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How to bleed a radiator with a combi boiler



If you have radiators that are getting nice and hot at the bottom, but remain cold at the top, it’s very likely that you have air trapped in the radiator and will need bleeding.

In this article, we will explain how a combi boiler works and explain what a pressurised system is.

We will also take you step by step through the process of bleeding your radiator so that you don’t need to call in a professional to do it for you.

What is a combi boiler?

A combi boiler
A ‘combi’ boiler is a single unit that provides a combination of two important functions.

  1. Provides the hot water required for your central heating system.
  2. Acts as a water heater for providing hot water to your taps and shower.

Space saving

The main benefit of a combi boiler is that they save a great deal of space because a separate water tank is not required to heat the water.

Traditionally, an old tank-fed system would require a separate tank in the loft or in a cupboard (a.k.a. the airing cupboard).

Instead of needing a large, cumbersome water tank with an indirect coil or heat element running through it, a combi boiler heats up water directly using a plate heat exchanger.

Constant hot water supply

The plate heat exchanger is a very important component of a combi boiler because it has the ability to transfer heat from one body of water to another very quickly – almost in real time.

In basic terms, this means that cold water can be made hot very quickly and created on demand, without the need for it to be heated separately in a large tank.

Not only does this make combi boilers highly efficient, it also avoids that classic problem of busy households running out of hot water in which to shower.

Separate water for heating & washing

In order to combine its two important functions, a combi boiler allows water to flow through it in two different routes, depending on what the water will be used for. Neither flow of water will come into contact with the other.

  1. Washing water – The cold water enters the boiler, through an isolating valve and flows across the pressure switch paddle which tells the boiler it needs to turn on. The cold water then enters the heat exchanger where it is heated up. The newly-formed hot water then leaves the heat exchanger, goes through a mixer and is distributed out to your taps.
  2. Central heating water – This comes out of the top of the boiler, and can either go off to the heating system or back to the heat exchanger. It then flows down into the pump, through the heat exchanger and back out of the boiler into the return.

How to repressurise your combi boiler

Combi boiler pressure gauge

Another benefit of combi boilers is that they are able to provide water to your taps and shower at the same pressure as the mains.

This means that you don’t need a hot water pump in order to have a powerful shower, as the pressurised system provided by the boiler takes care of this for you.

However, if you have air trapped in your radiators, it might be causing your central heating system to be overpressurised.

What does a ‘pressurised system’ mean?

If you look at the front of your combi boiler, you should see a gauge that denotes the pressure in your system.

An ideal reading with everything off should be about 1 – 1.5 bar – bar being the metric units of pressure.

When cold water gets hot inside your heating system, the molecules expand. When the water expands, it needs somewhere to expand (move) into. In a pressurised system, this place is known as the expansion vessel.

Inside the expansion vessel is a rubber diaphragm. On one side of the diaphragm is your heating system water, on the other side is some compressed air.

As the heating water gets hot and expands, it can expand into the compressed air but without allowing air into the heating system due to the rubber diaphragm in the way.

For your heating system to operate correctly, when your heating is off and you are not running any water, this compressed air should be pressurised to 1 – 1.5 bar.

If your reading is considerably higher than this, it quite possible that you need to bleed some air out of your radiators in order to bring the pressure reading down.

How to bleed your radiators

If your system pressure is too high, it’s likely that releasing some of the air out of the system will solve the problem for you and bring the pressure down to the required level.



At the top of each of your radiators will be a bleed valve. Depending on the type of bleed valve, you may need a radiator bleed key or a flat head screwdriver to undo it.

The next thing to do is switch your heating system off completely so that no problems are created by bleeding the air out while it’s still running.

Use a pair of grips to ensure that your valves at the bottom of your radiator are open so that water is able to flow into your radiator.

You can then slacken off your radiator’s bleed valve with your radiator key or flathead screwdriver.

If there is air in your radiator, as you slacken off the bleed valve, you will hear a hissing sound as the air escapes.

Have a cloth ready to catch any drips that come out after the air escapes. Once water starts coming out, you know that all of the air has been released from that radiator. Then tighten the bleed valve back up.

Go round each of your radiators and repeat the process as necessary and also check the pressure gauge on your combi boiler to see if it has returned to around 1 bar.

Add chemical inhibitor

Fernox Heating System Protector

One of the most common reasons for air getting into your system in the first place is due to a lack of chemical inhibitor in your radiators and pipework.

When there’s no inhibitor in your system, the water will have a chemical reaction with the inside of your radiators and create hydrogen, causing a build up at the top of your radiators and stopping it from working properly.

To see why it’s so important to add inhibitor to your system, see the informative video below, featuring James the professional plumber.


VIDEO: The importance of adding chemical inhibitor


If you believe that your inhibitor levels are running low, or you have no idea when they were last topped up, the video below will take you through the process step by step of adding inhibitor to a pressurised heating system.


VIDEO: How to add chemical inhibitor to a pressurised heating system


What if my heating pressure is too low?

If you’ve bled air out of your radiators but no water has come out and the pressure gauge on your boiler has dropped below half a bar, it is likely there is a another problem that is causing the pressure to be too low.

Below we’ll look at the possible reasons for a pressure drop and how you can solve them.

A leak in your system

To check for a leak in your system, it’s a good idea to look at all the valves on all the radiators in your house.

Lift up the heads with a pair of grips to check there’s no leakage on top of the valves.

You should also ensure that the compression fittings on both side of each valve are fully tighten and that there is no moisture seeping out.

Boiler pump is set too high

‘Cavitation’ is a situation where the pump is working too fast for the water to escape the boiler. The water and air is then separated and hydrogen is created.

The hydrogen will then travel round your system and escape out of the automatic air vent. This can then cause the pressure in your system to drop.

This situation can be avoided by reducing the speed on your pump.

Expansion vessel issues

It’s possible that the rubber diaphragm in your expansion vessel has perished and is no longer doing the job of keeping the hot water and compressed air separate.

You’ll initially see an increase in pressure before the pressure release valve kicks in and drops the pressure very low.

You can check if the rubber diaphragm is the problem by locating the valve (like a valve on a tyre) on your expansion vessel and letting the air out. If water starts coming out, it is likely a problem with your rubber diaphragm and your vessel will probably need replacing.

If water doesn’t come out, you can simply pump up the expansion vessel using a bicycle pump until the reading on your boiler is 1 – 1.5 bar again.

Pressure release valve issues

A pressure relief valve is a simple component that contains a rubber valve and a spring.

If the water in your system gets up to a certain pressure, the water will force open the valve and the the water will flow through an escape pipe and be released somewhere outside.

Over time, the strength of the spring and the rubber can get less and less causing the valve to provide less resistance to the water flow, dropping the pressure in your system.

If you suspect any of these issues to be affecting your combi boiler and central heating system, then it’s best to get the profesionals in to have a look.


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