Overflow into expansion tank

[Yeti]

When the heating system is on there is an almost continuous flow of hot water from the overflow pipe into the expansion tank. It only seems to do it when the heating is on, not when the hot water is being heated up. Should it run all the time? Its very noisy and sounds like a mini waterfall in the loft!

The other problem is that when the water starts to heat up the radiators go cold - don't know if that's related problem or something else?

[Gareth Thomas]

I know it sounds stupid, but from what you say it sounds like the pipework is plumbed up incorrectly, especially if as you say it's like a waterfall when you turn on the heating. Trace the pipework from the boiler, sounds like your flow/return pipe has been mixed up with your overflow or vent pipe.