Boiler pressure moves around slightly as a system heats up and cools down, and that’s normal. What isn’t normal is pressure that steadily drops when the heating is on, or a pattern where you top it up, run the heating, and then find it has fallen again.
Pressure loss nearly always means water is leaving the sealed system somewhere, or the system is behaving badly as it expands when hot. The reason people get stuck is that the leak or discharge isn’t always obvious, especially if it only happens when the system is warm and under higher pressure.
If you want a structured route based on what you’re actually seeing at home, it’s usually quickest to start with the house cold diagnostic and follow the branch that matches your symptoms rather than trying random fixes.
In a typical UK sealed system, pressure rises as water heats and expands. The expansion vessel is meant to absorb that change. If it isn’t doing its job, pressure can climb during a heating run and the pressure relief valve can discharge water to protect the system. From the homeowner’s point of view it looks like “pressure drops when heating is on”, because water is being released during the cycle.
The awkward bit is that you might not notice it. The discharge can route outside via a small copper pipe and only drip when the system is hot. If the ground is damp near that outlet, or you can see signs of intermittent dripping, that’s a strong clue.
Small leaks can also open up only when hot. A joint that looks fine cold can seep when the system is warm, and a radiator valve can weep just enough to lose pressure without leaving an obvious puddle. That’s why people often say “there’s no leak” even when the gauge proves otherwise.
Pressure problems often show up around bleeding and refilling, because that’s when air gets moved around and the system is disturbed. If the pressure issue started after bleeding radiators, it’s worth reading radiator not heating after bleeding, because the same chain of events can leave the system underperforming and confusing to diagnose.
If the pressure problem began after topping up or any refill, it can also come with odd radiator behaviour that wasn’t there before. The symptom in radiator cold after system refill is a good comparison point because it captures what homeowners often notice straight after a system change.
One important practical point is that repeatedly topping up isn’t neutral. Fresh water introduces oxygen and minerals, which can speed up corrosion and sludge formation over time. The goal isn’t to keep topping up and hope it settles. The goal is to stop the system losing water.
If you want the broader “where to put effort first” view for typical UK homes, the complete warm home guide helps you avoid chasing one number on the boiler while missing the reason the house still feels cold.
Pressure loss can be simple or it can be technical, and there’s a clear safety line. If you suspect the system is discharging, or you’re topping up regularly and can’t trace why, it’s usually worth involving a qualified engineer rather than running the system in a cycle of top-ups while the underlying fault gets worse.