How to Bleed Radiators with a Combi Boiler: A Step-by-Step Guide

Many homeowners experience the frustration of uneven heating in their homes, especially during the colder months. You might notice that despite having a combi boiler and a central heating system, some rooms remain stubbornly cold, or your energy bills are inexplicably high. This common issue often stems from air trapped in your radiators, hindering the efficient circulation of hot water.

Trapped air bubbles in your heating system not only lead to cold spots on radiators but also force your combi boiler to work harder, using more energy to generate heat. This inefficiency can significantly impact your comfort and wallet. Without addressing this, you could be facing a central heating system that’s both underperforming and overcharging.

Fortunately, there’s a straightforward solution – bleeding your radiators. This process involves releasing the trapped air from all the radiators, ensuring that hot water flows freely, heating your home evenly. In this guide, we’ll walk you through how to bleed radiators with a combi boiler, using simple tools like a radiator key. By the end of it, you’ll have a heating system that operates at peak efficiency, providing the warmth you need while potentially lowering your energy costs.

What Causes the Need for Bleeding Radiators?

Understanding the root cause of why radiators need bleeding is crucial in maintaining an efficient heating system. Here’s what leads to this necessity:

Formation of Air Bubbles

The primary culprit behind the need to bleed radiators is the formation of air bubbles within the central heating system. When your combi boiler heats water, it circulates through the radiators to warm your home. During this process, air can become trapped in the system, forming bubbles that rise to the highest points, typically the radiators.

Impact on Heating Efficiency

These air bubbles create problems for your heating system. Unlike water, air is a poor conductor of heat. When air occupies space within a radiator, it prevents hot water from filling that space, leading to cold spots. This phenomenon significantly reduces the radiator’s ability to emit heat efficiently.

Increased Energy Consumption

With trapped air in the system, your combi boiler and the entire central heating system have to work harder to achieve the desired temperature. This extra effort results in higher energy consumption, leading to increased energy bills. In essence, you’re paying more for less heat.

Uneven Heating Distribution

The presence of air bubbles can lead to uneven heating distribution throughout your home. Some rooms might feel adequately warm, while others remain cold, despite having all the radiators on. This inconsistency is a telltale sign that air is trapped somewhere in your heating system.

Long-Term System Strain

If not addressed, trapped air can cause long-term strain on your combi boiler and the entire heating system. Over time, this can lead to wear and tear, reducing the lifespan and potentially leading to the replacement of your boiler..

Identifying the Need to Bleed Your Radiators

Knowing when to bleed your radiators is key to maintaining an efficient heating system. Similar to what happens with towel rails, here are the signs that indicate it’s time to perform this essential task:

1. Cold Spots on Radiators

One of the most obvious signs that you need to bleed your radiators is the presence of cold spots, particularly at the top of the radiator while the bottom remains warm. This uneven heating is a clear indicator that air is trapped inside, preventing hot water from occupying the entire radiator.

2. Checking the Pressure Gauge

Your combi boiler’s pressure gauge is an invaluable tool in this regard. A normal pressure reading should be between 1 and 1.5 bar when the heating system is not in operation. If the pressure gauge shows a reading significantly higher or lower than this range, it could be a sign that air has accumulated in the system, affecting its efficiency.

3. Unusual Noises from the Heating System

Listen for any strange noises like gurgling or banging from your radiators or heating pipes. These sounds can be caused by the movement of air bubbles through the system and are a clear sign that you need to bleed a radiator.

4. Reduced Efficiency of the Hot Water Pump

The hot water pump is responsible for circulating water through your central heating system. If air is trapped in the system, the pump has to work harder to move the water, which can lead to reduced efficiency and increased wear on the pump.

5. Using the Radiator Bleed Key

If you suspect that air is trapped in your radiators, use a radiator bleed key to open the bleed radiator valve slightly. If air escapes (you’ll hear a hissing sound), it confirms that bleeding is necessary.

