Why do radiators need bleeding?

17 January 2023
Why do radiators need bleeding?

Why do radiators need bleeding?

Bleeding a radiator of airBleeding a radiator of air

When it comes to central heating systems, some things must be left to professionals particularly if repairs on your gas boiler are required. However, there are some small DIY jobs that you can perform on your radiators that are very simple, yet will make a huge difference to the smooth running and efficiency of your heating. Bleeding your radiators is one such task.

In this article, we'll look at why radiators sometimes need bleeding, some common signs to look for and how you can sort it out and get your rads up and running properly again.

What does 'bleeding a radiator' mean?

Despite the rather dramatic wording, bleeding a radiator does not involve cutting a radiator open and allowing the water to spill out onto the floor. Rather, it's the simple process of allowing air that has become trapped in the radiator to escape, usually by using a radiator bleed key to open up the radiator bleed valve.

When air gets trapped in a radiator, it actually stops the radiator from heating up properly, which in turn will cause more energy to be used as your boiler has to work harder to try and heat your home to a comfortable temperature. Releasing, or bleeding, the air out of the radiator will allow hot water to reach all parts of the radiator and heat up your room more efficiently.

How do you know if your radiator needs bleeding?

The most common symptom of air being trapped in a radiator is if the rad feels hot at the bottom but is cooler the closer you get to the top.

Air is lighter than water so if it gets into your radiator it will rise up to the top of the radiator's chamber and create an air pocket. Because radiators work by filling up with hot water and emitting heat out into the room, an internal air pocket will stop hot water from reaching the top of the radiator. The more air that is trapped in a radiator, the less hot water is in the radiator to heat the room.

If you have a thermostat set to a particular temperature, or thermostatic radiator valves turned to a particular setting, your boiler and pump will be constantly sending hot water into your radiator in order to raise the room temperature up to whatever you've set. A lot of trapped air will severely hinder the efforts of your boiler because the radiator is not operating to its full capacity.

What causes air to be trapped in radiators?

Air getting trapped in the top of radiators is definitely detrimental to the efficient running of your heating systems and there's a few reasons why it might happen.

Leaky pipes or valves

Over time, pipes and valves can develop small leaks due to corrosion or weakening of the joints. It doesn't necessarily have to be large leaks that involves water gushing everywhere as even a tiny leak is enough to allow air into the system. If you find that your boiler pressure regularly drops with the system requiring re-pressurising, you may have a leak in your system that requires further investigation.

Hydrogen build up

If you don't keep your chemical inhibitor levels topped up within your central heating system, the internal components of your system are going to be more prone to corrosion and rust. Over time, tiny rusty parts of metal can break off and settle at the bottom of radiators in the form of a rusty, brown sludge.

This rusty sludge can release a lot of hydrogen, which will rise to the top of radiators and create air pockets. This can have a double-whammy effect of sludge preventing hot water from heating the bottom of the radiator and the trapped hydrogen (air) stopping hot water heating the top of the radiator - both of which are very bad for the efficiency of your heating.

Pump location

The pump is the component of your heating system that helps water to flow around your pipe network between the tank, your boiler and the radiators. If your pump is above your supply pipe, then it's possible for air to enter the system there. This is a common feature of older properties where the water tank may also be in the loft.

How to remove air from your radiator

Switch your heating system off and allow 30mins for it to cool down.

Twist both radiator valves so that they are open.

Take a radiator bleed key and insert it into the bleed screw that you will usually find at the top of the radiator.

Turn the radiator bleed key slowly in a clockwise direction by about half a turn. You should be able to hear the hissing of the air escaping at this point. Have a cloth or towel ready as some water is likely to escape once the air has been bled.

Continue this process until water begins to squirt out and then retighten the screw until you can confirm there are no further leaks. Repeat this process on all of your radiators and then top up and repressurise your boiler if necessary.

Below is a video of a professional plumber showing you how to bleed radiator step by step

Regular heating maintenance

Once you've bled all your radiators, switch on your central heating and you should find that all of your radiators are heating up evenly and fully. If this is not the case, it's worth contacting a professional heating engineer to investigate if there are any other problems.

If you find that you're having to bleed your radiators on a regular basis, there's clearly a leak in your system where air is coming in. Again, this would be time to get in a professional to get to the bottom of it.

It's recommended to have your boiler and heating system inspected every 12 months. A heating engineer can come in, service your boiler, top up your chemical inhibitor levels and generally ensure your system is running smoothly and efficiently.

Annual heating maintenance is the best way to ensure your heating is doing what it should without running up unnecessarily large heating bills. Just like with a car, the more you look after your heating system, the less likely it is to break down when you most need it.

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