The answer to this is yes and no — but it’s all good news – yes you don’t have to drain the whole central heating system but no you don’t even have to connect the two pipes together. By closing the valves at both ends you can drain the water out of the radiator then take it away.
Start by getting a bucket or bowl that will fit under the radiator and close the control valve, that’s the larger one which might have a thermostatic valve on it. Then do to the other end, closing the smaller valve that’s known as the lockshield valve. You’ll need to remove the small white cap and get pliers or a small spanner to do this.
It’s important to count the number of turns to close the lockshield valve as you’ll need to set it back in the same place when you replace the radiator.
Disconnecting the Radiator
Place a number of old towels or dustsheets under the radiator to collect any water spills and put the bowl under the control valve. Then disconnect the union nut (the nut holding the radiator to the control valve) and water will begin to trickle out of the radiator into the bowl.
Once the radiator is fully drained you can undo the union nut at the other end, by the lockshield valve. Opening the bleed screw at the top of the radiator will increase the flow of water out of it. Then stuff paper into each end of the radiator to prevent drips and remove it from the brackets holding it onto the wall.
Replacing the Radiator
To replace the radiator on the wall, hang it on the brackets and reconnect the valves at both ends, cleaning the threads first and using PTFE tape on the threads. Then open the valves to allow water into the radiator, opening the lockshield valve the same number of turns as when you closed it. This will keep it at the same temperature as before.
Undo the bleed screw to allow air to escape fully although be careful to close it before water gushes out. You may have to bleed other radiators nearby if air gets into the pipes from the radiator you emptied. Check for leaks from the valves too.
Doing Away with the Radiator
If you don’t want to replace the radiator you can join the two valves together with a pipe and this is unlikely to harm the central heating system. Air would get into the pipes though and with no bleed valve you would have to chase the air through other radiators.
You will also have to re-balance the system as the removal of one radiator will upset the equilibrium which means that other radiators will be hotter or colder as a result. You must also protect or hide the pipe that has replaced the radiator as it will get very hot. In some way you need to prevent people from being able to touch it.
The correct solution, as you probably already know, is to re-route the pipework to remove the valves and the unused section of the pipe network.