Do this first
- Switch the boiler off and let it cool. Repressurising a hot system gives a false reading, because the water expands as it heats.
- Find the filling loop. On most combis it is a braided silver hose under the boiler with a valve at each end. On some Worcester and Ideal models it is a built-in key or a plastic lever behind the front panel instead.
- Open both valves slowly. You will hear water moving. Watch the pressure gauge, not the clock.
- Stop at 1.0-1.5 bar cold. The green band on the gauge is the manufacturer's target; 1.2 bar is a safe middle for most systems.
- Close both valves fully and detach the loop if yours is removable. A loop left open lets mains pressure creep into the system.
- Restart the boiler. Most clear a low-pressure lockout on their own; some need the reset button held for a few seconds.
If you overshoot, bleed a radiator briefly to drop back into the band.
The question that actually matters: how often?
A sealed heating system is closed. In normal use it should not lose water at all. A small drop over a year is ordinary, and a drop straight after radiators have been bled is expected.
Anything beyond that is a leak, and there are only so many places it can be going:
| How often you top up | What it usually means |
|---|
| Once or twice a year | Normal. No action. |
| Every month or two | A weeping joint, valve or towel rail, or a failed expansion vessel. Worth investigating. |
| Weekly | An active leak. Usually under a floor, behind a wall, or inside the boiler case. |
| Daily, or it will not hold at all | A significant leak, or a failed pressure relief valve dumping to the outside discharge pipe. |
The reason this matters more than the pressure reading: a system that needs topping up weekly is putting fresh, oxygenated mains water into the pipework every week. That accelerates internal corrosion, which creates more pinholes, which means more topping up. It is a loop that gets worse, and the water has to be going somewhere in the meantime.
Where the water is actually going
Somewhere you can see
Check, in this order: the pressure relief discharge pipe outside (a drip after the boiler fires means the PRV or expansion vessel has failed), under and behind the boiler, every radiator valve spindle and the nut where the tail meets the pipe, towel rails in bathrooms, and the underside of any radiator where a rust bloom has formed.
Somewhere you cannot
If all of that is dry, the loss is under a floor, in a wall, or inside the boiler. A pinhole in a 15mm pipe under a screed floor can lose enough to drop the gauge weekly while producing no visible damp for months, because the screed absorbs it and the heating dries it. By the time a floor lifts or a ceiling stains below, the leak has usually been running a long time.
The expansion vessel, which is not a leak
One case looks exactly like a leak and is not. If the pressure climbs high when the heating runs and drops below normal when it cools, the expansion vessel has probably lost its air charge. The system is not losing water; it has nowhere to put the expansion. That is a recharge or a replacement vessel, not a detection job, and it is worth ruling out before anyone lifts a floor.
When to stop topping up and call someone
- You are refilling weekly or more often.
- The gauge will not hold pressure for a full heating cycle.
- There is a warm or damp patch on a floor, or an unexplained warm spot underfoot.
- The discharge pipe outside drips whenever the boiler fires.
- Your boiler shows a pressure fault code that returns after every reset.
What a detection visit does that guessing does not: the system is isolated, pressure-tested to confirm the loss is real and to size it, then the run is traced with thermal imaging and, on a drained circuit, tracer gas. That locates the point of loss before anything is opened, which is the difference between lifting one board and lifting a floor.
What it costs
Repressurising is free and takes five minutes. Finding a leak that keeps emptying the system is charged at our hourly rate, and if we do not find it you do not pay for the time we spent looking. Most heating losses are located inside two hours.
