How to bleed a radiator

A huge 84% of UK households have gas central heating, making radiator maintenance an essential job for most of us. But while it's simple enough to carry out, it's a task we often overlook.

Heating and energy

20 March 2018

Knowing how to bleed a radiator can make your heating more energy efficient, keep your boiler in peak condition and it will save you money on gas heating costs. And a bit of basic know-how can prevent all kinds of problems; from inefficient heating to rust, leaks and dreaded boiler damage.

In this guide, learn why bleeding radiators is so important, how to check for signs of a gas heating problem and how to bleed a radiator.

When should you bleed a radiator?

If your gas central heating isn't warming your home properly, it could be because there are air bubbles trapped in the system. This is when you'd need to bleed your radiator. These bubbles prevent hot water from circulating effectively, so radiators aren’t as hot as they should be and take longer to warm your home.

Uneven heating can also lead to mould and damp, so if you think this may already be an issue, read our practical guides on dealing with condensation and mould

Before moving on to learning how to bleed a radiator, you'll need to check for blockages in the system. Firstly, turn the gas central heating on and make sure that each thermostatic radiator valve (TRV) is on full power and isn't obstructed. This is important because TRVs work by sensing the surrounding air's temperature, which will be affected if they're covered up. You can usually find the TRV at the top or bottom corner of your radiator. Once the heating has had time to warm up, put on some gloves and check how hot each of the radiators is in turn. Because air bubbles in the gas central heating system rise, you'll find air collects at the top of radiators and steadily gathers, making your home colder as the problem worsens. So if a radiator isn’t heating up, or is cool at the top and hot at the bottom, you'll need to learn how to bleed it.

However, do check for signs of a more serious problem before you start the bleeding process. In particular, look out for rust or water underneath or on the radiator, as this could be the sign of leaks in your heating system. Also check that your boiler power light is on (or the pilot light is lit), the boiler pressure is at the right level, and that your boiler isn't leaking. 

If there are no other obvious concerns, your home's heating will almost certainly benefit from bleeding the radiators.

How to bleed a radiator in six steps

  1. Start by turning the central heating off, as otherwise boiling water could spray out of the radiator when you bleed it.
  2. Gather your tools: raid the kitchen for a dry cloth like a tea towel, and a bowl or tray to catch water. The only specialist bit of kit you’ll need is a radiator key, which you can get cheaply at most DIY shops. A pair of strong, narrow pliers can do the job but risk damaging the valve, so use a radiator key if you can.
  3. Find the square bleed screw at the top corner of your radiator, and place the bowl or tray directly underneath it at the base of the radiator.
  4. Next, place the radiator key over the bleed screw (it should fit snugly), cover it with another cloth and slowly turn the key anti-clockwise for about half a turn. As the air releases, you'll hear a hissing sound. At this point, hold the cloth close to prevent water dripping or spitting onto the radiator.
  5. Once air stops hissing and water begins to trickle out steadily, tighten the screw again, being careful not to over-tighten and damage the valve. That's the basic process of how to bleed a radiator covered, but do wipe down the radiator to avoid leaving any moisture, which could cause rust.
  6. Switch the heating on again, and check the boiler pressure. If it's below the optimal level, you may need to boost it by using the filling loop on your boiler. This is usually a tap or lever on the main water supply to your boiler. Finally, check that all the radiators are heating evenly and that none of the bleed valves are leaking. You may need to bleed some radiators a second time, but if the problem still isn't resolved, contact a heating engineer.

What are the other long-term solutions to improving your home heating?

Over 50% of our fuel bills go towards heating and hot water. Knowing how to bleed a radiator can be invaluable for cutting down on heating costs, but there are lots of other quick fixes worth knowing that can make your gas central heating system more energy efficient, helping to save money.

Although it might seem unnecessary, do try to use your boiler in spring and summer, as well as in autumn and winter. Turning on the heating for just 10-15 minutes a month can prevent boiler breakdown and those surprise cold showers come December. It's also wise to keep an eye out for obvious signs of trouble. If the pressure repeatedly drops on your condenser boiler, or there is water dripping from it or pooling nearby, there may be a leak, in which case you'll need a Gas Safe-registered professional to take a look.

Trickier problems can usually be avoided by having your boiler serviced annually. You can also save money and energy by installing a thermostat onto your boiler. Set it to a maximum temperature of 60°C for cost-effectiveness – it could save you £30 a year. If you have a conventional boiler (not a condensing boiler), you might consider an insulation jacket too. They cost around £15, and save an average of £40 a year by preventing heat from escaping.

Proper insulation can also make a big difference to how energy efficient your home is. Installing loft insulation can save up to £250 a year, and draft-proofing for your windows and doors could save you up to £50 a year on your gas bills. Insulation is a great way to keep the house warm in a cost-effective way.

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