To ensure an even distribution of the hot water to the radiators in a system you’ll need to do what’s known as balancing the system. Because the water is pumped via a pipe, and branches to feed each radiator, the ones at the beginning of the ‘run’ tend to get more than their fair share.
Balancing radiators is achieved by using the lockshield valve on each to regulate the flow of water to the radiator. By partial closing of these valves the radiators nearest the pump can be restricted more than those further away.
The best way to get this right is by using radiator thermometers, which can simply be clipped to the flow and return pipe at either end, and measuring the difference. You won't often see a plumber balancing radiators using these though - experience enables them to adjust the valves without measuring the temperature.
Remember, the pipe to each radiator is a branch from the main flow pipe. Near the furthest radiator, the main flow pipe turns back towards the boiler and becomes the return pipe. The pipe from each radiator then feeds back into this.
The water does not go into the first radiator then out and on to the next etc. It actually flows from one main pipe feeding all of them, and then flows back out into the return coming back from all of them.
When balancing radiators you will need to turn the heating off well in advance, so that the water has a chance to cool right down. Each radiator has a control valve – the one you use to turn it on or off, and a lockshield valve – the one with a cover which you don’t normally turn. To open or close this lockshield valve, you need to remove the cover and use a pair of pliers. Open both valves on all the radiators.
Now, turn the heating back on and go to the first radiator. You are going to adjust them in the order in which they are served, so if you don’t know this, make a note of the order in which they heat up.
Fit the radiator thermometers to the flow and return. Turn off the lockshield valve then open it gradually again until the difference between the two thermometers is about 20 degrees Farenheight.
Move on to the second radiator on the system and do the same. If you repeat this for all of them in order, you should have a perfectly balanced system so that all radiators heat up efficiently. You may find the last one will need the lockshield valve fully open.