A central heating system may need to be drained down to carry out a repair or maintenance work or to make a modification to the system such as fitting a new radiator. This guide explains how to drain down and refill a sealed or unvented central heating system. For vented systems see Draining Down Vented Central Heating System.
Draining Down a Sealed Central Heating System
On a sealed system, as the name implies, there is no new water being added to the system on demand. Water is only added when necessary. Turn off the boiler, disconnect it from the power supply, and allow the water in the system to cool. Not only will this safeguard against scalding yourself but the pressure in the system will be reduced. When you open the drain point the water flow will be less powerful making it easier to manage.
Locate the drain point on the heating system. This will usually be at the lowest point of the system and will frequently be on the pipe work next to the lowest radiator in the house. Attach a length of hose to this drain valve and secure using a jubilee clip. Tightening the jubilee clip over the hose where it connects to the drain valve will prevent it coming loose. As a precaution, place a suitable container under the connection in case there is a small amount of leakage. Run the other end of the hose to a suitable drain off point outside. The hose should obviously run downhill to allow the water to flow.
Ensure that the valves are open on all your radiators so that all pipe work will be drained. Open the drain valve by turning the square head with a spanner or small wrench. If it’s tight, hold the body of the valve with a second wrench to prevent it twisting while being opened. Check that the water is flowing to the drain outside. Open the air vent at the top of the radiators to allow air in to replace the water. This will speed up the process.
Once the water has stopped flowing, the heating system will be empty and you can now carry out whatever repairs or maintenance work is required.
Ensure that all air bleed valves are closed on the radiators. Close the drain valve, undo the jubilee clip securing the hose to the drain valve and remove the hose. Hold the hose end up so that any residual water in it will drain to outside. Take the hose outside to coil it back up before storing away.
Open the filling loop by the boiler to allow water to fill up the system and continue filling until the pressure on the gauge reaches the level recommended in the instruction manual. This is usually around 1 bar.
Go round the radiators, one by one, opening the bleed valve and releasing any air. Do the downstairs radiators first and then the upstairs ones. This will minimise the chance of trapping air in the system.
Once you have released all air from the system go back and top up the system again using the filling loop by the boiler. If you have been carrying out repairs or alterations, double check your work to make sure there are no leaks.
Turn the boiler back on following the instructions in the manufacturer's manual, and allow the heating to run normally.
You’ll need to go round bleeding the air bleed valves again as the system heats up, and may need to do this a few times over the next day or two as the air from the water is driven out. Check the pressure gauge each time and top up as necessary.
Finally, once the system is up to temperature, inspect for leaks again. This is very important as a connection which is watertight when cold may expand and leak when warm.