How to bleed vertical radiators

17 January 2023

Bleeding a radiator with a bleed key and a cup to catch the water

From time to time, it may be necessary for your vertical radiators to undergo the bleeding process. While there are many heating-related jobs that should be left to a professional heating engineer, bleeding radiators is actually a task you can easily do yourself. Bleeding your vertical radiators will normally have an instant impact on their level of efficiency, which will ensure your heating is not using more energy than it needs to. In this article, we'll explain why radiators might need bleeding and how you can bleed a vertical radiator in your home.

What is 'bleeding a radiator'?

Although it sounds a little dramatic, bleeding a radiator is nothing more than letting out air that has built up at the top of your radiator.

Depending on the type of vertical radiator you have, you can either use a brass radiator key or a flathead screwdriver to gently undo the bleed valve and allow the air to escape.

Air that gets trapped in a radiator will stop it from heating up properly, which will make your boiler and heating system work harder in order to heat your home. Not addressing this issue will cause you to have unnecessarily high heating bills and, in extreme cases, could shorten the lifespan of your boiler. Bleeding your vertical radiator is a simple job, but can make a world of difference to the efficiency of your system.

Why does air get trapped in vertical radiators?

Trapped air in radiators is not a major problem if dealt with fairly quickly. There are a few reasons why it might occur.

Build-up of hydrogen

When left to their own devices, the internal components of your heating system are prone to corrosion and rust, due to a reaction between water, oxygen and the metal in your radiators. Over time, bits of corroded metal can break off and come to rest at the bottom of radiators in the form of a brown sludge.

The rusty sludge will often release hydrogen, which rises to the top of radiators and create air pockets. This can affect the performance of your radiators in two ways:

1. The sludge will stop the radiator getting hot at the bottom

2. The trapped hydrogen will stpo the radiator from getting ot at the top

Both of these scenarios will increase the energy usage of your heating system and push up your bills.

Internal corrosion can be prevented by ensuring your chemical inhibitor levels in your heating system are topped up on an annual basis.

Leaking valves or pipes

As part of the natural wear and tear of a heating system, valves and pipes can develop small leaks due a weakening of the joints or corrosion. In many instances, it will be small leaks that you aren't even aware of that can allow air into the system. It doesn't necessarily have to be caused by a huge noticeable leak with water spilling out everywhere.

Boiler pressure dropping on a regular basis is a common symptom of a potential leak and further investigation is required.

Location of heating pump

The pump is the part of your heating system that helps water to flow around your pipes and between the tank, your boiler and your radiators. If the pump is above your supply pipe, then it's possible for air to enter the system at that point.

This is a common feature of older properties where the water tank may also be in the loft and may need looking at by a professional to try and minimise the problem.

What are the symptoms of air trapped in a vertical radiator?

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.

How to bleed your vertical radiator

Below are the simple steps that will enable you to bleed the air out of your vertical 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
Top up and repressurise your boiler if necessary

For a more indepth look at how to bleed your radiators, please have a look at the step-by-step guide from a professional plumber in the video below

Annual heating maintenance

Once you've released the air from all your vertical and horizontal radiators, turn on your central heating again. You should notice that all of your radiators get hot fully and evenly. If you find to the contrary, get in touch with a professional heating engineer to investigate if there are any other issues in your system.

If you find that you are needing to bleed your radiators regularly, there is likely to be a leak somewhere in your system that is allowing air in. Again, this would be time to call a professional to get to resolve the problem.

It's recommended to have your boiler and heating system serviced every 12 months. A heating engineer can come in, perform maintenance on your boiler, check your chemical inhibitor levels and have a look to see that your system is running efficiently.

Yearly heating maintenance is the best way to make sure 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.