Ariston [109] fault codewhat it means and how to fix it

This is the opposite problem to a 108 and it gets looked up far less often, which is a shame because the underlying cause is frequently more significant. Ariston stop the appliance when heating circuit pressure exceeds 3 bar. Two situations produce that. The straightforward one is overfilling: somebody topped up past the mark, or left a filling loop valve slightly open so the system has been slowly equalising with mains pressure ever since.

The more consequential one is a failed expansion vessel. That vessel exists to absorb the expansion of water as it heats — water expands by roughly four per cent between cold and full operating temperature, and without somewhere for that volume to go the pressure rises steeply every time the heating runs. A system with a dead vessel shows a normal cold reading and a very high hot one, which is the pattern to look for. Left alone it discharges through the safety valve on every cycle, which is why so many overpressure faults eventually present as low pressure instead.

1. 1

   Check both filling loop valves are fully closed. If it is a braided hose with lever valves, both levers should sit across the hose, not along it.

2. 2

   Note the pressure cold, then run the heating for half an hour and note it again. A rise of a few tenths of a bar is normal; a rise of a bar or more points firmly at the expansion vessel.

3. 3

   If it is simply overfilled, bleed a small amount out of a radiator into a container until the cold reading comes back to about 1.2 bar. Go slowly — it comes out faster than you expect.

   Bleed from a radiator, never from the safety valve. Lifting a safety valve manually often leaves debris on the seat and it then weeps permanently, which turns a five-minute job into a part replacement.
4. 4

   If the cold reading is correct but it still climbs past 3 bar when hot, stop and book an engineer to check the expansion vessel charge. That work requires the system to be drained and is not a householder job.