Boiler pressure that drops overnight is a common issue in UK homes, and it often looks more serious than it actually is. The pressure usually appears fine during the day, then by morning the boiler is sitting below its normal range.
In many cases, this happens because the system cools down fully overnight. As water cools, it contracts slightly, which can cause a small drop in pressure. In a sealed system that’s already sitting close to the lower limit, this change becomes noticeable by morning.
Another common reason is a minor leak that only shows itself when the system is cold. Joints, valves, and seals can contract as temperatures drop, allowing tiny amounts of water to escape. These leaks are often too slow to spot during the day but add up over several hours.
Pressure relief valves can also play a role. If pressure rises too high when the system is running, the valve may release a small amount of water. By the time everything cools down, the pressure sits lower than expected.
Topping the pressure up occasionally can be normal, but frequent drops usually mean something else is going on. Understanding how pressure behaves across a full heating cycle is part of keeping running costs down, which is covered more broadly in this guide to keeping a UK home warm for cheap.
If pressure loss is happening alongside heating cutting out or radiators behaving oddly, it’s worth checking how the whole system is running rather than treating pressure as a standalone issue.