The short answer
on most combi and system boilers, the gauge should read between 1 and 1.5 bar when the heating is off, rising a little when it's on. If it's below 1, you can usually top it up yourself. If it keeps dropping, something needs fixing.
Always check your own boiler's manual. The right pressure is often marked in green on the gauge.
What's normal.
| Gauge reading | What it means |
|---|---|
| Below 1 bar | Too low. The boiler may stop working or lock out. |
| 1 to 1.5 bar (heating off) | Normal for most boilers. |
| Up to about 2 bar (heating on) | Normal. Pressure rises as the water heats. |
| Above 2.5 bar | Too high. |
A small rise and fall between hot and cold is fine. A steady drop over days or weeks isn't.
Why pressure drops.
- You've bled the radiators. Letting air out lowers the pressure. Top it up once and it should hold.
- A small leak. Look for damp patches or green or white crust around radiator valves, pipe joints and under the boiler.
- The pressure relief valve. If it's letting by, you may see a drip from the small copper pipe on the outside wall behind the boiler.
- The expansion vessel. If this has lost its charge, pressure will swing high when hot and low when cold.
How to top it up.
Most boilers are topped up using a filling loop: a short braided hose, or a built-in lever or key, under the boiler.
- Turn the boiler off and let the system cool.
- Find the filling loop. Check both ends are attached.
- Open the valve slowly. You'll hear water running.
- Watch the gauge. Stop at 1 to 1.5 bar.
- Close the valve fully. If it's a removable hose, close both valves.
- Turn the boiler back on. Reset it if the manual says to.
Boilers differ, so follow your manual. If you can't find a filling loop or you're not sure, leave it and ask an engineer.
If the pressure is too high.
Bleeding a radiator will bring it down a little. If the pressure keeps climbing on its own, the filling loop may not be fully closed, or there's a fault. That needs an engineer.
When to call someone.
- You're topping up more than once or twice a year.
- You can see a leak, or water dripping from the boiler.
- The pressure rises above 2.5 bar when the heating is on.
- The boiler keeps locking out.
Never take the boiler's casing off or try to repair it yourself. Only a Gas Safe registered engineer should work on a gas boiler.
If you smell gas, open windows, don't use switches or flames, leave the house and call the National Gas Emergency Service on 0800 111 999.
When to call FixAble.
Boiler repairs come under plumbing and heating, which is part of every FixAble plan. Labour is included, with up to £250 of materials on every job, and we book your appointment within 24 hours. Every plan includes an annual boiler service too: it is included on every plan.
Full boiler replacements aren't included. See what's not included.
FixAble works across Greater Manchester and surrounding areas, within 30 miles of Ancoats.
