Boiler Losing Pressure, What Should You Do (How To Fix)

If your boiler is losing pressure, it usually means there is a small leak, trapped air in the system, or a fault with an internal part. 

Low pressure can cause heating problems and, if ignored, lead to bigger issues inside your boiler. 

Our Gas Safe engineers explain the main causes, how to spot the signs, and the safest way to restore pressure.

Check out our expert guide on if a boiler can lose pressure without a leak!

Understanding boiler pressure

What boiler pressure means in a sealed heating system

Most modern boilers use a sealed system that relies on water pressure to circulate heat through radiators and pipework. 

When the pressure drops too low, the boiler struggles to push hot water around the system, leading to poor heating performance or shutdowns.

What is considered low boiler pressure

The pressure gauge on your boiler shows how much water is in the system.

When cold, it should read between 1 and 1.5 bar. If it falls below 1 bar, the system needs repressurising.

Why boiler pressure matters for efficiency, performance and safety

Stable pressure helps your boiler run efficiently and protects components from strain.

Low pressure reduces heating output, while excessive pressure can cause leaks or part failure.

Common causes of boiler pressure loss

1. Small leaks in the heating system

Minor leaks are the most common cause of pressure loss. They may go unnoticed under floors or behind walls.

Fix: Have a qualified engineer reseal or replace any damaged fittings.

Cost guide: £100 to £200

2. Internal boiler leaks

If pressure keeps dropping even after topping up, there may be an internal leak inside the boiler casing.

Fix: A Gas Safe engineer will inspect the boiler and replace any worn seals, valves or gaskets.

Cost guide: £150 to £300 depending on the part

3. Air in the system or recently bled radiators

Bleeding radiators releases trapped air but slightly lowers pressure.

Fix: Repressurise the system using the filling loop until the gauge reads 1 to 1.5 bar.

Cost guide: Free if done yourself

4. Faulty expansion vessel or pressure relief valve

The expansion vessel balances pressure as the system heats and cools.

If it fails or loses charge, pressure can rise too high and escape through the relief valve.

Fix: A Gas Safe engineer can recharge or replace the vessel and check the relief valve.

Cost guide: £250 to £450 depending on boiler type

5. Incorrect filling loop setup or valve left open

A filling loop left partially open can cause continuous changes in pressure.

Fix: Make sure both ends of the loop are fully closed once the boiler has been topped up.

Cost guide: Free if corrected early

6. Corrosion or system inhibitor issues reducing water volume

Without corrosion inhibitor fluid, sludge and rust can form inside pipes, creating leaks or blockages.

Fix: A system flush and new inhibitor will help prevent future problems.

Cost guide: £150 to £250

7. Seasonal temperature changes and system contraction

Cold weather can cause pipes to contract slightly, leading to small pressure dips.

Fix: Check the gauge regularly. Persistent pressure drops may indicate another issue.

Cost guide: None unless repairs are required

Signs of low boiler pressure

• Radiators cold at the top or uneven heating

• Gurgling or bubbling noises from pipework

• Pressure gauge reading below 1 bar

• Fault code or flashing pressure warning on display

• Boiler turning off unexpectedly

These are the most common signs noticed by homeowners before the heating fails completely. Low pressure can also affect hot water flow and shower performance, and our boiler and shower issue fixes guide explains how to sort these problems.

How to tell if your boiler pressure is too low

How to read the pressure gauge and what the numbers mean

A reading below 1 bar means pressure is too low. 

Between 1 and 1.5 bar is ideal, and around 2 bar when hot is normal.

Checking for visible leaks around the system

Inspect around radiators, valves and under the boiler for any damp areas or signs of corrosion.

Ensuring the filling loop is set up correctly

The filling loop is a flexible metal hose beneath the boiler. Both valves should be turned off after topping up the system.

Spotting recurring pressure drops versus one-off dips

A small drop after bleeding radiators is normal. Regular pressure loss suggests a leak or faulty part.

When low pressure triggers fault codes or boiler shutdown

Many modern boilers lock out when pressure is too low. Make note of any error codes to share with your engineer.

How to fix low boiler pressure

Step-by-step: how to repressurise your boiler safely

1. Turn off the boiler and let it cool.
   

2. Locate the filling loop under the boiler.
   

3. Slowly open both valves until you hear water entering the system.
   

4. Watch the gauge and close the valves once it reaches 1 to 1.5 bar.
   

5. Turn the boiler back on and check that pressure holds steady.
   Cost guide: Free
   

How to bleed radiators and remove trapped air

1. Switch off the heating.
   

2. Use a radiator key to open the bleed valve until water flows steadily.
   

3. Close the valve and check the boiler gauge, topping up if required.
   Cost guide: Free
   

Checking external relief pipes and leak points

If the copper pipe outside your home is dripping, the pressure relief valve may be discharging water.

This should be checked by an engineer.

Cost guide: £100 to £250 depending on repair

What to do if your boiler keeps losing pressure

When to contact a Gas Safe registered engineer

If pressure drops again within a day or two after topping up, or you notice damp patches near the boiler, contact a professional.

What an engineer will check (expansion vessel, valves, internal components)

An engineer will test the expansion vessel, inspect valves and seals, and look for any internal leaks.

Cost guide: Diagnostic visit £80 to £120

Repair or replace? When constant pressure loss signals a bigger issue

If your boiler repeatedly loses pressure despite several repairs, internal corrosion or wear could be the cause.

At that point, replacement may be more cost-effective.

Cost guide: Minor repairs £100 to £300. Replacement from £1,500.

Preventative maintenance to stop boiler pressure drops

Schedule annual boiler servicing and system checks

An annual service allows engineers to clean key parts, test pressure controls, and prevent small issues from becoming major faults.

Maintain system inhibitor levels and water quality

Inhibitor fluid helps prevent corrosion and sludge build-up. It should be checked and topped up during servicing.

Regularly check the filling loop and pressure gauge

Monitor your boiler’s pressure once a month, especially before winter. Early detection prevents breakdowns.

Early warning signs to look for

Look out for damp patches, minor drips or strange noises from the system.

These can all indicate developing leaks or trapped air.

Final thoughts on fixing and preventing boiler pressure loss

Boiler pressure loss is usually caused by small leaks, air in the system or a faulty expansion vessel. 

Repressurising can resolve minor drops, but recurring issues should always be checked by a Gas Safe engineer. 

Most low pressure problems are simple to fix once identified, and regular servicing keeps your system safe, efficient and reliable.

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