If you have a double feed vented system, the small tank in the loft is your feed & expansion tank. It does 2 things, feeds the central heating system with water if it needs it, and gives a space for the water in the central heating circuit to expand into as it comes on and warms up. So the portsmouth type float valve in the F&E tank should shut off when there isn't much water in the tank when the heating is off, perhaps only 20% full, not sure exactly.
So if your valve is working correctly and allowing enough free space in the tank to allow for expansion the only other thing I can think of is there is some sort of blockage in the feed pipe which is allowing the expanding water up the pipe into the F&E tank but not allowing it back down again? There shouldn't be any valves at all on the pipe from the F&E tank downwards to the heating bit.
|