Almost every sealed heating system has a small pressure gauge on or near the boiler, and almost every boiler will refuse to fire if that gauge falls far enough. It is the single most frequent cause of a heating system stopping, it is usually a five-minute job to put right, and it is worth knowing how before a cold evening rather than during one.
It is also the fault most commonly mishandled — not because people repressurise badly, but because they repressurise repeatedly. Topping up is the fix for a one-off drop. Doing it every week is not a fix at all, and this guide explains what it is hiding.
What the gauge should read
With the heating off and the system cold, most domestic boilers want somewhere between one and one and a half bar. Your manufacturer's figure is on the appliance or in its manual and takes precedence over any general advice, including this. Many gauges have a green band marked on them, and cold pressure sitting in that band is what you are aiming for.
The reading will move during normal operation, and that is not a fault. Water expands as it heats, so the pressure rises when the heating is running — often to two bar or a little above — and falls back as the system cools. What matters is where it sits cold. Below about half a bar most boilers lock out and display a low pressure fault code.
How to repressurise it safely
The filling loop is what puts water in. It is usually a braided silver hose with a valve at each end, sitting under the boiler, or on newer appliances a built-in key or lever behind a flap on the underside. Turn the boiler off first and let the system cool, so you are setting the cold pressure rather than a hot one.
Open the valves slowly and you will hear water flowing. Watch the gauge rather than the clock, and close the valves as soon as it reaches the middle of the target range. It rises faster than people expect and overshooting is easy. Then close both valves fully and, if the loop is a detachable hose, disconnect it — a filling loop left connected can allow mains water to creep into the heating system, which slowly raises the pressure and can contaminate the system water.
If you overshoot, bleed a small amount out of a radiator bleed valve with a cloth and a container underneath until the gauge comes back down. Then restart the boiler. If it had locked out, most appliances need a single reset once the pressure is back in range.
- Boiler off and system cold before you start
- Open the filling loop slowly and watch the gauge, not the clock
- Close both valves fully, and disconnect a detachable loop afterwards
- One reset if the boiler had locked out — not repeated resets
The part that matters: why it dropped
A sealed system is closed. In principle the same water goes round for years, so pressure should be stable and a fall means water has left. Where it went is the actual question, and there are four usual answers.
A radiator or valve is weeping somewhere — sometimes visible as a small stain or a rusty streak under a valve, sometimes hidden under a floor. A joint under floorboards is leaking slowly enough to evaporate without ever showing a puddle. The pressure relief valve is passing, which usually shows as an occasional drip from a small copper pipe outside the house. Or the expansion vessel has lost its air charge.
That last one is the most commonly missed. The expansion vessel is a sealed container with a rubber diaphragm and a cushion of air behind it, and its job is to absorb the expansion of the water as it heats. When the air charge leaks away over the years the cushion is gone, so pressure climbs sharply when the heating runs, the relief valve opens to protect the system, water goes out through the discharge pipe, and the pressure is low again once everything cools. The signature is exactly that: a big swing between cold and hot readings, and a slow loss over days. Recharging or replacing the vessel is the fix, and no amount of topping up will substitute for it.
Why repeated topping up quietly damages the system
Every time you refill, you introduce fresh mains water, and fresh water carries dissolved oxygen. Oxygen plus steel radiators is corrosion, and corrosion is magnetite — the black sludge that settles in radiator bottoms, blocks microbore pipework and eventually clogs a heat exchanger. Refilling also dilutes the corrosion inhibitor that was dosed into the system to prevent exactly that.
So a system topped up weekly for a year is not merely inconvenient. It is being actively degraded by the remedy, and the bill at the end of that process is a power flush and sometimes a heat exchanger. If you are topping up more than about twice a year, the leak is worth finding — it is nearly always cheaper than what the topping up is doing.