Where this starts
A sealed heating system is a closed loop at a set pressure. It should not need topping up, and a system that does is losing water somewhere or failing to accommodate the expansion of the water it has.
THE TIMING SEPARATES THE TWO FAMILIES. Watch the gauge through a heating cycle. If the pressure rises sharply as the system heats — sometimes far enough to discharge — and then sits lower than it started once cooled, the system has nowhere to put the expansion. That is the expansion vessel, and the usual cause is not a failed vessel but a lost gas charge behind its diaphragm.
If instead the pressure falls slowly and steadily regardless of whether the boiler has run, water is leaving the system. That can be a visible drip at a radiator valve, a joint, or the boiler itself, or an invisible one under a floor or in a wall.
The third thing people see is water discharging from a pipe outside the building. That is the pressure relief valve doing its job, and it is a symptom rather than a cause — either the pressure genuinely went too high (the vessel), or the valve seat has been damaged and now weeps below its setting.
WHY IT MATTERS BEYOND THE INCONVENIENCE: every refill introduces fresh, oxygenated, untreated water. Oxygen is what drives corrosion inside steel radiators, and corrosion produces the sludge that then blocks them. A system topped up weekly for a year is a system being slowly destroyed by the remedy.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Expansion vessel has lost its gas charge
Most commonPressure rises sharply while heating and sits low when cold — a big swing across the cycle. Often with discharge from the outside relief pipe when hot.
How to check: Watch the gauge cold, then at full temperature. A swing of more than about a bar across the cycle points here rather than at a leak.
A leak on the system
Most commonA slow steady fall that continues whether or not the heating has run, with no large swing across the cycle.
How to check: Note the pressure at the same time on consecutive days with the heating off. A fall with the system idle is water leaving it.
A weeping pressure relief valve
CommonWater or lime staining at the discharge pipe outside, sometimes with the pipe damp long after the system has cooled.
How to check: Feel or look at the outside discharge pipe. A wet or stained pipe means the valve has passed, whether or not it should have.
A leaking radiator valve or joint
CommonA steady fall with a visible damp patch, a stain on a floor, or corrosion at a valve — often small enough to evaporate before it pools.
How to check: Run a dry tissue around every valve, joint and the boiler's own pipework. A small weep shows on paper before it shows on the floor.
A leak under a floor or in a wall
Less commonA steady fall with nothing visible anywhere accessible, sometimes with a warm patch on a floor or unexplained damp.
How to check: Whether any area of floor is warmer than its surroundings with the heating on. A buried leak often shows as heat before it shows as water.
Air being released after a refill
Less commonA one-off fall over days after the system was filled or drained, settling once the air has been bled out and not recurring.
How to check: Whether it follows recent work. A single fall after filling is the system settling, not a fault.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It is the whole split between the two main families and costs one heating cycle. A large swing is expansion; a flat profile with a slow decline is a leak.
- 2
It separates a fall caused by the heating cycle from a fall that happens regardless, which is the difference between a vessel and a leak.
- 3
A wet or stained pipe tells you the system has been relieving pressure, which points at the vessel — and a pipe that drips when the system is cold points at the valve itself.
- 4
Small weeps evaporate before they pool and are invisible to the eye. Paper finds them, and most system leaks are at a valve rather than in a pipe.
- 5
Each refill adds oxygenated water, which corrodes the system from inside and creates the sludge that blocks radiators later. The remedy is doing damage.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Anything involving the boiler, its controls, its flue or the gas supply — in most jurisdictions that is restricted work, and it is not a DIY diagnosis in any case.
- Where the pressure is dropping enough to need topping up more than very occasionally, since the repeated refilling does more harm than the pressure loss.
- Where a leak cannot be found among accessible pipework, which means it is buried and needs tracing rather than searching.
- Where the relief valve is discharging regularly, because either the system is genuinely over-pressurising or the valve has failed, and both need resolving rather than watching.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.