How to Bleed Radiators and the Causes of Trapped Air in your Heating System

Bleeding a radiatorThere's one plumbing and heating job that we're all happy to do, even if you don't know the difference between your U-bend and your pipe-cutters, and that's bleeding the radiator. While we're all happy treating the symptoms, if you find yourself regularly bleeding the radiators in your home it might be worth calling us in to take a look and solve your radiator management problem!

Why do radiators need bleeding at all?

Radiators, and central heating systems in general, sometimes suffer from having trapped air in the system. It's not always just "air" of course, sometimes it can be hydrogen because of rust and muck inside the system. Either way – you know your radiators aren't working to full efficiency.

What are the most likely causes of trapped air in your system?

This air can come in if the central heating system has been installed with the pump above the supply pipe - which, while not the end of the world - does lead to some extra air being added to the system. Additionally, for non-combi systems - for example, those with an open tank in the loft for their immersion heater - this open tank can be a source of extra air being added to the system.

If you're having to bleed an old system often this could be related to a build-up of hydrogen in the system - which can occur when a pipe is rusty inside, or a build-up of "sludge" has occurred. While this isn’t dangerous, it does stop your heating from working at its best.

And finally, small leaks can let air in - and often these are so small it's hard for you to notice. This can especially be the case if you find yourself re-pressurising the boiler regularly.

How to check if your radiator needs bleeding?

You are most likely to bleed a radiator when you notice it's not as hot as it should be. The radiators that need bleeding the most are often those at the highest point in the house (as this is where the air rises to).

While the heating and radiators are on, place your hand on the top of the radiator and feel if it is hot. Run your hand down the front of the radiator and feel where the heat begins. The cold part of the radiator is the air trapped at the top of the system, and it is this that you will need to bleed.

How to bleed a radiator?

Once you've found the best radiator, or radiators, to bleed, you can do the following to bleed your radiator

Turn off the heating. You don't want the system to be pumping hard while you open it! It's also best to let the system cool before you begin to bleed the radiators. This helps in two ways:

You don't get scolded by hot water from the radiator and:

Cool water in the system takes up less room that hot water (as water expands when heated), this means you're letting out the optimal amount of air from your central heating system.

Once the system has cooled, you're ready to bleed the radiator using a radiator valve key. We suggest that you buy a decent on of these, once, and keep it safe. While you can get cheap radiator keys, they have been known to "split" on stubborn valves. You should also have a towel or rag handy to help you catch any escaping water from the system.

Apply the radiator key to the valve at the end of the radiator at the top - it should only fit one way. Hold the rag or towel below the valve against the radiator so as to avoid too much mess.

Turn the radiator valve key slightly to open the valve just enough for the air to escape. Be ready to close it quickly however once the air has been removed, as the water from the system will shortly follow.

Once the air is bled from the system, tighten up the valve and wipe down any excess dribbles from the radiator. Once you've bled one radiator you might find the air still moves about the system, sometimes back to the same place. Don't fret, though - this is just how it works.

You will need to re-pressurise your system and turn it back on again to check whether there are any more air pockets in the system. If you find more in the system once it has settled you will need to repeat the process.

And that's it!

If you find yourself constantly managing the radiators in your house, and endless re-pressurising your boiler, then it might be a sign of a bigger problem. If that's the case, why not get in touch and we'll see what we can do to diagnose and alleviate the problem.