How to drain a radiator system

Occasionally, you'll need to drain your entire heating system rather than just a single radiator. This might be necessary if your boiler needs to be replaced, you've noticed multiple leaks in your radiator network, or you're carrying out work on your central heating.

While this is a more involved process than draining a single radiator, it's still manageable if you follow these eight steps carefully.

1. Switch off your heating system

Before you do anything else, switch off your boiler completely using the main power switch. Then, wait until all your radiators and pipes have cooled completely.

If you have a solid fuel boiler, you'll need to extinguish the fire safely and wait until the boiler is completely cold, which can take several hours. After a while, give the pipes a careful touch to make sure they feel cool.

2. Turn off the water supply

Next, you need to stop any new water from entering your heating system while you drain it. How to do this depends on which type of boiler you have:

• Combi boiler – If you have a combi boiler, the hot water comes directly from the mains, so you'll need to close the stopcock. This is usually under your kitchen sink, but it might also be in a utility cupboard or near your front door. Turn it clockwise until it stops.
• Hot water cylinder boiler – If you have a boiler with a hot water cylinder, look for the pipe running from your cold water storage cistern (which is usually in the loft) to the cylinder. There should be a valve on this pipe which you can close by turning clockwise.

"Double check that you've definitely closed the right valve. If you're not sure which valve controls your heating system, turn on a tap and see if water still comes out – if it does, you've closed the wrong one," says Brian.

3. Attach your hosepipe

Now you need to find the radiator in your home that has a drain-off valve. This can usually be found on the radiator at the lowest point in your house, with the valve sticking out of the radiator at about a 45-degree angle from the bottom left or right side.

Take your hosepipe and secure it around the drain-off valve using a jubilee clip. The other end of the hosepipe should either go into a large bucket or run outside to a drain – but don't let the water spill out onto plants or grass, as your radiator system likely contains chemicals that can harm them.

Brian explains: "Tighten the jubilee clip properly using a flat-head screwdriver; a loose connection could end up leaking everywhere and make a mess of your floor. Give the hose a gentle tug once it's attached – if it comes off easily, it needs tightening more. And if you're draining into buckets, make sure you've got plenty of them lined up and ready because you could be looking at 100 litres or more depending on the size of your system."

4. Bleed your radiators

Before opening the drain valve, walk around your home and ensure all your radiator valves are in the open position. Then, return to the radiator with the hosepipe attached and open the drain-off valve by turning it anti-clockwise. Water will then start flowing out through the hosepipe.

Brian says: "To speed up the process, bleed your radiators one at a time – starting with the ones upstairs. Insert your radiator key into each bleed valve and open it up, and you'll hear air being sucked into the system. This builds up pressure in the radiator, pushing the water out faster."

"This is where you'll really see the benefit of draining your system. That first bit of water that comes out can be absolutely filthy – thick, dark, and full of sludge. That's all the gunk that's been building up and impacting your heating efficiency. Once it starts running clearer, you know you're getting somewhere," he adds.

5. Drain the system

Give the system enough time to drain completely. Depending on how large your heating system is, this can take anywhere from 30 minutes to an hour or more. You'll know it's finished when the water stops flowing out of the hosepipe, and you can't hear any more water moving through the pipes.

6. Close the valves

Once the system is fully drained, go back around and tighten up all the bleed valves you opened earlier. Make sure each one is fully closed to avoid any leaks. Then, close the drain-off valve where your hosepipe is attached and carefully remove the hosepipe, but keep your bucket and towels handy to mop up any residual drips.

"Double-check every bleed valve is closed before you start refilling your heating system," Brian warns. "It's easy to forget one, and when you turn the water back on, you'll have water spraying out all over your bedroom or living room. I always do a quick walk around and give each one a gentle turn just to make sure."

7. Turn on the water supply

Now you can turn the water supply back on. Go back to whichever valve you closed earlier (either the mains stopcock for a combi boiler, or the valve on the pipe to your hot water cylinder) and turn it anti-clockwise to open it back up.

8. Refill the boiler

If you have a combi boiler, go to the filling loop (usually underneath the boiler) and slowly open the valves to let water back into the system. Watch the pressure gauge on your boiler – you want it to reach between 1 and 1.5 bar while the system is cold. Once it reaches the correct pressure, close the filling loop valves.

If you have a classic system boiler, the water cylinder should refill automatically once you've opened the same valve you closed earlier.

Once your system has refilled, switch your boiler back on. Go around and check that all your radiators are heating up properly. You'll likely need to bleed each radiator again to remove any air that's entered the system during refilling. Also, don't forget to check underneath each radiator for any leaks from the valve connections – if you spot any drips, tighten the nuts slightly.