istor he260 and manifold to underfloor heating?

our boiler provides hot water to the tap shows a c on the front control which i believe means the heating is on but the water heats in the flow pipe and doesnt get past a valve to heat the undefloor manifold,
this element has a red base which turns at 1 degree c plus in one direction and - 1 in the opposite direction.god knows what its called.
any suggestions