Boiler Losing Pressure? What It Means and How Urgent It Is
A sealed system does not consume water.
What the gauge should read
Cold, between 1 and 1.5 bar. Running, it will rise to somewhere between 1.5 and 2.5 bar as the water expands, and that is normal. Below about 0.8 bar most boilers lock out and you get no heat and no hot water. The number itself is less useful than how fast it falls.
How fast is it dropping?
Once a year is normal and probably just air being released. Once a month suggests a small leak or a tired expansion vessel. Every week, or every few days, means water is actively leaving the system and something will get damaged, that is worth a call now rather than in the spring.
The three usual causes
A leak on the pipework or a radiator tail, often under a floor and invisible. A failed expansion vessel, where the internal diaphragm has gone and the system cannot absorb expansion. Or a pressure relief valve passing, which discharges outside through a small copper pipe on the external wall.
Check the outside pipe
This is the check almost nobody makes. Find the small copper pipe coming through the wall near the boiler. If it is dripping, damp, or has a limescale trail beneath it, your pressure is going out of the building through the relief valve, and no amount of topping up will fix it.
How to repressurise safely
Boiler off and cold. Open both filling loop valves slowly and watch the gauge, it moves quickly. Close at 1.2 bar, then close both valves fully and, if it is a removable loop, disconnect it. Leaving a filling loop connected and slightly open is a mains-to-system cross-connection and it is a Water Regulations breach.
When to stop topping up and call
If you are repressurising more than once a month, adding water is treating the symptom. Every top-up introduces fresh oxygenated water, which corrodes radiators from the inside and produces the black sludge that eventually blocks the heat exchanger. The leak is cheaper to find than the damage is to undo.
What the fault code is really telling you, what's safe to try, and when to stop.
Five checks that fix a surprising share of no-hot-water mornings for free.
Valve, spindle, tail or the panel itself, each one leaks differently, and only one of them means a new radiator.
Immersions, multipoints and wall heaters fail in predictable ways. What to check before replacing anything.
Sediment makes cylinders rumble, slows heating and kills elements. Flushing it is routine maintenance.