How to Drain a Central Heating System
DRAINING AND FLUSHING THE SYSTEM
Draining the central heating water from the system is done via the system drain cock. Normally there is only one — fitted on the boiler or one of the lowest radiators — but systems with complicated pipe runs may have more (in which case drain each in turn). Avoid airlocks by opening the radiator bleed valves (see below), but give some of the water a chance to escape first.
Check the colour of the system water as it drains out: a deep black is normal; rust red points to severe corrosion — flush the system before refilling, and add a corrosion inhibitor.
Flushing and refilling
To flush, leave the drain cock open and restore theat the expansion tank. Let the water run through the system for at least 30 minutes, then turn off at the tank again.
To refill, close the drain cock and open the tank stop-valve part-way so that the water enters slowly. Then, starting on the ground floor, close the radiator bleed valves one by one as the system starts to fill.
1. Switch off the power, then turn off the water supply to the expansion tank. If there is no stop-valve, tie up the tank’s ball-valve.
2. Identify the system drain cock, which has a square shaped head and a small spout. Look for it near the boiler, or on the lowest pipe in the system.
3. Connect one end of a hose to the spout and run the other end to a nearby drain. Turn the drain cock head anticlockwise with an adjustable spanner.
4. Starting at the top of the system, open the radiator bleed valves one by one to release any air trapped during the draining process.