Introduction
High boiler pressure can be a frustrating issue for homeowners in East Meon, leading to safety cut-outs, leaks and an inefficient heating system. In many cases, the culprit is a fault with the expansion vessel. Whether you live near the South Downs or just outside Petersfield or Liphook, understanding how this small component affects your boiler’s pressure is key to keeping your heating running smoothly.
What Is an Expansion Vessel?
An expansion vessel, also known as an expansion tank, is a sealed chamber partially filled with air and water. It absorbs the extra volume of hot water when your heating system warms up, preventing excessive pressure build-up inside the boiler.
How Does an Expansion Vessel Work?
- When cold, water occupies most of the vessel and the air cushion is compressed.
- As the system heats up, water expands and enters the vessel, compressing the air further.
- The vessel’s air cushion absorbs the extra volume, maintaining steady pressure.
Why Is It Important?
- Protects boiler and pipework from damage due to overpressure.
- Prevents the pressure relief valve from constantly bleeding off water.
- Ensures consistent heating performance and system longevity.
Symptoms of Expansion Vessel Problems
When the expansion vessel loses its air cushion or becomes waterlogged, you may notice:
- Repeated pressure spikes above 3 bar.
- Frequent activation of the pressure relief valve.
- Leaking from the pressure relief pipe.
- Cold spots on radiators despite high system pressure.
- Long warm-up times or noisy pipework.
These issues are common in GU32 properties and are often misdiagnosed, leading to unnecessary new boiler installation quotes.
How to Check and Reset Boiler Pressure
Before assuming the worst, you can perform some simple checks:
- Switch off the boiler and allow it to cool completely.
- Locate the pressure gauge—ideally it should sit between 1.0 and 1.5 bar when cold.
- If it’s above 2 bar, use the filling loop to gently release excess water until the gauge reads 1.2 bar.
- Bleed a radiator to check if the pressure drops rapidly. A quick drop indicates a waterlogged vessel.
- Turn the boiler on and observe the gauge. If it climbs above 3 bar, the vessel needs attention.
Never force the gauge or tamper with sealed components—these steps are for basic pressure checks only.
Common Causes and Solutions
1. Waterlogging
Over time, the membrane inside the expansion vessel can deteriorate, allowing water to fill the entire chamber. This eliminates the air cushion and causes immediate pressure spikes.
Solution: A Gas Safe engineer can replace or recharge the vessel. In older vessels, a full replacement is often more cost-effective than a repair.
2. Loss of Air Charge
If the pre-charge valve leaks, the air cushion escapes. This leads to gradual pressure increases as the system heats up.
Solution: The engineer will check the air pressure (typically 0.75 bar cold) and top up if necessary. Regular boiler servicing helps catch this early.
3. Incorrect Vessel Sizing
Smaller or ill-specified vessels struggle to handle the water volume in larger or extended heating systems, common in properties around Haslemere and Liss.
Solution: Upgrading to a correctly sized vessel matched to your system’s capacity resolves recurring pressure issues.
4. Persistent Leaks
Leaking pipework or radiators can cause the boiler to lose water, triggering an automatic refill that pushes pressures too high.
Solution: Locate any leaks and repair or replace faulty components. A well-maintained system with full boiler cover plans ensures leaks are fixed quickly.
Maintenance Tips to Avoid Expansion Vessel Failures
- Schedule an annual service to catch membrane wear and air charge loss.
- Check boiler pressure monthly and record readings.
- Bleed radiators at least twice a year to remove trapped air.
- Insulate external pipework in areas like Farnham to prevent freezing and cracking.
- Invest in a magnetic filter to reduce debris that can damage components.
When to Call a Professional
While simple pressure checks can be DIY, expansion vessel work carries risks. Contact a qualified Gas Safe engineer if:
- Your boiler pressure spikes recur after resets.
- You notice waterlogging in the vessel or hear banging noises.
- The system requires more than one refill a month.
- You’re planning major boiler upgrades or system extensions.
Frequently Asked Questions
Q: How long does an expansion vessel last?
A: Typically 5–10 years, depending on system usage and maintenance quality. Regular servicing extends its life.
Q: Can I recharge the expansion vessel myself?
A: It’s not recommended. Specialist equipment and expertise ensure the correct air charge and safe handling of pressurised parts.
Q: What if my vessel needs replacing?
A: Replacing the vessel restores pressure control. Your Gas Safe engineer will choose the right size for your system’s volume.
Q: Will a new boiler eliminate pressure issues?
A: A new boiler alone won’t fix a faulty expansion vessel. Ensure any replacement includes a properly sized vessel and magnetic filter.
If you need help with your heating or boiler, our local engineers are here to help. Call us on (01420) 558993 or book online at embassygas.com/book.
Written by Embassy Gas — your local heating experts covering Bordon, Alton, Liphook, Farnham and surrounding areas.