Boiler Pressure FAQs
Understanding and managing your boiler's pressure.
What pressure should my boiler be at?
When the heating system is cold, your boiler pressure should sit between 1 and 1.5 bar on the pressure gauge. When the heating is running and the system is warm, pressure may rise to 2–2.5 bar, which is normal expansion. If cold pressure is consistently below 0.8 bar or above 2.5 bar, action is needed.
What happens when boiler pressure is too high?
When boiler pressure exceeds 2.5–3 bar, the pressure relief valve (PRV) will open to release water and protect the system. You may notice water dripping from the overflow pipe outside. Consistently high pressure can indicate a faulty expansion vessel that is no longer absorbing system expansion, or an overfilled system. If the PRV is frequently discharging, call an engineer — do not ignore it.
What happens when boiler pressure is too low?
When boiler pressure drops below 0.5–0.8 bar, most boilers will display a low pressure error code and lock out. You'll have no heating or hot water until the pressure is restored. Low pressure is usually caused by a small leak in the system, or by air having been released when bleeding radiators without subsequently topping up the pressure.
How do I repressurise my boiler?
Most combi and system boilers can be repressurised via a filling loop — a flexible braided hose connecting the heating system to the mains cold water supply. Open both valves slowly, watching the pressure gauge, until it reaches 1–1.5 bar, then close both valves and ensure the filling loop is disconnected (on external loops) or valves are fully closed. Your boiler manual will show the specific procedure. Call us if you're unsure.
Why does my boiler keep losing pressure?
If your boiler loses pressure repeatedly (more than a few times a year), there is almost certainly a leak somewhere in the system. Leaks can be obvious (visible water) or very small (pinhole leaks in pipework or radiators that evaporate before pooling). A failed expansion vessel can also cause repeated pressure drops. A Gas Safe engineer can carry out a system pressure test to locate the source.
What is a pressure relief valve (PRV)?
The pressure relief valve is a safety device that automatically opens if boiler pressure rises too high (typically above 3 bar), releasing water to prevent damage to the system. It is a critical safety component. A PRV that discharges regularly indicates an underlying pressure issue that should be investigated, not simply ignored. If a PRV fails to re-seat after discharging (continues to drip), it should be replaced.
When should I call an engineer about boiler pressure?
Call an engineer if: the pressure keeps dropping despite topping up; the PRV is frequently discharging water; the pressure rises above 3 bar; there are visible water leaks anywhere on the boiler or heating system; or if you cannot locate the filling loop or are not confident repressurising the system yourself. We're happy to talk you through a simple repressurise over the phone on 01483 676108.
What is an expansion vessel and when does it need replacing?
The expansion vessel is a sealed tank containing a rubber diaphragm and nitrogen gas. As the heating system warms, water expands into the vessel, preventing pressure from rising. Over time, the nitrogen charge or the diaphragm can fail, meaning the vessel no longer absorbs expansion effectively. A failed expansion vessel causes pressure to rise on every heating cycle and the PRV to discharge. Replacement is a straightforward repair for a Gas Safe engineer.
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