This often catches people out because the boiler seems fine during normal use. You top it up, the heating runs, and everything appears stable. Then the following morning, the pressure gauge is back where it started.
The most common cause is a small leak somewhere in the system. This doesn’t have to be visible. Micro-leaks at radiator valves, pipe joints, or under floorboards can lose just enough water to matter once the system cools. Expansion and contraction exaggerate the drop.
Another possibility is the expansion vessel losing its charge. When this happens, pressure rises too quickly during heating and falls too far once the boiler switches off. Over time, this creates a repeated overnight drop pattern.
If radiators also struggle to stay warm or heat inconsistently, pressure loss can affect circulation. That overlap is covered here: Why Your Boiler Fires Then Switches Off.
Where pressure loss becomes frequent, it often connects to this wider issue: Why Your Boiler Keeps Losing Pressure.
If you want a broader understanding of how pressure, circulation, and heat retention all interact, the main reference point is here: How to Keep a UK Home Warm for Cheap.
If you’re not sure where to start diagnosing it, the step-by-step logic is laid out here: House Cold Diagnostic Tool.