks there is a draw on the hot water supply. The boiler is only 18 months old and was working perfectly until this work was done. The bathroom and kitchen installers just seem to be scratching their heads and don't seem to know why this could be happening. I'm no plumber but I do know that the boiler should not be wanting to switch itself on everytime I flush the toilet or fill the kettle up from the cold water tap!

Any help or advice would be much appreciated.
Thanks

[wuzza]

There's a possibility that the pipes may be crossed, when the cold water is being drawn it sounds like its drawing slightly on the hot side, therefore kicking the boiler in.

[htg engineer]

They don't have to be crossed, just the flow of water/change in pressure can sometimes be enough to operate the Microswitch/hall effect sensor.

The pipework must have been altered, if you have tee'd into the cold mains very close to the boiler, try teeing in away from the boiler, see if this helps.

htg