Boiler pressure too low or too high: what to do?

Your boiler's pressure gauge is a key indicator of system health. Pressure that is too low or too high can prevent the boiler from working or, worse, damage components. This guide explains what to do safely, but always prioritise your safety: if you smell gas or see water leaking near electrical parts, treat it as an emergency.

What is happening?

Boiler pressure refers to the water pressure inside your central heating system. When the system is cold, most boilers should read between 1 and 1.5 bar. Pressure rises when the system heats up, which is normal, but it should not exceed the manufacturer's limit (often around 2.5-3 bar). Low pressure usually means water has been lost through a leak or recent bleeding of radiators. High pressure can be caused by overfilling, a faulty filling loop, or a problem with the expansion vessel.

Emergency warning

If you smell gas or suspect a gas leak, do not touch electrical switches, do not use a phone indoors, and call the gas emergency service immediately from outside or a neighbour's phone. In the UK, call the National Gas Emergency Service on 0800 111 999. In other countries, use your local gas emergency number. Also, if water is leaking near electrical components, turn off the power at the mains and call a professional.

What to do first

1. Check the pressure when the system is cold. Turn off the heating and wait for the system to cool. Read the gauge or digital display. Normal cold pressure is typically 1-1.5 bar.
2. If pressure is low (below 1 bar): Look for visible leaks around radiators, valves, and pipework. If no leak is found, you may need to top up the system using the filling loop (see below).
3. If pressure is high (above 2.5 bar when cold): Do not run the boiler. Bleed radiators to release pressure (see below) and check if the filling loop is letting water through.
4. If you are unsure or unable to safely perform these steps, contact a qualified heating engineer.

How to top up low pressure

Only attempt this if you are confident and have a filling loop (a flexible hose or built-in valve). Refer to your boiler manual for specific instructions. Typically:

• Ensure the boiler is off and cool.
• Connect the filling loop if it is detached (some are permanently connected).
• Open the valve(s) slowly and watch the pressure gauge.
• Stop when the pressure reaches 1-1.5 bar.
• Close the valve(s) firmly and disconnect the hose if applicable.

If pressure drops again soon after, there is likely a leak that needs professional attention.

How to reduce high pressure

High pressure can be relieved by bleeding radiators. This releases trapped air and slightly reduces system pressure. Use a radiator key to open the bleed valve at the top of each radiator until water dribbles out, then close it. Start with the radiator furthest from the boiler. If pressure remains high, the filling loop may be passing water or the expansion vessel may be faulty; call an engineer.

What NOT to do

• Do not add water to a hot system; always let it cool first.
• Do not overfill; stop at the recommended cold pressure.
• Do not ignore persistent pressure loss or rise; it indicates a fault.
• Do not attempt repairs on gas or electrical components yourself.

When to call a professional

Call a qualified heating engineer if: pressure keeps dropping, pressure remains high after bleeding, you see water leaks, the boiler locks out, or you are not comfortable performing any step. For gas-related work, ensure the engineer is Gas Safe registered (UK) or locally certified.

This guide is general information and not a substitute for professional advice. Always follow your boiler manufacturer's instructions and local regulations.