How to read your boiler's pressure gauge, why pressure drops or rises, how to repressurise a sealed system using the filling loop, and when to call a Gas Safe engineer.
Combi and system boilers run on a sealed heating system that needs the right water pressure to work. If pressure falls too low, the boiler may lock out and stop heating; if it rises too high, a safety valve releases water. This guide explains how to read the gauge and what you can safely do yourself.
Reading the gauge
Most boilers have a dial or digital display showing pressure in bar. Many manufacturers recommend around 1 to 1.5 bar when the system is cold, rising a little when the heating is on. Your boiler's manual gives the exact range, and many dials have a green zone. Below about 1 bar, many boilers show a fault code or stop working.
If you have a regular (heat-only) boiler with a feed tank in the loft, it is likely to be an open-vented system and does not need repressurising.
Why pressure drops?
- Bleeding radiators, which releases water.
- A leak somewhere in the system: check radiator valves, pipe joints, under the boiler and for damp patches.
- The pressure relief valve letting water out, often through a pipe going outside.
- A faulty expansion vessel inside the boiler, which can cause pressure to swing high when hot and low when cold.
How do you repressurise: step by step?
Always follow your boiler's manual, as filling systems differ. A typical external filling loop works like this:
- Turn the boiler off and let the system cool.
- Find the filling loop, usually a braided flexible hose with a valve at each end, under the boiler. Some boilers have a built-in filling key or lever instead.
- Make sure the hose is connected at both ends.
- Open both valves slowly. You will hear water flowing in. Watch the gauge.
- Close both valves when the gauge reaches the level your manual recommends, typically about 1 to 1.5 bar.
- Disconnect the filling loop if it is the detachable type, as the regulations expect, catching any drips.
- Turn the boiler back on and reset it if it shows a fault, following the manual.
If you overfill, you can reduce pressure by bleeding a radiator a little.
If pressure is too high
High pressure (often above about 2.5 to 3 bar) can mean the system was overfilled, the filling loop valves are letting water through, or the expansion vessel has lost its charge. Check the filling loop valves are fully closed. If pressure stays high, call a Gas Safe registered engineer. Do not tamper with the pressure relief valve.
If pressure keeps dropping
Needing to top up more than a couple of times a year suggests a leak or a faulty component. Look for:
- drips or corrosion at radiator valves and visible pipe joints;
- damp patches on ceilings or floors;
- water dripping from the condensate or pressure relief pipe outside.
Get the system checked. Small leaks can damage floors and ceilings, and repeated topping up brings in fresh oxygenated water that can cause corrosion.
What you should not do?
Do not remove the boiler casing or attempt repairs inside the boiler. In Great Britain, work on gas appliances must be carried out by a Gas Safe registered engineer. Check an engineer's ID card and registration on the Gas Safe Register website.
Keep your boiler healthy
- Have it serviced annually by a Gas Safe engineer, which many manufacturers require to keep the warranty valid.
- Check the pressure gauge every month or so.
- If you rent, your landlord must arrange a gas safety check every year.
- Fit a carbon monoxide alarm near gas appliances.
Frequently asked questions
Is low boiler pressure dangerous?
It is not usually dangerous, but the boiler may not work. Repeated drops need investigating.
Sources and further reading
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.