How to Top Up Boiler Pressure

Your heating system is a sealed loop of water under pressure — normally between 1.0 and 1.5 bar when cold. Over time, small amounts of water are lost through evaporation, tiny leaks, or when you bleed radiators, and the pressure falls. When it drops below about 0.6 bar, most boilers lock out for safety and won't fire. Topping it back up restores normal operation.

A one-off top-up is normal. But if you're repressurising every few days or weeks, there's a leak somewhere — and it's worth finding before it causes damage:

• • A tiny leak on a radiator valve or joint (often hidden under a floor or behind a wall).
• • A faulty pressure relief valve (PRV) discharging water outside through the condensate or a copper pipe.
• • A leaking heat exchanger inside the boiler.
• • Air in the system — bleeding radiators lowers pressure, so top up afterwards.
• • An expansion vessel that's lost its charge, causing pressure to rise high then drop to zero.

• • 1. Read the gauge — note the current pressure. If it's between 1.0 and 1.5 bar, don't top up; it's already healthy.
• • 2. Find the filling loop — a braided silver hose connecting two pipework points, usually underneath or beside the boiler. It has one or two small tap handles (or screwdriver-slot valves).
• • 3. Make sure both filling-loop valves are fully closed before you start, so you don't overshoot.
• • 4. Slowly open both valves. You'll hear water hissing in and see the gauge rise.
• • 5. Watch the gauge and stop at 1.2 bar (between 1.0 and 1.5 bar). Never go above 2.0 bar — if you overshoot, bleed a radiator briefly to release pressure.
• • 6. Close both valves fully. A leaking filling loop is a common cause of pressure creeping too high, so make sure they're shut.
• • 7. Reset the boiler if it had locked out on low pressure. It should now fire normally.

Topping up via the filling loop is safe and intended for the homeowner. But if pressure rises above 2.5–3.0 bar on its own, or the PRV is dripping outside, don't keep bleeding off pressure — the expansion vessel or PRV needs an engineer. And never remove the boiler casing to 'find' the filling loop; if you genuinely can't locate it, book an engineer to show you.

• • Pressure drops again within a few days of topping up — there's a leak to find.
• • Pressure rises above 3 bar when the heating is on — the expansion vessel may need recharging.
• • Water is dripping from the copper PRV discharge pipe outside — the relief valve may be passing.
• • The gauge reads zero and the filling loop won't raise it — the loop may be blocked or disconnected.
• • You can't locate the filling loop or your boiler uses a keyed internal filling link.

If topping up doesn't hold, our Gas Safe Registered (594547) engineers find the cause — a hidden leak, a passing PRV, or a flat expansion vessel — and fix it properly across Surrey, Berkshire, Buckinghamshire, and West London. We can also show you exactly how your filling loop works on the first visit so you're confident doing it yourself next time.