Vaillant [F.24] fault codewhat it means and how to fix it

Vaillant define F.24 as the temperature increasing too quickly. It is closely related to F.23 and F.20, and the three together describe one problem observed at three different points. Rather than measuring the gap between flow and return, or waiting for an absolute temperature limit, F.24 watches the rate of change. When the burner fires into a properly circulating system the temperature climbs at a predictable, fairly gentle rate because there is a large volume of moving water to warm up.

When circulation has failed there is only the small volume sitting in the heat exchanger, and its temperature shoots up almost immediately. The appliance can detect that within seconds, well before anything reaches a dangerous level. That makes F.24 genuinely useful: it is an early warning. An appliance showing F.24 has a flow problem that has not yet become an overheat, and dealing with it now is materially cheaper than dealing with it after several weeks of resets.

1. 1

   Check the pressure is 1.0 to 1.5 bar cold and top up if not.

2. 2

   Open all thermostatic radiator valves fully. If the fault appears mostly in mild weather when the house is already warm, this alone may be the whole answer.

3. 3

   Bleed the radiators from the ground floor upwards, finishing with the highest, and repressurise afterwards.

4. 4

   Listen at the appliance during the first thirty seconds of a call for heat. A healthy pump gives a steady low hum; silence or a buzz without the hum points squarely at the pump.

5. 5

   Reset once. If F.24 persists, book an engineer while it is still an early-stage circulation fault rather than an overheat.