Error 108 or 1P4: low heating pressure

What it means

The heating circuit is a closed loop: the water goes to the radiators or underfloor heating and back to the boiler. It needs a minimum pressure to work. With the heating cold, the pressure gauge usually reads between 1 and 1.5 bar.

When the pressure falls below the minimum, the boiler stops so that the pump and the heat exchanger don't run without enough water.

Most likely causes

1. Bleeding the radiators. Letting the air out lowers the pressure. That's to be expected: it just needs topping up.
2. A leak in the system: a radiator, a valve, a fitting or the underfloor heating. If you have to top up more than once or twice a year, there's most likely a leak.
3. The expansion vessel has lost its charge. Water expands as it heats up and the vessel absorbs that increase. Without its charge, the pressure rises sharply when the system is hot, the pressure relief valve discharges water and, once it cools, the pressure drops.
4. The pressure relief valve is dripping because of wear or limescale.

What you can check yourself

Without tools and without opening the unit.

1. Check the pressure gauge with the heating cold and note down the pressure. On many models it's also shown on the display.
2. Look for traces of water under the boiler, on the radiators and around the valves.
3. Watch the pressure with the heating running. If it rises to nearly 3 bar and then drops, tell us: it points to the expansion vessel.
4. Topping up is easy, but the filling valve must be closed properly afterwards. If it's left open, the circuit keeps filling and ends up discharging through the pressure relief valve: water on the floor.

Call us before you touch anything. If you want to top up yourself, we'll take a minute on the phone to explain where your model's filling valve is and how to close it properly. A water damage claim costs far more than a visit. And if you have to top up often, book an engineer: there's most likely a leak or the vessel has lost its charge.