Boiler pressure too high or keeps dropping: what to do

Read the gauge cold, then hot. The numbers tell the story.

The answer in 2 lines. On most combi and system boilers, the gauge reads about 1 to 1.5 bar when the heating is off and cold, a little higher when it is running (your manual has the exact figure). Pressure that keeps falling usually means a small leak or a tired expansion vessel; pressure that climbs towards 3 bar often means the filling loop is letting water in or the expansion vessel needs attention. Top up once if the manual shows how, but if you are topping up every week or the relief valve is dripping, call a Gas Safe registered engineer.

The first 3 steps

Read the pressure gauge with the boiler cold, then again an hour after the heating comes on, and write both numbers down.
Check the filling loop under the boiler (one or two small taps, or a lever or key) is fully closed: it is sometimes left slightly open after a top-up.
Look for water under the boiler, under radiators and their valves, and at the end of the small pipe that runs outside from the pressure relief valve.

Stop: danger

Smell of gas: do not use switches, flames or your phone indoors; open doors and windows, turn off the gas at the meter if you can, get out and call the free National Gas Emergency Service on 0800 111 999 (England, Scotland and Wales), 24/7.
Headaches, dizziness, nausea or tiredness, especially in several people at once or easing when you go outside: think carbon monoxide. Open windows, switch the appliance off if you can, get out into fresh air and call 999 if anyone feels unwell (or NHS 111 for advice).
Gauge at or above 3 bar, or the relief valve pipe pouring hot water: switch the boiler off, keep away from hot pipes and call a Gas Safe registered engineer.
Never take the casing off or try to re-pressurise the expansion vessel yourself.

What the gauge is telling you

About 1 to 1.5 bar when cold: normal for many boilers. Check your manual: some say 1 to 2 bar.
A bit higher when the heating runs: normal, as hot water expands.
Close to 3 bar when hot (the red zone on many gauges): too high. Often the expansion vessel, or a filling loop letting water through.
Below 1 bar, again and again: water is escaping somewhere (a weeping valve, a radiator, the relief valve) or the expansion vessel has lost its air charge.
No gauge and a small tank in the loft: you probably have an open-vented system, which does not run under this kind of pressure. This guide is for sealed systems.

Pressure too low: topping up safely

Switch the boiler off and let it cool.
Find the filling loop, usually underneath the boiler. Your manual shows it; British Gas has a step-by-step guide.
Open it slowly, watch the gauge rise to the cold pressure in your manual (often 1 to 1.5 bar), then close it fully.
Switch back on and reset if needed. Check the gauge again over the next few days.
Not sure which tap is the filling loop, or the loop is missing? Do not guess: call a Gas Safe engineer.

Pressure too high: what you can try

Make sure the filling loop is closed.
If it is only slightly high when cold, some manufacturers (for example Viessmann and Vaillant) say you can let a little water out by bleeding a radiator, heating off and radiators cold, watching the gauge as you go.
If it climbs back up, goes near 3 bar when hot, or the relief valve drips: that is a job for a Gas Safe engineer (expansion vessel, filling loop or valve).

What not to do

Topping up every few days without finding out why: fresh water brings in air and oxygen, which wear the system.
Opening the filling loop with the boiler hot, or going above the figure in the manual.
Draining radiators at random to bring a high pressure down.

Good to know

Keep a note of the pressure and the date by the boiler: it helps the engineer find the cause faster.
Bleeding radiators always makes the pressure drop a little: check the gauge afterwards.

Who to call and who pays

Gas Safe registered engineer: A gas engineer must be on the Gas Safe Register (it is the law). Ask to see their Gas Safe ID card and check it on the register: how to check an engineer (HSE). Choose one whose card lists the work you need (for example "boilers" for a gas boiler).
If you rent: in England your landlord must keep the installations for heating and hot water in repair and working (Landlord and Tenant Act 1985, section 11), and must have gas appliances checked every year by a Gas Safe engineer and give you the record (HSE). Report the fault in writing (email or text is fine) and keep a copy. If nothing happens, contact your council's private housing (environmental health) team; Shelter England explains your options.
Council or housing association tenants: under Awaab's Law, in force since October 2025, social landlords must deal with emergency hazards within 24 hours (GOV.UK guidance for tenants). A second phase covering more hazards starts on 30 November 2026 (GOV.UK announcement).
Water leaking into a neighbour's flat: tell them straight away and contact your home insurer.

In Bristol, Sophie notices her combi boiler going from 1.5 bar to nearly 3 bar every time the heating runs, and a trickle from the little pipe outside. She switches it off and notes the readings. The Gas Safe engineer finds the expansion vessel has lost its air charge, re-pressurises it, and the gauge stays steady.

A made-up story, for illustration.

Bobby

Sources, verification, confidence

Verified on 1 November 2026, for England. Confidence: high for the safety steps and the landlord duties in England; medium for the typical pressure values (about 1 to 1.5 bar cold, relief valve around 3 bar), which come from boiler manufacturers and vary by model: your manual comes first. We found no official statistics on boiler pressure faults (checked 1 November 2026). This guide describes common cases, not a diagnosis.