A dropping pressure gauge that won’t stay put
Few things are more frustrating than topping up your boiler’s pressure, only to watch the needle sink again a day or two later. A modern sealed heating system shouldn’t need regular topping up, so if yours does, something is quietly going wrong. Here’s what’s likely behind it and what you can do about it.
What is boiler pressure, and what’s normal?
Combi and system boilers run on a sealed, pressurised water system. That pressure keeps water circulating properly around your radiators. You can see it on the gauge on the front of the boiler, or on the digital display.
As a guide:
- When cold (heating off): around 1.0 to 1.5 bar.
- When hot (heating on): it rises slightly, often up to around 2 bar — this is normal.
If the pressure regularly drops below 1 bar, the boiler may lock out and refuse to run to protect itself. Consistently above around 2.5–3 bar suggests a different fault worth investigating.
The most common causes of pressure loss
A leak somewhere in the system. This is the most frequent culprit. Water can escape from radiator valves, pipe joints, the pump or the boiler itself. Even a tiny weep — too small to leave an obvious puddle — will slowly bleed pressure away. Check under radiators, around visible pipework and near the boiler for damp patches or staining.
Recently bled radiators. If you’ve just bled your radiators, you’ll have released air and lowered the pressure. This one is harmless — simply top the system back up.
A faulty pressure relief valve (PRV). This safety valve releases water if pressure gets too high. If it sticks or wears, it can weep continuously instead of sealing. A tell-tale sign is water dripping from the small discharge pipe that exits through an outside wall.
A failing expansion vessel. This vessel absorbs the extra volume as water heats and expands. When it fails, pressure swings dramatically — high when hot, low when cold — and the system struggles to stay stable. This is a common cause of stubborn, recurring pressure loss and needs a professional to recharge or replace it.
What you can safely fix yourself
Repressurising your boiler is a routine job most homeowners can do, using the filling loop — usually a silver braided hose with a valve or tap at each end beneath the boiler.
- Turn the boiler off and let it cool.
- Check both ends of the filling loop are securely attached.
- Open both valves slowly. You’ll hear water entering the system.
- Watch the gauge and close the valves once it reaches around 1.0–1.5 bar.
- Close both valves fully and switch the boiler back on.
Always check your boiler’s manual first, as the exact process varies by model. Never leave the valves open, as over-pressurising can force water out of the PRV.
When repressurising isn’t enough
Here’s the key point: if you top up the pressure and it drops again within a day or two, there’s an underlying fault. Repeatedly refilling a sealed system is not a fix — it introduces fresh, oxygen-rich water that accelerates internal corrosion and sludge build-up, storing up bigger problems.
At that point it’s time to call in a professional to trace the leak or test the expansion vessel and PRV.
DIY vs calling a Gas Safe engineer
You can safely repressurise the boiler, bleed radiators and look for obvious leaks. Beyond that, tasks such as:
- Recharging or replacing the expansion vessel,
- Replacing the pressure relief valve,
- Tracing a hidden leak under floors, and
- Any work involving the gas supply or internal boiler components
should be left to a Gas Safe registered engineer. Working on gas appliances without qualification is illegal in the UK and genuinely dangerous. If you ever smell gas, leave the property and call the National Gas Emergency line on 0800 111 999.
For homeowners across Norwich and Norfolk, our advice is simple: if your gauge keeps dropping, don’t keep firefighting it — get the root cause diagnosed once and properly.
Tired of topping up your boiler? We’ll find the cause
If your boiler keeps losing pressure, the team at Norfolk Heating Ltd can trace the fault and fix it for good. Our Gas Safe registered engineers will find the leak, test your expansion vessel and valves, and get your system holding pressure the way it should.
Call us on 07751 261207, email enquiries@norfolkheatingltd.co.uk, or visit Prestige House, Avian Way, Norwich NR7 9AR. We’re open Monday to Friday, 7:30am–5:00pm — get in touch and put an end to the constant top-ups.