6. Performance of the Best Combi Boiler

Even the best combi boiler models can fall victim to the effects of trapped air in the system. If you notice a decline in your boiler’s performance, it’s worth checking if bleeding the radiators could resolve the issue.

Tools Required for Bleeding Radiators

Before you begin the process of bleeding your radiators, it’s important to gather the right tools. Having these items on hand will make the task smoother and more efficient:

1. Radiator Bleed Key

This small, simple tool is essential for bleeding radiators. The radiator bleed key is designed to fit into the bleed valve of your radiator. If you don’t already have one, they are readily available at hardware stores.

2. Container for Catching Water

As you bleed your radiators, some water may escape. Have a container, like a bucket or a bowl, ready to catch any drips or spills. This will help prevent water damage to your floors or carpets.

3. Cloth or Rag

Keep a cloth or rag handy to clean up any water that escapes from the radiator. This is also useful for gripping the radiator bleed key if it becomes slippery or if water starts to spray out.

4. Gloves

It’s a good idea to wear gloves to protect your hands from any hot water that may escape from the radiator. This is especially important if you’re bleeding the radiators shortly after the heating system has been on.

5. Screwdriver (Optional)

In some cases, a flathead screwdriver can be used instead of a radiator bleed key, depending on the type of bleed valve your radiators have.

6. Torch or Flashlight (Optional)

A torch or flashlight can be helpful, especially when locating the bleed valve on radiators in dimly lit areas.

With these tools at your disposal, you’re ready to effectively and safely bleed your radiators. This simple maintenance task can significantly improve the performance of your heating system, ensuring that your home is heated evenly and efficiently. Next, we’ll guide you through the step-by-step process of how to bleed your radiators, ensuring you can confidently carry out this task.

Step-by-Step Guide to Bleeding Radiators

Bleeding your radiators is a straightforward process that can significantly improve the efficiency of your heating system. Here’s a comprehensive guide to help you through each step:

Step 1: Preparation

  • Ensure Safety: Before starting, ensure your heating system is turned off. This is important to avoid burning yourself with hot water.
  • Gather Tools: Have your radiator bleed key, container for catching water, cloth, and gloves ready.
  • Identify Radiators to Bleed: Check each radiator for cold spots, particularly at the top, to identify which ones need bleeding.

Step 2: Locating the Bleed Valve

  • Find the Bleed Valve: The bleed valve is typically located at the top corner of the radiator. It’s a small valve, often with a square bit in the middle that fits the radiator bleed key.
  • Position Container and Cloth: Place your container under the bleed valve to catch any water and have your cloth ready to handle any spray of water.

Step 3: Bleeding the Radiator

  • Insert Radiator Key: Fit the radiator bleed key onto the valve. Ensure it’s seated correctly.
  • Turn the Key Clockwise: Gently turn the radiator key clockwise to open the valve. You should hear a hissing sound as air escapes. This is the trapped air being released from the radiator.
  • Monitor Water Escape: Keep an eye on the water escaping. Initially, there may be a mix of air and water, but you’re looking for a steady stream of water, which indicates that most of the air has been released.
  • Close the Valve: Once water starts flowing in a steady stream without air, close the valve by turning the radiator key clockwise. Be careful not to overtighten, as this can damage the valve.

Step 4: Checking Water Pressure

  • Monitor Boiler Pressure: After bleeding each radiator, check the water pressure on your boiler’s pressure gauge. The ideal boiler pressure is typically between 1 and 1.5 bar. If the pressure is too low, you may need to repressurise your system.
  • Repressurising the Boiler: If the boiler pressure is too low, you’ll need to add water to the system. This is usually done using the filling loop on your boiler. Open the loop to allow water into the system, watching the pressure gauge until it reaches the desired level, then close the loop.

Step 5: Repeat for All Radiators

  • Work Through Each Radiator: Repeat the bleeding process for each radiator that needs it. Start with the radiator furthest from the boiler and work your way to the closest. This ensures that you effectively remove air from the entire system.

