Clear it first
Glow-worm's own homeowner guidance calls F22 the easiest code on the boiler to fix yourself: it indicates a fault in the heating system water pressure and can usually be resolved by repressurising the system. On older Glow-worm appliances the same fault shows as F9.
- Switch the boiler off and let it cool. A hot system reads high and you will undershoot.
- Open both valves on the filling loop slowly. On most Glow-worm combis this is a braided hose under the boiler with a valve at each end.
- Stop between 1.0 and 1.5 bar cold, close both valves fully, and detach the loop if yours is removable.
- Restart. Most units clear F22 on their own once the pressure is back; some need the reset button.
If you overshoot, bleed a radiator briefly to come back down. The step-by-step method is here.
What Glow-worm publishes about F22
The trade fault-finding table for the Energy range gives F.22 as "Safety switch-off. Low water pressure in the boiler", with the causes listed as no or insufficient water in the product, a defective water pressure sensor, or a loose, disconnected or defective cable to the pump or to the water pressure sensor. On the Betacom A range the same code reads "Low water pressure or ignition temperature rise too slow".
That second half of the cause list is the part nobody mentions, and it changes what you should pay for. F22 does not prove the system has lost water. It proves the boiler believes the pressure is low. A dead sensor or a disconnected cable produces the identical code on a system that is perfectly full.
| What the gauge reads | What F22 is telling you |
|---|
| Below about 0.6 bar | Genuine pressure loss. Top up, then count how long it holds. |
| Normal, 1.0-1.5 bar cold, and F22 persists | Sensor, wiring or PCB. Glow-worm also publishes F.75 as a pressure sensor fault and F.24 for a water pressure sensor that is not connected or has short-circuited. Not a leak. |
| Normal after topping up, F22 back within days | Active water loss. This is the one worth investigating. |
How often is too often
A sealed heating system is closed. Refilling it once or twice a year is ordinary, and a drop straight after bleeding radiators is expected. Anything more frequent is water going somewhere it should not.
The reason to act rather than keep topping up: every refill introduces fresh mains water carrying dissolved oxygen into a circuit designed to have none. Oxygen corrodes radiator steel and copper joints from the inside, which produces more pinholes, which means more topping up. Meanwhile the water that left is sitting in a joist, a screed bed or a ceiling void.
Where the water goes on a Glow-worm system
Things you can see
Start outside. If the pressure relief discharge pipe on the external wall drips whenever the boiler fires, the system is pushing water out through the relief valve and no top-up will hold. Then check under the boiler case, every radiator valve spindle and tail nut, the underside of radiators for a rust bloom, and bathroom towel rails, which corrode faster than anything else on the circuit.
Things you cannot
If all of that is dry, the loss is under a floor, inside a wall, or in the boiler. A pinhole in a 15mm heating pipe under screed can drop the gauge enough to trip F22 weekly and still produce no visible damp for months, because the screed holds the water and the heating dries the surface. On underfloor heating the pipe run is far longer and the same loss is spread across a whole floor, which is why guessing there is expensive.
Two faults that empty a system without leaking
Rule these out before anyone lifts a board. If the pressure climbs sharply when the heating runs and then drops below normal as it cools, the expansion vessel has probably lost its air charge and the system is venting the expansion through the relief valve every cycle. No water is leaking into the building; it is going out of the discharge pipe. That is a recharge or a replacement vessel.
The second is debris on the seat of the pressure relief valve itself, which lets it weep continuously. Both look exactly like a leak from the gauge, and both are cheaper to fix than to search for.
When to call, and what it costs
Call when the system will not hold pressure for a full heating cycle, when F22 returns weekly, when there is an unexplained warm or damp patch on a floor, or when the discharge pipe outside drips on every firing. Do not call if the gauge sits happily at 1.2 bar and F22 is still showing, because that is a sensor fault and a heating engineer should replace the part.
A detection visit isolates and pressure tests the circuit first, to prove the loss is real and to size it, then traces it with thermal imaging and, on a drained circuit, tracer gas, so the floor is opened at one point rather than searched. We work to a fixed hourly rate, settled before we attend rather than after, and if we do not find the leak you do not pay for the time we spent looking. We then carry out the repair and put the floor, wall or ceiling back.
If your boiler shows something other than F22, this list sorts the codes that mean water loss from the ones that do not.
