Glow-worm [F.20] fault codewhat it means and how to fix it

On the current ranges Glow-worm print F.20 as "Safety shutdown: Overheating temperature reached", and the Compact manual calls it the "temperature cut-out safety shutdown". The water in the heat exchanger got hotter than the board will tolerate, and rather than let it boil the boiler stopped.

The installation manuals list the electrical explanations — an incorrect earth connection between the harness and the appliance, a flow or return sensor loose or defective, a stray spark from the ignition cable, plug or electrode upsetting the reading — but Glow-worm's homeowner fault-code page is where the everyday causes are: "system water pressure below normal range of 1 to 1.5 bar", "circulation fault, pump, valve, blockage", and airlocks. It is also the page that tells you what you may do: "check the water pressure and refill the system if needed. Look for leaks... Ensure radiator valves are open, and external pumps and zone valves are working correctly. Make sure there are no airlocks in the system", and then to treat it as a case for an installer in the first instance because it "can relate to how the boiler is working with the heating system".

On the two-digit Ultracom2, Betacom 24a and Betacom2 the same F20 is "overheating fault — overheating safety device activated (97 °C)", with the pump, the harness and closed flow and return isolation valves as the checks. On the Ultracom, Flexicom and Ultrapower the overheat is F5 and their guides say "allow to cool then press reset button", and on those boilers a 20 means something else entirely — software incompatibility on the cxi, an incompatible user interface on the CI range.

1. 1

   Let the boiler cool for twenty minutes before anything else. On an Ultracom, Flexicom or Ultrapower showing F5, Glow-worm's guides make this the first instruction.

2. 2

   Check the pressure and, if it is below the 1.0 to 1.5 bar the homeowner page names, top up as the F.22 page describes. Look for any sign of a leak while you are there.

3. 3

   Walk round the radiators: are the valves open, including any thermostatic heads turned to frost? If you have zone valves or an external pump, are they moving and running when the heating calls? Bleed any radiator that is cold at the top.

4. 4

   Reset once as your guide describes. If F.20 returns, Glow-worm say this is a case for your installer in the first instance — leave it off and book the visit rather than repeating the cycle.

   Do not touch the pipework under the boiler while the code is showing; the flow pipe can be very hot for some minutes after a cut-out.