Why is my boiler losing pressure?

A sealed heating system should sit at roughly 1 to 1.5 bar when cold, rising to somewhere under 2.5 bar when hot. If you're reaching for the filling loop every week or two, water is either escaping the system or the system has lost its ability to absorb expansion. Neither fixes itself.

What the gauge is telling you

• Slow, steady drop over weeks — almost always a small leak somewhere in the system.
• Drop straight after bleeding radiators — normal, and needs one top-up.
• Pressure very high when hot, very low when cold — a failed expansion vessel.
• Drop only when the heating runs, plus a dripping outside pipe — the pressure relief valve is discharging.
• Sudden, large drop — a significant leak. Isolate and call an engineer.

The common causes in detail

A small leak. Radiator valves and their gland nuts, towel rail joints, pump unions, and buried pipework under floors are the usual culprits. Look for green or white staining on brass fittings, rust marks under valves, and damp or discoloured patches on floors and ceilings. A leak losing 0.2 bar a fortnight can be a few drops a day — small enough to evaporate before you see a puddle.

Recently bled radiators. Bleeding releases water as well as air, so pressure drops. Perfectly normal. Top back up to 1.2 bar and move on. If air keeps returning within weeks, something is drawing air in and that needs investigating.

A failed expansion vessel. The vessel contains a rubber diaphragm with an air charge behind it, which absorbs the expansion of heated water. When the charge is lost, there is nowhere for the expansion to go: pressure spikes when hot, forces water out of the relief valve, and reads low again when cold. Recharging the vessel is a straightforward engineer job; a perished diaphragm means a replacement vessel.

A weeping pressure relief valve. Check the small copper pipe that exits through an outside wall near the boiler. If it drips or shows lime staining, the PRV is passing — usually either because of the vessel fault above, or because grit has caught on the seat.

A leaking heat exchanger or diverter valve. Less common, more serious. Signs include damp inside the boiler casing, corrosion under the appliance, or steam from the flue that doesn't clear. This needs an engineer straight away.

Auto air vents. A stuck automatic air vent, often in the airing cupboard or loft, can weep continuously into an unnoticed corner.

How to top up safely

1. Turn the boiler off and let the system go cold.

2. Locate the filling loop — a braided flexible hose with a valve at each end, or an internal keyed filling device.

3. Open both valves slowly. You'll hear water flowing.

4. Watch the gauge and close the valves at 1.2 bar.

5. Close both valves fully, and if it's a flexible loop, disconnect one end.

6. Restart the boiler and bleed any radiators that need it, then re-check the pressure.

Never leave the filling loop open. An open loop can over-pressurise the system, dump water through the relief valve, and in a sealed system it is also a cross-contamination risk.

Why repeated topping up is bad for the system

Fresh mains water carries dissolved oxygen. Every top-up feeds the corrosion reaction that produces magnetite sludge, which settles in radiator bottoms and heat exchangers, blocks flow and shortens the boiler's life. It also dilutes the corrosion inhibitor. Topping up monthly is not maintenance — it's a slow way of wrecking a heating system.

When to call us

Book a visit if:

• You're topping up more than once a month
• The relief pipe outside drips after the heating runs
• Pressure swings from near zero cold to over 2.5 bar hot
• There's any damp, staining or corrosion near pipework or under the boiler
• The boiler locks out on low pressure repeatedly

We trace leaks with pressure testing and thermal imaging where pipework is buried, so we can find the fault without lifting an entire floor. Chasing a small leak early costs a fraction of replacing a corroded heat exchanger later — and our care plans include the callout.