Combination boilers are pressurised sealed systems. Unlike older open-vented setups with a loft tank, a combi relies on a fixed volume of water at roughly 1–2 bar. If pressure falls too low, the boiler may lock out and you can lose heating and hot water.
What is normal pressure?
Most combis are set to run at 1.0–1.5 bar when cold. Check your installation manual at BoilerManuals.com for the exact range — some models allow up to 1.8 bar cold.
- Pressure rises slightly when the system is hot — this is normal.
- Below 0.5 bar (or the manufacturer’s minimum), the boiler will often display a fault and refuse to fire.
- Above 3 bar, the safety pressure relief valve may discharge water — investigate promptly.
How to read the gauge
The pressure gauge is usually on the boiler front panel or on the bottom manifold. It may share a dial with temperature. Note the reading when the system has been off for at least an hour (cold).
How to top up pressure (filling loop)
Repressurising is normally a homeowner task, but only if you are confident following the manual. General steps:
- Locate the filling loop — often a silver braided hose with valves connecting mains cold water to the heating return. Some boilers use built-in filling keys or levers instead.
- Ensure the boiler is off or in standby (not firing).
- Open the valves slowly (or turn the key/lever as instructed) while watching the gauge.
- Raise pressure to about 1.2 bar cold — do not overfill.
- Close the valves fully. If the loop is a removable hose, disconnect it if your installer left it that way.
- Reset the boiler if a fault code is showing, then check that pressure holds over the next few days.
Important: A permanently connected filling loop left open is a common cause of slow over-pressurisation and repeated pressure relief discharge. Isolate it when not in use.
Why pressure drops
- Small leak — Radiator valves, towel rail connections, or micro-leaks at fittings. Pressure falls over days or weeks.
- Recent bleeding — Venting air from radiators removes water and lowers pressure. Top up after bleeding.
- Expansion vessel — A failed or under-charged vessel can cause pressure to swing high when hot and very low when cold.
- Pressure relief valve passing — Water dripping from the blow-off pipe outside may indicate the PRV has lifted or is faulty.
- Recent work — System draining for repair or a new radiator needs refilling and correct inhibitor dose.
When to call an engineer
- Pressure drops again within hours or days after topping up — treat as a likely leak.
- Water dripping steadily from the pressure relief discharge pipe (often a white plastic pipe outside).
- Gauge stuck at zero but radiators feel hot (faulty sensor) or pressure swings wildly.
- You smell gas, see water pooling under the boiler, or are unsure how your filling arrangement works.
Gas boiler repairs must be carried out by a Gas Safe registered engineer. Leak detection and vessel recharging are not DIY tasks.
Pressure and hot water
Very low system pressure can stop both heating and hot water. If heating works but hot water does not (or is weak), the cause is more often a diverter valve, scaled plate heat exchanger, or mains flow issue — see our hot water problems guide.
Related guides
Installation best practice covers filling loops, pressure testing, and inhibitor dosing. For annual checks, see BoilerService.com.
Always follow the manufacturer’s instructions for your specific boiler model. If in doubt, contact a Gas Safe registered engineer.