How to Bleed Radiators With a Combi Boiler


From time to time, you may find that you need to bleed your radiators to release air bubbles trapped inside. This might sound incredibly complicated, but it really isn't. If you can bleed your radiators yourself, you can save yourself an avoidable callout charge.

These days, more and more homeowners are opting for a gas combi boiler. While that might sound great, you might be wondering how this affects the radiator bleed process.

So let's take a step-by-steplook at how to bleed radiators with a combi boiler.

What is a combi boiler?

What is a combi boiler

First of all, what is a combi boiler? A gas, LPG, oil or electric combi boiler is a single-unit heating system with two important functions:

Providing hot water for your central heating system

Heating the water for your hot water taps and shower

A combi boiler is different from a traditional central heating system because it doesn't require a separate hot water tank. Instead, a combi boiler uses a plate heat exchanger to heat water instantaneously.

When to choose a combi boiler

The primary benefit of a combi boiler is space. Combi boilers are a great choice when it comes to saving space as they only require a single unit with no additional water tanks in the attic or airing cupboards.

What is a pressurised system?

The pressure is the balance of air and water within your boiler.

Your combi boiler requires adequate pressure to function at its optimum capacity. If you look at the front of the boiler, a pressure gauge should indicate an ideal pressure of 1-1.5 bar. When cold water molecules or air bubbles become trapped, they can cause pressure to build.

When is the heating pressure too high?

When is heating pressure too high?

If the pressure gauge on your boiler is reading 2.5 bar or higher, it might be a good idea to bleed your radiators and release the trapped air. The pressure gauge drops as the water pressure reading returns to normal.

When is the heating pressure too low?

Even when switched off, your boiler pressure should sit at around 1 bar. If it falls below this, the pressure is too low, which can affect the heating performance in a whole other way. Decreased water pressure is often caused by a leak somewhere in the system. If this might be the case, check the pipes and floor for signs of moisture and damp patches.

Alternatively, the water pressure may have dropped too low if you have recently bled your radiators. In this case, you can use the filling loop to add more water. Keep an eye on the pressure gauge, so you know when it has reached an optimum level!

Low pressure is not likely to cause damage to your combi boiler, but it might prevent it from working properly. In this case, you might find that your central heating is under strain and burning through more energy, ultimately costing you more energy bills.

How to bleed your radiators in 6 easy steps

Bleeding radiators is not that hard as long as you are confident in what you're doing. If you are willing to follow a simple guide and give it a try yourself, you could save yourself a little extra money.

What you need

  • A bleed key
  • A container of some sort to catch water
  • Insulated gloves to prevent hot water burns
  • Some cloth or old rags to absorb water droplets

Step 1

Turn your combi boiler on and make sure each of your radiators are on. Wait a few minutes for them to heat up.

Step 2

Carefully check all of the radiators you turned on to see if they feel a little cool and listen for any strange sounds. Both of these are indicators that air pockets might be trapped inside.

Make a note of any you think might need to bleed. Then turn your combi boiler off.

Step 3

It's important that you wait for the radiators to cool!

Once cool, place your container beneath the radiator you want to bleed and locate the bleed valve on the top or side of the radiator.

Step 4

Insert your radiator key into the bleed valve and turn the key anti-clockwise. Refrain from turning it too much, or you might damage the radiator key or the bleed valve; usually, no more than a half-turn is required.

If the radiator did need bleeding, you would hear a hissing noise as the trapped air escapes.

Step 5

When the hissing stops, close the bleed valve and wipe away any water droplets. Repeat this process with all the radiators on your list.

Step 6

The next step is to check if you need to re-pressurise your boiler. Combi boilers are sealed central heating water systems, and while it is fine to bleed them yourself, they need to be topped up afterward.

There should be a lever located near the water supply which you can use; watch the pressure gauge so you know when the water pressure reaches 1-1.5 bar.

Every boiler is different - make sure you check the instruction manual for directions on re-pressurising your boiler.

Why do I need to bleed my radiators?

There are several reasons why you might need to bleed a radiator, namely if your central heating system is making unusual noises or suddenly seems to be taking longer to heat water.

It is often worth bleeding your radiators to see if you can solve the problem yourself without spending money straight away on a professional engineer.

When air bubbles become trapped in a radiator, they can compromise the efficiency of the entire heating system. Air bubbles expand, and after a while, they block the pipes and prevent the hot water pump from circulating water the way it is supposed to.

As well as being annoying, it means your home will take longer to heat up and might not warm up as much as you would like. This will also cause your heating system to work harder, burning more energy and ultimately causing your energy bills to rise.

Cold water molecules can expand and cause the boiler pressure to rise. This is not a good thing. At best, you might find your shower a little sharp. At worst, your boiler might take early retirement, break down or leak.

When to bleed radiators

When to bleed radiators

Technically, you can bleed your radiators any time of the year. It is generally advised that you seek to do this at the start of winter though, as the ambient cold temperatures will make it easier to tell if your radiators are heating evenly.

Of course, if you think there might be trapped air inside one or more of your radiators, it is unnecessary to wait until winter to fix the problem!

There are a few key indicators that your radiators need bleeding.

Does your radiator feel cool when the central heating system is switched on? If so, this might indicate that trapped air bubbles are preventing the hot water flow.

Is your radiator making some strange sounds? No, it isn't haunted; it's just letting you know that you need to bleed out some extra air.

Check your boiler's pressure gauge; if the reading is 2.5 bar or higher, it's time to try bleeding your radiators.

How to tell if bleeding radiators worked?

When you have bled all your radiators, turn your combi boiler on. Then turn all the radiators to their highest setting and wait for them to heat up.

Check your radiators for cool spots or strange noises; if there are none of either, good job!

If there are, then maybe bleeding your radiators wasn't enough, and it might be time to call a professional engineer.

Explore our range of radiators and combi boilers

If you've tried bleeding your radiators and that hasn't worked, you might need to look for replacement parts for your central heating system.

At PlumbNation, we stock a range of radiators including central heating radiators, towel radiators, and designer radiators.