Over the years, sludge, rust and scale build up in the water of a heating system. They settle at the bottom of radiators, so the radiator only heats at the top, the boiler is overworked and gas bills rise. Flushing restores heat output and protects the boiler from damage.
Below is a manual, radiator-by-radiator flush for a house with its own boiler. In a block of flats with central heating, agree the work with the building management, as only they can isolate and drain the riser.
What you will need
Important: Only start with the system switched off and completely cold: hot water and steam under pressure are dangerous. A steel radiator full of water is heavy, so remove it with a helper.
Step 1
Get ready
Switch off the boiler and wait until the system has cooled completely. Ventilate the room, put on old clothes and gloves and lay towels and rags on the floor under the radiator.
Decide which radiators to flush. Start downstairs and work your way up. Before closing the lockshield valve, count how many turns open it is so you do not upset the system balance later.
Step 2
Drain the radiator
Close both radiator valves. Place a bucket under the joint and start undoing the union nut between the radiator and the pipe. If the nut is stuck, spray it with WD-40 Smart Straw: it penetrates rust and grime and lubricates the thread. For really corroded joints, use WD-40 Specialist Penetrant and give it 10-15 minutes.
Once the nut is loose, open the bleed valve at the top: air gets in and the water drains freely into the bucket.
Step 3
Remove and tilt the radiator
When the flow slows, undo the second nut, lift the radiator off its brackets and tilt it over the bucket to empty the rest. Plug the open tails with a rag so you do not mark the floor.
Step 4
Flush the inside
Take the radiator outside or to the bath. Connect the hose to one tail and run water through until it comes out clear at the other end. For a better result, tap the sections gently with a rubber mallet to break up compacted sludge.
Black water means corrosion sludge, orange means rust. If there is a lot of dirt, it is a sign the whole system and boiler should be power-flushed by a professional.
Step 5
Refit the radiator
Hang the radiator back on its brackets, reconnect the nuts, renewing the seal if needed, and tighten them with two spanners. Open the valves, returning the lockshield to its original setting. The radiator will slowly refill.
Step 6
Bleed and restart the heating
Slowly turn the bleed valve anticlockwise with the key until you hear a hiss, and keep it open until a steady trickle of water appears. Close it and add a drop of WD-40 Multi-Use to protect it from rust.
Check the boiler pressure gauge, top up if needed, switch the heating on and inspect the joints for leaks after an hour.
Tip: After flushing, add a central heating corrosion inhibitor compatible with your boiler. It slows the formation of new sludge, so the next flush will be needed much later.
What next
Flush the system every 2-3 years or as soon as radiators go cold at the bottom. In between, bleed the radiators each autumn and treat the valves with WD-40 Multi-Use so they do not seize.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
More guides in «How to use».