On this page
- How pressure behaves in a sealed system
- Cause 1: overfilled system
- Cause 2: filling loop open or passing
- Cause 3: expansion vessel lost its charge
- Cause 4: a leaking plate heat exchanger on a combi
- How to bring the pressure down safely
- What happens if pressure stays high
- Book with Gasmaster
- Frequently asked questions
Quick answer
On most sealed heating systems, pressure should sit around 1 to 1.5 bar when cold and rise a little, often to around 2 bar, when the system is hot. If it climbs towards 2.5 to 3 bar, something is wrong. The three usual causes are the system being overfilled, a filling loop that has been left open or is letting water through, and an expansion vessel that has lost its air charge. You can safely release a small amount of water from a radiator to bring pressure down, but a recurring rise needs an engineer.
Key takeaways
- Normal cold pressure on a sealed system is typically 1 to 1.5 bar, rising a little when hot.
- Pressure near 3 bar is high enough for the pressure relief valve to open and discharge water outside.
- Check both filling loop valves are fully closed; a tiny leak through them slowly overfills the system.
- An expansion vessel that has lost its charge causes pressure to rise sharply when the system heats.
- Releasing water via a radiator bleed valve lowers pressure in the short term but does not fix the cause.
How pressure behaves in a sealed system
Water expands as it heats. In a sealed system, the expansion vessel (a small tank with an air cushion behind a rubber diaphragm) absorbs that expansion. That is why pressure rises modestly as the heating warms up and falls again as it cools. When something upsets this balance, pressure either climbs too high or drops too low. We covered the opposite problem in boiler losing pressure but no visible leak.
Cause 1: overfilled system
The most common cause is simply too much water added when topping up. People often fill to 2 bar when cold, then the pressure rises to near 3 bar when the heating is on. Check the reading with the heating off and the system cold.
Cause 2: filling loop open or passing
The filling loop connects the mains to the heating system. Both valves should be fully closed after topping up. If one is slightly open, or its internal seal has worn, mains water can creep in and pressure rises over days. Some combis have an internal filling key or lever rather than an external loop. Our repressurising guide shows where these are.
Cause 3: expansion vessel lost its charge
If pressure is normal when cold but shoots up when the heating runs, the expansion vessel may have lost its air charge or the diaphragm may have failed. Without that cushion, heated water has nowhere to go and pressure spikes. An engineer can recharge the vessel with a pump and check the diaphragm. Some vessels are inside the boiler and some are separate; replacing an internal one is more involved.
Cause 4: a leaking plate heat exchanger on a combi
Less commonly, a combi’s plate heat exchanger can develop an internal leak between the mains water and heating sides, pushing mains water into the heating circuit. This needs an engineer to diagnose.
How to bring the pressure down safely
- Turn the heating off and let the system cool.
- Put a cloth and container under a radiator bleed valve.
- Open the valve slowly with a radiator key and let a little water out.
- Keep an eye on the gauge and close the valve when it reads about 1 to 1.5 bar.
If the pressure rises again afterwards, you have not fixed the cause.
What happens if pressure stays high
At around 3 bar the pressure relief valve opens and discharges water through a copper pipe outside. Once a relief valve has operated, it may not reseal perfectly and can keep dripping. See pressure relief valve dripping outside.
Book with Gasmaster
Pressure keeps climbing? Gasmaster Plumbing & Heating, Gas Safe registered (220932), repairs expansion vessels, filling loops and boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service or book through our boiler service booking page.
Frequently asked questions
Is 2.5 bar too high for a boiler?
When hot, 2 bar is common and 2.5 bar is on the high side. When cold, 2.5 bar is too high and should be reduced.
Can high boiler pressure cause an explosion?
Modern boilers have a pressure relief valve and other safety devices to prevent dangerous pressure. High pressure is a fault to fix, not an emergency in itself.
Why does my boiler pressure go up when the heating is on?
A small rise is normal. A large rise usually means the expansion vessel needs attention.
Sources
- Manufacturer user guides for sealed-system combi and system boilers
- Gas Safe Register (gassaferegister.co.uk)
- Energy Saving Trust, heating system guidance (energysavingtrust.org.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
