How to Bleed your Radiators on a Combi Boiler System

It’s frustrating when you turn on the heating on your combi boiler and your home takes forever to get warm. It’s a miserable experience when it doesn’t warm up at all, or when you expect to have a hot bath or shower and you’re greeted with chilly water instead.

If you’ve found yourself in such situations, you might need to bleed the radiators on your combi boiler system.

What is happening to my radiators that they need bleeding?

Air bubbles are naturally produced when water is heated inside your central heating system. These air bubbles are trapped within the system and build up over time, rising to the top of the radiators. Eventually, they take up enough space that they start affecting the boiler system’s performance.

Air is not as good a conductor of heat as water. Therefore, the space occupied by air bubbles doesn’t heat up like the rest of the affected radiator. The heating system then has to use more energy to generate heat, which means you end up paying more than usual for your energy bill for less heat.

What does it mean to bleed a radiator?

Bleeding a radiator means getting rid of the trapped air bubbles. By releasing air, this lets the water flow throughout the entire radiator once again, restoring the radiator’s efficiency in generating heat.

How do I know for sure that I need to bleed my radiators?

  • Presence of cold spots

The telltale sign of a radiator that needs bleeding is the presence of cold spots at the top. You can feel this when you touch the top of an affected radiator and it is considerably cooler than the rest of the radiator. Just be careful to turn off the heating before you do this to avoid accidentally burning your hand.

  • High pressure

You can also figure out if your radiators need bleeding by checking your boiler’s pressure gauge. A combi boiler that is working properly should show a pressure gauge reading of 1 to 1.5 bar with the heating turned off and water isn’t running. If the pressure is higher, you need to bring it down by bleeding the radiator.

  • Strange sounds

Yet another indication of a radiator that needs bleeding is when you hear strange sounds like rattling and gurgling when you use it.

What do I need for bleeding my radiators?

Before you start bleeding your radiators, you need to prepare some tools.

The most important tool is the radiator bleed key. Your combi boiler should come with one. If you can’t find it, don’t worry. You can get a new one at just about any DIY store.

If it’s absolutely urgent and you can’t get a radiator bleed key anytime soon, a flathead screwdriver or small pliers could work in a pinch. Even a 12-point socket set could be used. However, we highly recommend you use a radiator bleed key.

The next thing you need is a pail or any other similar container. It is for collecting any water that might drip from your radiators during the process of bleeding.

You will also need a cloth to prevent water from potentially spraying out of the radiator when you bleed it. It’s also for wiping down the radiator afterwards.

Finally, a pair of thin gloves will keep you from scalding your hands when feeling radiators for cold spots.

How do I bleed my radiators?

        1. Turn up your combi boiler to the highest heat setting
          All your radiators should be on. Wait until each one is fully heated. This is how you diagnose your radiators for air bubbles and build enough pressure to push out the air when you start bleeding.
        2. Check each radiator for cold spots
          Feel the top of each radiator with your gloves on. If you notice a cold spot, that radiator needs bleeding.
        3. Turn off your combi boiler
          Once you’ve determined which of the radiators need bleeding, you should shut off the heating before you start bleeding. Wait until all the radiators cool off. You don’t want a stream of hot water rushing out, burning you, and making a mess.
        4. Set up the area for the radiator you’re bleeding
          Look for the radiator bleed valve. You can find it on top and to the side of your radiator. Your radiator bleed key should slot into it. Place your container beneath the valve.

          how-to-bleed-radiators-on-combi-boiler-system-02-set-area
          (Source: Flickr)

          The order in which you should bleed your radiators should be starting from the one furthest from the boiler and ending with the one closest to the boiler.

        5. Place your radiator bleed key and start bleeding
          To start bleeding, turn the valve anti-clockwise using the radiator bleed key or whatever tool you have on hand. A quarter to a half turn should be good. Air will hiss as it escapes from the radiator.

          how-to-bleed-radiators-on-combi-boiler-system-03-place-bleed-key
          (Source: Wikimedia Commons)

          Use the cloth to help you grip the key and valve. It should also cover enough of the valve to prevent water from potentially spraying from the valve.

          Turn the key clockwise once water starts trickling steadily to close the valve. Make sure to not close the valve too tightly, as this could damage the valve. The steady flow of water means all the air from the radiator has been released.

          Wipe down whatever water has run down on the radiator to prevent rusting.

          Repeat this process for every radiator with air bubbles.

        6. Turn your heating back on
          With your heating turned back on, check the radiators again for any cold spots or strange sounds. You might have to bleed some of the radiators again.
        7. Check your pressure gauge
          Combi boiler systems tend to lose pressure when you bleed their radiators. Your pressure gauge might show a bar reading below 1. In that case, you need to re-pressurise the boiler.

          how-to-bleed-radiators-on-combi-boiler-system-04-check-pressure-gauge
          (Source: pxfuel)
        8. Re-pressurise your combi boiler
          To repressurise your boiler, start by switching off your heating.Look for your boiler’s filling key and corresponding valve with a keyhole or its filling loop with two valves. Older combi boilers have the key and valve setup, while modern combi boilers often have a filling loop.

          how-to-bleed-radiators-on-combi-boiler-system-05-repressurise-01
          Filling loop (Source: Amazon)
          how-to-bleed-radiators-on-combi-boiler-system-05-repressurise-02
          Filling key (Source: Amazon)

          The valve or the filling loop is typically found below the boiler.

          If your combi boiler has a valve, use the key to turn the valve until you hear water flow through the pipes. If your combi boiler has a loop, open both valves.

          Check your pressure gauge as you perform either method. Once you get a 1.5 bar reading, close the valves.

          If you aren’t sure which method to use, look for your boiler’s physical or digital instruction manual. Contact the boiler’s manufacturer if you can’t find the information you need.

If bleeding radiators doesn’t fix the problem of your combi boiler system heating up very slowly or not heating up at all, then give us a call here at Ambient Heating (if you’re in the Bristol area that is – if not call a professional and experienced plumber).

To keep on top of problems, we recommend checking the radiators for trapped air bubbles every few months.