It always seems to happen on the same morning. The school run is in fifteen minutes, the kids haven't had breakfast, and the Worcester Greenstar on the kitchen wall is flashing a quiet little F22. Cold radiators. No hot water. Mild panic.
Good news: F22 is, in the trade, the friendliest fault code Worcester Bosch makes. It almost never means a new boiler. It almost never means a parts visit. In the 1,200+ Wembley boilers I've put my spanners to, about 90% of F22 calls are cleared in under sixty seconds with a filling loop, a torch, and a calm cup of tea.
This guide walks you through exactly what to do — and, just as importantly, the three signs that mean you should put the spanner down and ring me instead.
What does F22 actually mean?
Every modern combi boiler has a pressure sensor on its primary heat exchanger. When the water pressure in the central heating circuit drops below about 1.0 bar, the sensor tells the burner to stop firing. The reason is sensible: at low pressure, the boiler can't move water through the heat exchanger fast enough to carry away the heat, and you'd be looking at a melted aluminium block — about £600 of damage — within minutes.
So F22 isn't your boiler giving up. It's your boiler protecting itself. Worcester's logic is unusually conservative on this code, which is partly why it's so common.
F.22 or EA on older Junior models)How to clear F22 in 60 seconds
This works for the vast majority of Wembley homes I attend. If your boiler is the older Junior range with a key-and-tap arrangement instead of a braided silver hose, the principle is identical — open both valves, watch the gauge, close them again.
// The repressurise sequence
- Switch the boiler to standby. Press the standby/clock button on the front so the burner is off. Leave the boiler powered (don't pull the spur).
- Find the filling loop. Look directly underneath the boiler casing for a silver braided hose with two black or grey handles. It connects the cold mains pipe to the heating return.
- Open both valves a quarter turn. Turn them so the handles sit parallel to the hose. You'll hear water flowing — that's mains water entering the circuit.
- Watch the pressure gauge climb to 1.2 bar. Stop the moment the needle reaches between 1.0 and 1.5 bar (cold). 1.2 is the manufacturer-recommended sweet spot. Do not exceed 2.5 bar.
- Close both valves fully. Handles back to perpendicular. Snug, but don't force them.
- Reset. Press and hold the reset button on the boiler for three seconds. The F22 should clear and you should hear the burner light within about thirty seconds.
If you can't find a filling loop under the boiler, your engineer has probably fitted an internal filling key instead — common on Greenstar 4000 and 8000 series. The procedure is the same, but the key lives in a small drawer underneath. Don't force anything; if it doesn't look like the photos in your boiler manual, stop and ring.
Three signs you should call a Gas Safe engineer instead
The repressurise trick works because the water you've just added stays in the system. Sometimes it doesn't — and that's when you need a Gas Safe pair of eyes on it. Watch out for these three patterns:
1. The pressure drops back within a few hours
Topped it up, gauge sat at 1.2 bar, F22 cleared. Six hours later, the F22 is back. That is a leak. It might be a millimetre of weep on a radiator valve in the spare bedroom. It might be a pinhole in a buried pipe under the floor. Either way, you'll keep losing pressure faster than you can fill it. Don't repressurise more than twice in 24 hours.
2. You can see water dripping from the white pipe on your outside wall
That white plastic pipe (the "PRV discharge pipe") drips when your pressure relief valve has either given up or has been blown by over-pressurising. If you topped up past 2.5 bar and now there's a drip outside, the PRV is gone — about £85 in parts and an hour of labour. Don't keep topping it up; you're just feeding the drip.
3. The pressure is suddenly very high when hot, not low when cold
If F22 alternates with high-pressure faults (sometimes shown as A1 or a flashing 3-bar reading), your expansion vessel inside the boiler has likely lost its air charge. It's a 90-minute fix with a foot pump and a schrader valve — and it's not a homeowner job because the vessel sits behind the front panel of a live appliance.
Not playing ball after a top-up?
I cover all HA & NW postcodes — most Wembley homes are a 45-minute drive away. Fixed £45 call-out, credited toward the repair.
Why your Worcester is losing pressure in the first place
Even when there's no obvious leak, every central heating system loses a tiny amount of water over time — through micro-evaporation at valves, slight weeping at radiator joints, and the occasional bleed when air gets in. Losing about 0.1 bar over a few months is normal. Losing 0.4 bar in a week is not.
In Wembley, the three most common real causes I find on Worcester boilers are:
- Towel rail valves that haven't been touched in fifteen years. The packing in older chrome valves dries out, and they'll weep a thimble of water a day. You won't see a puddle because it evaporates off the warm pipe — but the gauge keeps creeping down.
- Auto air vents on the airing-cupboard tank. The little brass nipple on top fails closed (or open) and starts dribbling. Cheap part, easy fix.
- Microleaks in the heating circuit under solid floors. Rare, but every Wembley terrace that's had a kitchen redone has plumbing buried in screed. A leak detector and a thermal camera tell us which run is the culprit.
What pressure should a Worcester Bosch combi sit at?
What if my pressure gauge has gone to zero?
Same fix, just slower. You'll need to top up gently because the system has to expel air through the radiator bleed valves as it fills. Open both filling-loop valves about an eighth of a turn each — you want a trickle, not a torrent — and let the pressure climb steadily to 1.2 bar. Then go round each radiator with a bleed key, releasing any trapped air, and top up again to 1.2 if it's dropped. Don't be tempted to overshoot.
Not sure if your pressure-loss pattern is normal? Send me a photo of your gauge and your filling loop on WhatsApp — 07780 381514. I'll usually reply within the hour with a call-or-fix verdict, no charge.
Frequently asked questions
What does F22 mean on a Worcester Bosch boiler? +
F22 indicates low system pressure — the water pressure in your central heating circuit has dropped below approximately 1.0 bar, so the boiler has shut down the burner to protect itself. Repressurising via the filling loop clears the fault in about 90% of cases.
What pressure should a Worcester Bosch combi boiler be at? +
When the system is cold the pressure gauge should read between 1.0 and 1.5 bar. 1.2 bar is the manufacturer-recommended sweet spot. When the heating is hot, expect the needle to climb to around 1.8–2.0 bar — that's normal expansion.
Why does my Worcester Bosch keep losing pressure? +
Repeated pressure loss is not normal. It usually means one of three things: (1) a leak on a radiator valve, towel rail or pipe joint, (2) a failed expansion vessel inside the boiler, or (3) a faulty pressure relief valve dumping water out of the white pipe on your outside wall. Each requires a Gas Safe engineer.
Is it safe to repressurise my own boiler? +
Yes — repressurising via the filling loop is a homeowner task, not a Gas Safe task. The only things to watch are: never exceed 2.5 bar (you can blow the expansion vessel) and close both filling-loop valves fully when finished, otherwise the boiler will slowly over-pressurise.
How much does it cost to fix a Worcester F22 fault? +
If it's just a top-up — nothing. If there's a leak on a radiator valve, typically £85–£140 for a same-day fix. A failed expansion vessel is £140–£180. A failed PRV (the leaky white pipe outside) is £85–£120. All fixed in one visit.
Written by Jamil — Gas Safe registered engineer (Reg. 930067), based in Wembley HA9. Covering all HA & NW postcodes in Brent for boiler repair, installation and CP12. Last updated 12 May 2026.