Low boiler pressure is one of the most common reasons heating stops working. The good news is that it's often something you can check yourself. Here's what's normal, how to top up the pressure safely, and when it's a sign of a bigger problem.

What should my boiler pressure be?

Most modern sealed-system and combi boilers have a pressure gauge on the front. When the heating is off and the system is cold, the needle should normally sit at around 1 to 1.5 bar. When the heating is running, it will rise a little. Check your boiler's manual, as the recommended figure can vary slightly by model.

If the pressure drops below about 1 bar, many boilers will lock out and show a fault code until the pressure is restored.

Common causes of low pressure

  • Bleeding the radiators – letting air out also lets some water out, which lowers the pressure
  • A small leak – check under radiators, around valves and along visible pipework for damp patches, drips or rust marks
  • A faulty pressure relief valve – if the outside pipe that runs from the boiler through the wall is dripping, the valve may be letting water out
  • A failing expansion vessel – if the pressure jumps up high when the heating is on and then drops back down, the expansion vessel inside the boiler may need re-pressurising or replacing

How to top up your boiler pressure

Many boilers are topped up using a filling loop, usually a flexible silver hose or a built-in key or lever under the boiler. The exact method depends on your model, so always follow the manufacturer's instructions. In general:

  • Turn the boiler off and let the system cool
  • Open the filling loop valves slowly and watch the gauge
  • Close the valves when the needle reaches the recommended pressure, usually around 1–1.5 bar
  • Turn the boiler back on and reset it if it shows a fault

If you're not comfortable doing this, or you can't find the filling loop, call an engineer. Never open the boiler casing yourself – only a Registered Gas Installer should work inside a gas boiler.

When to call an engineer

Topping up once after bleeding radiators is normal. You should call an engineer if:

  • You have to top up the pressure more than once every few weeks
  • You can see water leaking from the boiler or pipework
  • The pressure climbs above 2.5–3 bar when the heating is on
  • The boiler keeps locking out even with the correct pressure

Constantly topping up hides the problem and can damage the boiler over time. A leak or a faulty part is usually a straightforward repair.

If you ever smell gas, don't touch the boiler. Leave the property and call the Gas Networks Ireland 24-hour emergency line on 1800 20 50 50.

GB Gas Services repair all major boiler brands across Dublin. Book a boiler repair or call us on 0872 855 624.