Glow-worm F22 Fault: Low Water Pressure — Causes & What To Do

F22 indicates a safety switch-off caused by low water pressure in the boiler — the system pressure has fallen below the safe operating level. The boiler shuts down to protect the pump and heat exchanger from running dry. You'll usually have no heating or hot water.

The most common causes, in order of how often we see them:

• • Low system pressure — pressure has simply dropped below the safe level (usually below 1 bar) and needs topping up via the filling loop. The most common F22 cause.
• • A leak on the system — if pressure keeps falling after a top-up, water is escaping somewhere: a radiator valve, joint, or hidden pipe under a floor.
• • Faulty water pressure sensor — the sensor can fail or drift, reporting low pressure even when the system is full.
• • Air in the system — trapped air causes erratic pressure readings and can contribute to low pressure.

Repressurising the system to around 1.2–1.5 bar via the filling loop and resetting the boiler usually clears F22. If pressure holds, the fix is complete. If it drops again within hours or days, there's a leak or a faulty sensor — a reset won't help. Book an engineer.

• • Check the pressure gauge — a healthy system sits between 1.0 and 1.5 bar when cold. If it's below 1 bar, low pressure is your F22 trigger.
• • Repressurise via the filling loop to around 1.2–1.5 bar, then reset the boiler. If the fault clears and pressure holds, the fix may be complete — but watch for it dropping again.
• • Bleed radiators if they're cold at the top or you hear gurgling — trapped air causes erratic readings. Re-check pressure afterwards, as bleeding lowers it.
• • If pressure keeps dropping after a top-up, there's a leak or a faulty pressure sensor — book a Gas Safe engineer to find the source.

The water pressure sensor sits inside the boiler casing. Replacing it, or any work inside the casing, is gas/electrical work that must be done by a Gas Safe Registered engineer. If there's no visible leak and pressure still falls, the leak may be under a floor — that needs professional leak detection, not DIY.

• • Confirm system pressure on the gauge and with a calibrated reading.
• • Test the water pressure sensor's reading against actual system pressure.
• • Check for leaks — visual inspection plus leak detection if the source is hidden.
• • Inspect the filling loop, pressure relief valve and expansion vessel.
• • Replace the pressure sensor if faulty, then re-commission the system.

Our Gas Safe Registered (594547) engineers repair every Glow-worm model — Betacom, Ultracom, Flexicom and Energy — across Surrey, Berkshire, Buckinghamshire and West London, and carry common sensors, fans and ignition parts on the van. Most Glow-worm fault-code repairs are completed in a single visit, with any part quoted and agreed before fitting.