What the gauge should read
Most domestic systems sit at roughly 1 to 1.5 bar when the heating is cold, and rise a little when it is hot. Your boiler's manual will give the exact figure, and there is usually a marked band on the gauge itself.
A reading that climbs well above 2.5 bar when hot, then falls back low when cold, is a different fault from one that simply drifts down over weeks. The two point at different things, so it is worth watching the gauge cold and hot before you call anyone.
The three things that actually cause it
Almost every case comes down to one of these.
- A leak somewhere in the system. Often a radiator valve weeping, a joint behind a panel, or — very commonly in older housing with solid floors — buried pipework you cannot see. A leak big enough to drop the pressure can be small enough to evaporate before it ever shows as a wet patch.
- The pressure relief valve discharging. There is a small copper pipe running outside from the boiler. If it is wet or has limescale streaks down the wall below it, the valve is letting pressure out and the system is refilling the gap.
- A failed expansion vessel. The vessel absorbs the expansion as water heats. When it fails, pressure spikes hard when hot, forces water out through the relief valve, and reads low when cold. That is the classic high-when-hot, low-when-cold pattern.
Why topping it up repeatedly is a bad idea
Every time you refill, you introduce fresh, oxygenated mains water into a sealed system. Oxygen is what drives corrosion inside radiators and pipework, and corrosion is what produces the black sludge that blocks things up later.
Topping up once or twice a year after bleeding radiators is normal. Topping up monthly is the system telling you something is wrong, and doing it for a year quietly creates a second, more expensive problem on top of the first.
What an engineer will actually do
Pressure-test the system to confirm it is losing water, check the expansion vessel charge, inspect the relief valve, and trace the leak. Buried leaks under solid floors are traced rather than guessed at, and the repair is often a re-route rather than lifting a floor.
Common questions
Can I top the boiler up myself?
Topping up via the filling loop is a normal householder job and your boiler manual will show you how. What you should not do is treat it as the fix. If you are topping up more than a couple of times a year, get the cause found.
Is low boiler pressure dangerous?
Low pressure itself is not dangerous — the boiler will usually just lock out and stop heating. The concern is what is causing it: an unfound leak can damage floors and ceilings, and repeated refilling corrodes the system from the inside.
Why does the pressure only drop in winter?
Because the heating is running. A leak that barely moves the gauge in July shows up quickly when the system is hot, at pressure and cycling several times a day.
Related guides
Radiators cold at the bottom, hot at the top What is actually included in a boiler service? Your boiler stops every time it gets cold — here is why
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