In a central heating system the radiators closest to the boiler will naturally be warmer than those farther away. When the system is first installed, the radiator valves will be adjusted to ensure that all radiators in the system maintain similar temperatures. But, when changes to the system are made over time this balance may be affected, with some radiators being cooler than others when they're supposed to all be at the same temperature. It is a little time-consuming, but perfectly possible, to rebalance the radiators yourself to optimise their energy efficiency.
Switch off the central heating and wait until the radiators are cool before starting. Identify the flow valve and the return valve at the bottom ends of each radiator by turning the heating on and feeling which valve heats up first. This is the flow valve. Mark the flow and return valves with different coloured tape.
Open both valves fully on every radiator in the system by turning them counterclockwise with a radiator spanner. Return to the radiator nearest the boiler and clip a radiator thermometer to the pipe connected to the flow valve at a point just below the valve. Clip a second radiator thermometer to the pipe connected to the return valve.
Switch the central heating back on. Wait 5 minutes and then close the return valve by turning it clockwise using the radiator spanner. Slowly open it up again while watching the thermometers. Stop turning the valve when the temperature differential between the two is -11.1 degrees C Centigrade.
Move to the next radiator from the boiler. Attach the two thermometers as before. Close the return valve and then slowly open it again until the temperature difference is -11.1 degrees C Centigrade. Repeat this for each radiator in the heating system.
If you're not sure of the order of the radiators in the system relative to the boiler, get someone to help you identify which ones heat up first when the heat is switched on. The return valve is often located under a plastic cover that easily pushes off.
If a valve leaks water when fully opened there may be a problem with the valve, in which case replace it before trying to balance the radiators.