Why Does My Boiler Keep Losing Pressure?

A boiler that keeps losing pressure can quickly become frustrating, particularly when it leaves you without reliable heating or hot water. While topping the pressure back up may temporarily solve the problem, repeated pressure loss usually indicates that something within the heating system needs attention.

In this guide, we explain the most common reasons boilers lose pressure, what you can safely check yourself and when it is time to contact a qualified heating engineer.

What Should Your Boiler Pressure Be?

Most sealed central heating systems should display a pressure reading of approximately 1 to 1.5 bar when the boiler is switched off and the system is cold.

The pressure may rise slightly when your heating is running. However, if the gauge regularly drops below 1 bar, your boiler may stop working or display a low-pressure error code.

You can usually find the pressure reading on the front of your boiler, either on a traditional gauge or a digital display.

Common Reasons Your Boiler Is Losing Pressure

1. A Leak Somewhere in the Heating System

One of the most common causes of low boiler pressure is a small water leak.

The leak may be coming from:

• A radiator valve

• A pipe joint

• The boiler itself

• A towel radiator

• Pipework beneath the floor

Some leaks are easy to spot, while others may only produce a small amount of moisture. Check around visible pipes, radiator valves and beneath the boiler for damp patches, staining or signs of corrosion.

Avoid removing the boiler casing yourself, as internal checks should only be completed by a suitably qualified engineer.

2. A Faulty Pressure Relief Valve

The pressure relief valve is designed to release water when the pressure inside the system becomes too high.

If the valve becomes damaged or fails to close correctly, it may continue releasing small amounts of water. This can cause the boiler pressure to gradually fall.

You may notice water dripping from the external overflow pipe, which is usually located on the outside wall near the boiler.

3. A Problem With the Expansion Vessel

The expansion vessel helps manage changes in pressure as the water inside your heating system heats up and cools down.

If the vessel loses its air charge or develops a fault, the boiler pressure may rise sharply when the heating is switched on and then drop significantly once the system cools.

A faulty expansion vessel should be inspected and repaired by a heating engineer.

4. Air Has Been Released From the Radiators

Bleeding your radiators releases trapped air, helping them heat more evenly. However, releasing air can also reduce the pressure within the heating system.

If you have recently bled one or more radiators, you may simply need to repressurise the boiler afterwards.

Always follow the instructions provided by the boiler manufacturer. If you are unsure how to repressurise your system, contact a professional rather than risking overfilling it.

5. The Boiler Was Recently Repaired or Serviced

Boiler pressure may occasionally drop after maintenance work, radiator replacement or repairs to the heating system.

This can happen when water is drained from the system during the work. In many cases, the engineer will restore the correct pressure before leaving, but it is worth checking the gauge afterwards.

If the pressure continues dropping following a repair, contact the company that completed the work.

Can I Repressurise the Boiler Myself?

Many boilers have a filling loop that allows the homeowner to restore the correct pressure. The exact process varies depending on the boiler model.

Before attempting this, check the manufacturer’s instructions and make sure you understand which valves need to be adjusted.

The pressure should normally be restored to around 1 to 1.5 bar while the system is cold. Be careful not to add too much water, as excessive pressure can create further problems.

Repressurising the boiler repeatedly is not a permanent solution. If the pressure keeps falling, there is likely an underlying issue that needs to be investigated.

When Should You Call a Heating Engineer?

You should arrange a professional inspection if:

• The pressure drops repeatedly

• You can see water leaking from the boiler

• The pressure rises above the recommended level

• The boiler displays a recurring fault code

• Your heating or hot water stops working

• You notice water coming from the external overflow pipe

• You are unsure how to safely repressurise the system

Ignoring repeated pressure loss could allow a small problem to develop into a more expensive repair.

Local Boiler Repairs in Chislehurst

If your boiler keeps losing pressure, Chislehurst Plumbers Ltd can inspect your heating system, identify the source of the problem and recommend the appropriate repair.

We provide professional boiler and central heating services throughout Chislehurst and the surrounding areas.

Contact Chislehurst Plumbers Ltd today to arrange an appointment or request a quote.