Glow-worm [F.22] fault codewhat it means and how to fix it

F.22 — F22 on the older two-digit displays — is the boiler refusing to run a heat exchanger it believes is short of water. Glow-worm print it in every user guide as a fault the householder deals with, and they are more specific about the numbers than most manufacturers, which is worth having because the numbers change with the range. The Ultimate3 manual says the value flashes below 0.5 bar, the boiler stops below 0.3 bar and the display then reads 0.0 whatever the true pressure, so a refill appears to do nothing until it passes 0.3.

The Energy7 guide gives a working band of 0.5 to 2.7 bar rather than the Ultimate3's 1.0 to 1.5. The Ultracom2 guide flashes the pressure and stops the boiler below 0.4 bar and tells you to fill to between 0.8 and 1.2. The Flexicom guide, where F22 reads "low water pressure or ignition temperature rise too slow", says fill until 1.0 bar shows. And the Betacom2 guide gives no filling instruction at all for lack of water below 0.5 bar — it tells you to contact your installer.

On every range the installation manual adds two causes the gauge cannot show: a defective water pressure sensor, and a loose or damaged cable to the pump or to that sensor, which is why a boiler that shows F.22 at a healthy reading is a sensor job and not a filling one.

1. 1

   Read the pressure cold. On the Energy, Ultimate3 and Betacom4 it is on the display, and if the flow temperature is showing instead, the guides say to hold both temperature buttons together for more than five seconds to bring the pressure up. Older ranges show it on the display or a gauge under the casing.

2. 2

   Find the filling arrangement your installer left. Glow-worm boilers have a filling loop or tap beneath the appliance; the Flexicom guide calls it the filling loop under the boiler with two taps, the Ultracom2 guide a filling tap located under the appliance.

3. 3

   Fill slowly to the figure your own guide names: 1.0 bar on a Flexicom or Ultrapower, 0.8 to 1.2 on an Ultracom2, 1.0 to 1.5 on an Ultimate3 or Betacom4, anywhere in 0.5 to 2.7 on an Energy7 with 1.0 to 1.5 the sensible target. On an Ultimate3 the display sits at 0.0 until you pass 0.3 bar, so keep going.

   Close the taps and remove a loop hose when you finish — the Ultrapower guide says the flexible hose must be disconnected to comply with the water regulations, and a loop left open lets mains pressure into the heating.
4. 4

   Press reset once if the code has not cleared by itself. If the fault persists after that, or if you are refilling more than once or twice a year, the guides all say the same thing: have the system checked for a leak.