Improving the appearance of my garden in winter

How To Drain and Refill a Sealed Central Heating System

10 January 2017 by Paul Harrison

If you want to fit a new radiator or replace and old one in a sealed central heating system, you will first need to drain it. Here's a quick guide on how to do it.

Draining a Sealed Central Heating System

1. Start by turning off the boiler and disconnecting it from the mains power supply.  
2. Locate the lowest radiator in the house; you should find the system's drain valve here.  
3. Fix a length of hose to the drain valve and make it secure with a jubilee clip. To catch any leakage, put a small container underneath the valve.  
4. Now take the end of the hose and run it out of the window to ground level; gravity will carry out the drain work for you.  
5. Go around your house opening all the radiator valves, ensuring that all the pipework will be thoroughly drained. You should be able to open the drain valves by using a small spanner or pliers if the valve is tight.  
6. When the water has stopped flowing down the hose and the system is empty, you can fit your new radiator.  
7. To refill the system, close all the radiator valves, including the drain valve.  Remove the hose, allowing any residual water to run down to the outside.  
8. Open up the filling loop (next to your boiler), and allow the system to fill with water. Wait until the pressure gauge reaches the level shown in your boiler's manual, generally around 1 bar.  
9. Beginning downstairs and working your way up; open the bleed valve on each radiator to release any air that may be trapped in the system.  
10. Check your repairs to make sure there are no leaks.  
11. Switch the electricity back on and turn on your boiler, allowing your central heating to operate normally.  
12. As the radiators heat up, you may need to bleed them again. Check the pressure gauge on the boiler and top-up the system if necessary.  
13. Lastly, when the system reaches the temperature you want, go around and check again for leaks. This is necessary with a sealed system as a previously watertight cold connection will expand and when warm, possibly causing leaks.

If you need to carry out maintenance work or replace a broken radiator, you'll need to drain down your heating system first, using the above guide to help you. Always consult your boiler manual before attempting to switch off and drain the system.