Boiler Losing Pressure? What It Means and How Urgent It Is

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.

The expansion vessel case, which people miss

If there is no damp patch anywhere and the pressure still falls, suspect the expansion vessel before tearing up floors. The vessel is a sealed chamber with a rubber diaphragm and a charge of air behind it, and its job is to absorb the expansion of water as the system heats. When the charge is lost, heated water has nowhere to expand into, so pressure spikes well above 2.5 bar, the pressure relief valve opens, and water discharges through a copper pipe outside. The system then cools and reads low. Nothing leaked indoors, the water left through the wall. Checking the discharge pipe outside for dripping or limescale staining takes thirty seconds and settles it.

Tracing a genuine leak on the heating circuit

Real leaks on a sealed system are usually undramatic. Radiator valve glands weep just enough to evaporate before it puddles, leaving a faint white or rusty crust on the valve body. Towel rails in bathrooms are a frequent culprit for the same reason. Pipework buried in a solid floor is the harder case, and it shows up as a warm patch underfoot rather than visible water. If nothing is visible anywhere, an engineer can isolate the boiler from the circuit overnight, if the pressure holds with the boiler isolated, the fault is on the pipework, and if it still drops, the fault is inside the appliance.

Why topping it up repeatedly makes things worse

Every top-up introduces fresh mains water, and fresh water carries dissolved oxygen. Oxygen is what drives the corrosion that produces magnetite sludge, which then blocks heat exchangers, pinholes radiators from the inside, and wrecks pumps. A system topped up weekly for a year has taken in a substantial volume of oxygenated water and diluted whatever inhibitor was in it. So the habit that seems to be managing the problem is quietly creating a second, more expensive one.

What a proper repair looks like

Find the cause, fix it, then restore the system properly: refill to 1.0-1.5 bar cold, bleed the radiators, re-check the pressure, test the system water, and dose inhibitor to the correct concentration. If the water sample comes back dirty, a power flush and a magnetic filter belong in the same visit, otherwise the sludge already circulating carries on doing damage regardless of the leak being fixed. Ask for the inhibitor level to be recorded on the invoice; it is the number nobody checks and the one that predicts the next five years.

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