Is your boiler constantly flashing a low-pressure fault code? Having to top up the system via the filling loop every couple of days is not normal. While your first thought might be a leak under the floor, the truth is often hidden inside the boiler itself.
The most common cause: A flat expansion vessel
When the water in your central heating system heats up, it expands. Your boiler contains an expansion vessel—a tank with a rubber diaphragm and a pocket of air—designed to absorb this extra volume. Over time, that air charge can slowly bleed out.
When the vessel goes flat, the heated water has nowhere to expand into. The pressure spikes rapidly, forcing a safety valve to open and dump water outside. When the boiler cools down again, the pressure plummets to zero.
Why you shouldn't just keep topping it up
Constantly adding fresh tap water into the system is a recipe for disaster. Every time you top it up, you're diluting the central heating inhibitor chemical. This causes the inside of your radiators to rust, creating thick black iron oxide sludge that will eventually block the boiler's heat exchanger.
Getting it fixed
To fix this, a Gas Safe engineer will simply attach a pump to the vessel and recharge it with air. It usually takes less than an hour, with average repair costs sitting around £80-£120. Don't ignore it—a ruined heat exchanger means buying a whole new boiler!