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Updated: October 5, 2024

Many homeowners are surprised to find their combi boiler losing pressure without an obvious leak. Pressure loss in a boiler is a common issue, but it’s not always linked to a visible leak. In this guide, we’ll explore the causes behind this pressure drop, how to diagnose the issue, and the best solutions to restore optimal performance.

What is Boiler Pressure?

Boiler pressure refers to the balance of air and water in your heating system. This pressure must remain within a specific range—typically between 1 and 2 bars on the boiler’s pressure gauge. If it falls below this range, the system may not function properly, leading to cold radiators and inconsistent hot water.

However, when pressure drops, many homeowners immediately assume there is a leak in the system. While a leak is a common culprit, there are other reasons why your boiler could be losing pressure without one.

Causes of Boiler Pressure Loss

A combi boiler can lose pressure for several reasons, even if you can’t detect a visible leak. Here are some of the most common causes of pressure loss.

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1. Small Leaks in the System

Leaks can be hard to detect because not all of them are immediately visible. Minor leaks in the pipes or radiators may not cause noticeable water damage but can still lead to a loss of pressure. These micro-leaks might occur at joints or seals, and over time, they can significantly impact your boiler’s efficiency.

2. Natural Loss of Pressure

Over time, boilers can experience a gradual decline in pressure due to normal operation. This is common in older systems and doesn’t necessarily indicate a problem. If your boiler hasn’t been repressurized in a long time, it might simply need a top-up.

3. Bleeding Radiators

If your radiators have air pockets, they may not heat up evenly, which can cause pressure loss in the boiler. Bleeding the radiators releases trapped air and can help restore the system’s pressure. Regular bleeding can prevent this issue from recurring.

4. Faulty Pressure Relief Valve (PRV)

A faulty pressure relief valve could be responsible for losing pressure. The PRV is designed to release pressure if it gets too high, but if it’s malfunctioning, it could be allowing pressure to escape even when it’s within a safe range.

5. Auto Air Vent Issues

Modern boilers come equipped with an auto air vent to expel air from the system. If this vent malfunctions, it can lead to air pockets forming, which ultimately causes a pressure drop. This issue is more common in newer systems, and the fix typically requires a Gas Safe engineer.

6. Faulty Pressure Gauge

Another rare but possible issue is a malfunctioning pressure gauge. If the gauge isn’t working correctly, it may give you a false reading, making it appear that the pressure is low when it isn’t. If you suspect a faulty gauge, it’s essential to have it checked by a professional before attempting any fixes.

7. Expansion Vessel Problems

The expansion vessel in your boiler regulates pressure within the system by absorbing excess water when it heats up. If the vessel loses pressure or becomes damaged, the entire system will lose pressure as well. This issue requires a qualified engineer to repressurize or replace the vessel.

8. Schraeder Valve Leaks

The Schraeder valve, located on the expansion vessel, can also be a source of pressure loss. Similar to a tire valve, if this part is faulty, air may escape from the system, leading to a drop in pressure. A professional is required to repair or replace this component.

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How to Repressurize a Combi Boiler

Repressurizing a boiler is one of the simplest ways to resolve pressure loss, and it’s a task many homeowners can perform themselves. However, always refer to your boiler’s manual for specific instructions related to your model. Here’s a general guide:

  1. Turn Off the Boiler: Before doing anything, ensure the boiler is switched off and has had time to cool down.
  2. Locate the Filling Loop: This is typically a small hose located underneath the boiler. Ensure both ends are securely attached.
  3. Open the Valves: Turn both filling loop valves to allow cold water to enter the system. As the water enters, monitor the pressure gauge.
  4. Close the Valves: Once the pressure reaches the manufacturer’s recommended level (usually around 1.5 bars), close both valves.
  5. Turn the Boiler Back On: After repressurizing, turn the boiler on and check the pressure gauge again to ensure it’s within the correct range.

If the pressure goes too high, you can bleed a radiator to bring it back down.

Common Boiler Pressure Loss Fixes Requiring a Professional

While repressurizing the system is a DIY-friendly solution, some boiler pressure loss issues require a certified heating engineer:

  • Fixing a Leak in the System: Professional repair is needed if there’s a significant leak.
  • Repairing or Replacing the Pressure Relief Valve (PRV): A faulty PRV should only be handled by a professional.
  • Fixing an Auto Air Vent Leak: Modern boilers with automatic air vents can develop leaks over time.
  • Repressurizing the Expansion Vessel: This task is complex and needs a qualified engineer.
  • Repairing or Replacing Faulty Soldered Joints: Older boilers may suffer from deteriorating soldered joints, which require professional repairs.
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Costs of Hiring a Professional

The cost of fixing a combi boiler that’s losing pressure varies depending on the complexity of the job. On average, you can expect to pay between £30 and £60 per hour for labour in the UK. Below is a breakdown of typical costs for common boiler pressure-related issues:

  • Fixing a Leak: £60 – £220
  • Repressurizing a Boiler: £30 – £60
  • Bleeding Radiators (5-10): £70 – £100
  • Replacing a Faulty PRV: £130 – £220
  • Fixing an Auto Air Vent Leak: £30 – £60
  • Replacing a Pressure Gauge: £50 – £160
  • Repressurizing an Expansion Vessel: £30 – £120
  • Replacing an Expansion Vessel: £130 – £420
  • Fixing a Leaking Schraeder Valve: £35 – £125
  • Fixing Leaking Soldered Joints: £30 – £220

In rare cases where the boiler has multiple issues, replacement may be more cost-effective. Boiler replacement costs can range from £800 to £2,800 depending on the model and complexity of the installation.

Final Thoughts on Boiler Pressure Loss

While pressure loss is often associated with leaks, this is not always the case. Boilers can lose pressure for several other reasons, many of which are relatively simple to fix, like repressurizing the system or bleeding the radiators. If you suspect a more complex issue, such as a faulty PRV or a problem with the expansion vessel, it’s essential to consult a qualified heating engineer.

For homeowners dealing with persistent pressure loss issues, regular maintenance and timely repairs are the best way to ensure the longevity and efficiency of your boiler. Keep an eye on your pressure gauge and don’t hesitate to call in a professional when necessary.

If your boiler is old and inefficient, it may be time to consider upgrading to a more modern, energy-efficient model. Be sure to explore financing options if upfront costs are a concern.

Get an Instant Boiler Quote

If your boiler is consistently losing pressure and you’re considering an upgrade, we offer fast, reliable installation with flexible payment options. Visit our website to get an instant fixed-price quote and have a Gas Safe engineer install your new boiler within 24 hours.