Valves are sold in pairs, what is the other valve for?
The wheelhead valve is the temperature control, the other is called a lockshield valve and is used to balance the radiators in the system so that they all heat up at roughly the same rate. Simply put, the radiator closest to the boiler if left unchecked would heat quicker than the one sited at the other end of the house, so the lockshield valves should be set opened at increasing amounts the further away from the boiler the radiator is.
