Boiler losing pressure: causes and what to do
A sealed heating system should hold roughly 1 to 1.5 bar when cold. If the pressure keeps falling, water is leaving the system: usually a small leak on a radiator valve or joint, a pressure relief valve that is passing water outside, or an expansion vessel that has lost its charge. Top it up once using the filling loop. If it drops again within days or weeks, have the cause found rather than topping up repeatedly.
Key facts
- Normal cold pressure
- Roughly 1 to 1.5 bar (check your boiler's manual)
- Boiler stops firing
- Usually below about 1 bar
- Relief valve opens
- Around 3 bar
- Normal top-ups
- Rare — once after bleeding radiators
- Work inside the boiler
- Gas Safe registered engineers only
What the pressure is telling you
| Symptom | Most likely cause |
|---|---|
| Pressure drops slowly over days | A small leak on a radiator valve, joint or towel rail |
| Pipe outside drips after a top-up | The system was overfilled or the relief valve is passing |
| Pressure high when hot, low when cold | The expansion vessel has lost its charge |
| Pressure dropped after bleeding radiators | Normal — air was released; top up once |
How to top up a combi boiler
- 1Switch off and let it coolTurn the boiler off and wait for the system to cool.
- 2Find the filling loopUsually a braided hose with two valves under the boiler, or a built-in key or lever.
- 3Open the valves slowlyWatch the gauge rise to around 1.2 to 1.5 bar.
- 4Close both valves fullyMake sure no more water is entering the system.
- 5Restart the boilerReset any fault code following the manufacturer's instructions.
What the gauge should read
Most combi and system boilers have a pressure gauge on the front or underneath. Cold, the needle normally sits around 1 to 1.5 bar; with the heating running it rises a little as the water expands. Check your boiler's manual for its exact range.
Below about 1 bar many boilers stop firing and show a low-pressure fault code. That is a safety lockout, not a dangerous situation — the boiler is protecting itself.
Why the pressure keeps dropping
A sealed system is closed: if the pressure falls, water is escaping somewhere or the system cannot cope with expansion. The usual causes are:
- • A small leak — a weeping radiator valve, a joint under the floor, or a towel rail union. Look for drips, staining and corrosion at radiator valves.
- • A passing pressure relief valve — water dripping from a small copper pipe outside, near the boiler, often after the system was overfilled.
- • A failed expansion vessel — pressure climbs high when the heating is on, the relief valve opens, then pressure is low when the system cools.
- • Recently bled radiators — releasing air lowers the pressure; one top-up afterwards is normal.
How to top up the pressure
Most combi boilers are topped up through a filling loop — a braided hose with two valves under the boiler, or a built-in key or lever on some models. With the boiler off and cool, open the valves slowly, watch the gauge rise to around 1.2 to 1.5 bar, then close both valves fully. Follow the manufacturer's instructions for your model.
Do not overfill. Pushing the pressure towards 3 bar makes the relief valve open and you lose the water again.
When to call someone
Topping up once after bleeding radiators is routine. Topping up every week, or watching the pressure swing high and low as the heating runs, means a fault that will not fix itself. Finding a heating leak is plumbing work; replacing a relief valve or recharging an expansion vessel inside the boiler is work on a gas appliance and must be done by a Gas Safe registered engineer.
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