Draining & Refilling a Heating System

You may need to drain and refill your central heating system if you want to move radiators or replace them with others of a different size, requiring changes to the pipework, or if you want to treat corrosion.

A central heating system that uses water (the vast majority) will have at least one drainvalve that allows you to empty out the water. The drainvalve will usually be at the lowest point in the system, possibly under the floorboards, but there may be additional drainvalves by the boiler or where pipes are looped down from the first floor to a ground floor radiator - often the case with kitchen radiators.

Draining the System

1. Switch off the central heating.
2. Close off the water supply to the feed-and-expansion cistern to stop the system re-filling. To do this you can either turn off the water at the main stopvalve or tie up the ballvalve in the feed and expansion cistern - a better option since it allows you to continue using the kitchen cold tap. You can release the ballvalve any time you want to flush the system through with fresh water. You might need to do this after using a cleanser prior to adding corrosion proofer.
3. At each drainvalve (it doesn't matter where you start) attach a hosepipe to the outlet of the valve and lead the hose outside or into a bath.
4. Using a small spanner, unscrew the valve, allowing water to flow out.
5. Unscrew the air bleed valve on every radiator to make sure all the water is removed. You will need to include the air bleed valve on the hot water cylinder. This is usually located on the highest point on the upper of the two pipes that lead from the boiler into the side of the cylinder.

While the system is empty, you might want to remove individual radiators, take them into the garden and flush them through with water from a garden hose.

Refilling the System

1. Again with the heating system turned off, close all the drainvalves and air bleed valves and release the ballvalve in the feed-and-expansion cistern.
2. As the system fills, bleed each radiator in turn, starting with those on the ground floor and working upwards. You may need to go round the whole system twice, not forgetting the air bleed valve on the hot water circuit next to the hot water cylinder. Corrosion proofer is added on the final fill after cleansing and flushing (check instructions supplied with corrosion proofer for details).
3. When the system is full, turn the boiler on and check that there are no leaks from any of the air bleed valves or drainvalves - tighten any that are leaking.