Potterton [E117] fault codewhat it means and how to fix it

E117 is the opposite of the fault everybody looks up: pressure too high rather than too low. Potterton set the trip at around 2.9 bar. Two things cause it. The trivial one is overfilling — the last part of the range fills far faster than people expect and it is easy to overshoot, or to leave a filling loop valve fractionally open so the system slowly equalises with mains pressure. The significant one is a failed expansion vessel. Water expands by roughly four per cent between cold and full operating temperature, and the vessel exists to absorb that. Without it, pressure climbs steeply every time the heating runs. The diagnostic is simple and you can do it yourself: note the pressure cold, run the heating for half an hour, and note it again. A rise of a couple of tenths is normal. A rise of a bar or more means the vessel. This is also, quietly, the explanation for a great many low-pressure complaints — a system that over-pressurises and discharges through the safety valve every cycle ends up reading low once it cools.

1. 1

   Look underneath the appliance for a filling key still in place, and for lever valves sitting along the hose rather than across it. This accounts for more E117s than everything else combined.

2. 2

   Take two readings and write them down: one with everything stone cold, one after the heating has run for half an hour. The gap between them is the whole diagnosis.

3. 3

   A gap of two or three tenths of a bar is healthy. A gap approaching a full bar means the expansion vessel is no longer absorbing anything and needs an engineer.

4. 4

   If the cold figure alone is too high, let a little out of a radiator bleed screw into a jug until it settles at 1.2 bar.

   Use a radiator, not the safety valve. Lifting a safety valve by hand tends to leave grit on the seat, after which it weeps permanently and needs replacing — you would have converted a free job into a paid one.