Should your heating be on or off when you bleed radiators? I asked the experts why it's so crucial to get this step right

After a very mild start to autumn, I am definitely starting to notice temperatures dropping. While I've been using an oil-filled radiator for the odd days it's been a bit chilly, I am now contemplating putting my central heating on for the first time this season.

Before I do that though, I always like to bleed my radiators so I know they'll offer even heat. While this is a task well within the skillset of even the most novice DIYers, there are a few key things you need to know before you start, from whether you should bleed upstairs or downstairs radiators first, to whether your heating should be on or off.

Should your heating be on or off when you bleed a radiator?

While having your heating on can help identify radiators that are warm at the bottom and cold at the top (a clear sign of air build up), for the actual act of bleeding your radiators, it is essential that your heating is off and your radiators cold.

'If you’re going to bleed your radiators, make sure the heating is switched off and, ideally, wait until your radiators have gone completely cold.,' says Nick Duggan, Director of The Radiator Centre. 'You don’t want to be opening the bleed valve while there’s hot water circulating, as you could end up with hot water coming out and potentially burn yourself. Letting the radiator cool down also gives the air and water inside a chance to settle, so you’ll get a better result when you bleed it.'

As well as the risk of scolding, trying to bleed a radiator while the heating is on means water will come out faster, making it a rather messy business. Plus it's less effective and you'll likely find your radiators have cold spots again.

How long after bleeding your radiators can you turn the heating back on?

While there's no set time period as to how long you should wait before turning your heating back on after bleeding your radiators, experts do suggest making one quick check before you turn it back on.

'Once you’ve finished, you don’t need to leave the heating off for ages,' explains Nick. 'Give it a few minutes, check the boiler pressure, and then you can turn the heating back on. If you’ve bled a few radiators, the pressure may have dropped slightly, so it’s worth checking it’s still within the recommended range before you do.'

How do you identify which radiators need bleeding?

While a cold spot at the top of a radiator is a common sign a specific radiator needs bleeding, you might not be able to feel all the signs of trapped air in the system.

'Trapped air can create cold spots, reduce heating performance and, over time, contribute to corrosion within the system,' explains Darren Danks, Head of Training at Worcester Bosch. 'It's also worth noting that small pockets of air don't always produce noticeable cold spots, so a system can still contain air even if every radiator feels warm.'

So while you could just bleed the radiator that has a noticeable cold spot at the top, it's worthwhile taking a couple of extra minutes to bleed all of your radiators to release any hidden trapped air from the system.

How long does it take to bleed a radiator?

The good news is it only takes about 30 seconds to bleed a radiator, so you can likely bleed all of the radiators in your home in around 15 minutes. It might take a bit longer if you have radiator covers that need to be removed first.

But once you have the tools you need, and have the process nailed, it's definitely not a long and laborious task. This is what you'll need: