You may need to empty the water from the central heating system so that you can carry out repairs and maintenance or modify pipe work and fit a new radiator. This guide explains how to drain down and refill a vented central heating system. For sealed or unvented systems see Draining Down Sealed Central Heating System.
Draining Down a Vented Central Heating System
On a vented system, there will be a feed and expansion cistern (header tank as it is sometimes called) which allows for the changing water level in the system as the water heats and cools. This cistern also allows the water to be topped up as required by means of a float valve. Turn off the boiler, disconnect it from the power supply, and allow the water in the system to cool. This will safeguard against scalding yourself.
Turn off the supply of water to the system by closing the valve on the inlet to the feed and expansion cistern. If the valve is seized, you can tie up the float valve so that no water is supplied by the inlet instead. If this is the case, take the time to replace the seized valve before refilling the system
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 upstairs radiators to allow air in to replace the water. This will speed up the process. As the level of water in the system drops below the height of the downstairs radiators, you can go round doing the same thing for the lower radiators. If the level hasn’t dropped far enough, water will be pushed out of the vents rather than air sucked in.
Once the water has stopped flowing, the heating system will be empty and you can now carry out whatever repair or maintenance work is required. It is always possible that some water has been airlocked, so take care.
Refilling a Vented Central Heating System
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 valve on the inlet to the feed and expansion cistern so that water can now flow into the system again. If you tied up the float valve on the inlet, release this allowing water to flow into the cistern (and therefore the heating system) again.
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 check that the feed and expansion cistern has water in it and that this is at its normal cold level. When cold, the level in this is fairly low as there needs to be room for the water to expand as it heats up. 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.
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.