Open vented systems only
Occasionally you may need to empty the system in order to carry out some work on it – maybe to repair a leak or fit another radiator. Before doing so, you must switch off the boiler, disconnect it from the power supply and turn off the gas supply or, in the case of a solid fuel boiler, ensure that the fire is out.
Turn off the water supply to the feed and expansion tank. If you don’t have a valve for this, or simply can’t find it, tie up the ball cock with a piece of string round a length of wood across the top of the cistern.
Connect a hose to the drain cock which is normally found at the lowest point in the pipe work. Use a jubilee clip to secure it. Place the other end of the hose outside to discharge into a gulley.
Open the drain cock using a pair of pliers and allow the water to begin emptying. Now, go round opening the air bleed valves on the upstairs radiators to allow air in to replace the water. If you don’t, it’s a bit like holding a drinking-straw full of water with your finger over the top end – the water won’t come out. There may be additional vent points in some systems so check for these as well.
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.
When no more water comes from the hose, the system should be empty. However, it is always possible that some water has been airlocked, so take care.
Close the drain cock and all the air bleed valves which were opened to help drain the system. Turn the water supply to the cistern back on.
Now, open and bleed the downstairs air bleed valves first, then the upstairs ones. Once all this has been done, the cistern will stop filling as the ballcock closes off the water. The level of water should be just enough to float the ballcock. More than this, and there will not be enough room for expansion as the system heats up. If necessary, adjust the ballcock.
Check any work that you have carried out to ensure that there are no leaks, then reconnect the power supply and turn the gas cock back on. Re light the pilot light following the boiler maker’s instructions and fire up the boiler. Remember to turn up the thermostat if needs be.
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.