Step 6: Final Checks

  • Turn the Heating Back On: Once all radiators have been bled and the boiler pressure is correct, turn your heating system back on.
  • Check Radiators Again: After the system has been running for a while, check each radiator again for cold spots. This ensures that no air pockets remain and that your efforts have been successful.
  • Monitor Boiler Pressure: Keep an eye on the boiler pressure over the next few days. It’s normal for the pressure to fluctuate slightly after bleeding radiators, but it should stabilize within the normal range.

Additional Tips

  • Regular Maintenance: Regularly check your radiators for cold spots and your boiler pressure to determine if bleeding is needed. This proactive approach can save you from larger issues down the line.
  • Professional Help: If you’re unsure about any part of the process, or if bleeding your radiators doesn’t resolve your heating issues, it’s wise to consult a professional. They can provide expert advice and service.

By following these steps, you can ensure that your radiators and combi boiler operate efficiently, providing consistent heat throughout your home. Regular maintenance, including bleeding your radiators, is key to a well-functioning heating system and can help you avoid unnecessary energy costs.

Re-pressurising Your Combi Boiler

After bleeding your radiators, it’s common for the water pressure in your combi boiler to drop. This is because air, once occupying space in the system, has been released, and water has been expelled. Re-pressurising your boiler is a crucial step to ensure it operates efficiently and safely. Here’s how to do it:

Understanding Boiler Pressure

  • Ideal Pressure Range: The optimal operating pressure for most combi boilers is between 1 and 1.5 bar. This information is usually found in the boiler’s manual.
  • Effects of Low Pressure: If the boiler pressure is too low (below 1 bar), your heating system may not function properly, leading to inadequate heating and potential boiler damage.

Locating the Filling Loop

  • Find the Filling Loop: This is usually a silver or grey flexible hose with a valve at each end, located underneath the boiler. Some models have a built-in filling key instead.
  • Ensure Boiler is Off: Before re-pressurising, make sure the boiler is off and cool to avoid any risk of injury.

Re-pressurising Process

  • Open the Valves: Slowly open the valves on the filling loop to allow cold water into the system. You will hear water flowing into the system.
  • Monitor the Pressure Gauge: Watch the boiler’s pressure gauge closely as you do this. You should see the pressure rising steadily.
  • Close the Valves: Once the pressure reaches between 1 and 1.5 bar, close both valves. It’s important to do this promptly to avoid over-pressurising the system.
  • Check for Leaks: After re-pressurising, inspect the area around the boiler and the filling loop for any leaks. If you find any, it’s best to call a professional.

After Re-pressurising

  • Restart the Boiler: Turn your boiler back on. It may have a reset function that needs to be activated.
  • Check Radiator Performance: Once the boiler is running, check the radiators again to ensure they are heating up evenly and there are no further issues.
  • Regular Monitoring: Keep an eye on the boiler pressure over the next few days. A slight fluctuation immediately after re-pressurising is normal, but it should stabilize soon.


  • Pressure Drops Again: If the pressure drops significantly after re-pressurising, there may be a leak in the system. In this case, it’s advisable to contact a heating engineer.
  • Over-Pressurised System: If you accidentally raise the pressure too high (above 1.5 bar), you’ll need to release some water from the system using a radiator bleed valve.


Bleeding radiators and maintaining the correct pressure in your combi boiler are more than just routine maintenance tasks; they are essential practices for ensuring the longevity and efficiency of your home heating system. By following the comprehensive steps outlined in this guide, you can address common issues like cold spots and uneven heating, ultimately enhancing the comfort of your home.

This guide not only provides practical steps but also empowers you, the homeowner, with the knowledge to tackle common heating system issues. Understanding how to bleed your radiators and re-pressurise your combi boiler puts you in control, enabling you to respond quickly to changes in your system’s performance.

Regular maintenance, including bleeding radiators, directly impacts the efficiency of your heating system. By ensuring that hot water circulates freely without the hindrance of trapped air, your combi boiler doesn’t have to work as hard, leading to reduced energy consumption and, consequently, lower energy bills.