Central heating is a circuit, boiler has a hot out and a cold in.
On the out hot pipe there are valves, one to the rads and one to the hot water cylinder, then return back to the boiler to be reheated.
Each one opens as it's needed.
You can also have a 3 port valve, which does the same as 2 2 port valves in one unit.
yea so the hot out pipe goes to the first valve, and splits to second valve yea?and then they both go to the rads and the tank coil yea?
and on the return do they join back up together is this correct?
No, on a 2 zone valve system the hot water splits first, one to the hot water zone valve and then on to the hot water cylinder/coil. The other split to the radiator zone valve and then on to the rads.