Why Does My Boiler Keep Losing Pressure?

What normal pressure looks like

Most sealed central heating systems should sit between 1 and 1.5 bar when cold, with the needle on your pressure gauge in the marked green zone. It'll creep up a bit when the heating's running and settle back down once it cools off, and that's normal.

A slow drift down over several months, needing a top-up every now and then, is also fairly normal since every sealed system loses a small amount over time. What isn't normal is the pressure dropping to zero, or needing topping up every few days. That's the boiler telling you something's wrong.

The usual causes

In order of how often I actually find them on a callout:

• A pinhole leak somewhere in the pipework or a radiator, often small enough that you never see a puddle, just a damp patch, a bit of discolouration on a joint, or nothing visible at all if it's under a floor
• A leaking pressure relief valve, which normally drips outside through a small pipe near an external wall. If that pipe's dripping, the valve is losing water and pressure with it
• Radiators bled without repressurising afterwards. Bleeding a radiator releases air, but it also lets a bit of water and pressure out, and it's easy to forget to top back up
• A faulty expansion vessel, the part that absorbs pressure changes as water heats and cools. When it fails, the boiler can't regulate pressure properly and it drops or spikes
• A loose fitting or joint, sometimes one that was never quite right from installation, working itself looser over months or years
• The filling loop not fully closed after last being used. A slow weep here is easy to miss

How to top it up safely

If the pressure's low and there's no visible leak, topping up is straightforward on most systems. Turn the boiler off. Connect the filling loop (usually a flexible braided hose under the boiler, sometimes it's a lever instead) at both ends. Slowly open the valve or lever until the gauge reads around 1.3 bar, then close it again at both ends. Turn the boiler back on.

The key word is slowly. Overshoot the pressure and you'll need to bleed a radiator to bring it back down, which just adds another job. If you're not sure which valves are which, check the boiler's manual or give me a call and I'll talk you through it over the phone.

When to stop topping up and call someone

If you've topped up once and it holds for months, that's nothing to worry about. If you're doing it every week, or the pressure drops noticeably within a day or two, stop topping up and get it looked at. Repeatedly filling a system that's actively leaking can mask a bigger problem and, in some cases, cause its own damage.

Other signs worth mentioning when you call: damp patches near pipework or under the boiler, a dripping pressure relief pipe outside, or kettling and banging noises alongside the pressure loss. All of those point to something specific an engineer can usually diagnose quickly rather than guess at.

Finding a small pinhole leak in buried pipework can take a bit of detective work, but it's rarely as drastic or expensive as people expect. Most repeat pressure-loss callouts are sorted in a single visit.

Common questions