"bell" wrote in message
om...

Hi, Does anyone know what's inside one of these?

nothing. it is empty, just a neutral point.

It's a bit of kit to link up two boiler systems. My dad's doing a self
build and he's going to need one but is curious as to how they work.

He needs one? On a selfbuild he is better off using a thermal store/heat
bank, which also acts as a neutral point and gives a hell of a lot more,like
mains pressure hot water, than the Dunsley.