Draining and re-filling a sealed heating system


If you want to fit a new radiator or replace an old one in a sealed gas central heating system, you will first need to drain it. If you don’t drain the radiator prior to removing it, it will leak water all over your floor!

Draining a Sealed Central Heating System

Start by turning off the boiler and disconnecting it from the mains power supply. You will then need to locate the lowest radiator in the house; you should find the system’s drain valve here. 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.

Now take the end of the hose and run it out of a door to ground level; gravity will carry out the drain work for you. 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. now vent all of the radiators starting from the highest first then when the water has stopped flowing down the hose the system is empty, you can fit your new radiator.

To refill the system, close all the radiator vent valves, including the drain valve. Remove the hose, allowing any residual water to run down to the outside. 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.

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. Check your repairs to make sure there are no leaks. Switch the electricity back on and turn on your boiler, allowing your central heating to operate normally. 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.

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.

Contact the experts

If you’re not confident in attempting this work yourself, contact the experienced experts at Shaw’s Plumbing and Heating for assistance.
0800 8 247 585

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