Why does my bath mixer start dripping fast when my boiler comes on to make hot water?

[FarmerBrian]

I've had new washers put on my bath mixer tap but when the boiler comes on to make hot water, the bath mixer starts dripping fast. I have to go and turn it off very tightly. I don't understand what's happening. The bath mixer doesn't drip otherwise. Any idea what's going on? When I've felt the dripping water, it's cold not hot..

My basin taps do this sometimes. I turn the bath taps on and I hear whooshing behind me and the other taps have come on. It doesn't happen all the time I usually mutter about the DC not turning them off fully, but it has only happened since we had a new boiler fitted November 19.

No help, but it would be good if anyone else could explain why.