Boiler Losing Pressure? What’s Normal and When to Worry

What should my boiler pressure be?

Most combi and system boilers have a pressure gauge on the front or on the display. With the heating off and the system cold, it should normally read between 1 and 1.5 bar. It will rise a little when the heating is on, because water expands as it warms. Your boiler’s user manual gives the exact range for your model.

If the pressure drops too low, most modern boilers lock out and show a fault code. That’s a safety feature, not a breakdown.

Common reasons pressure drops

• You’ve bled the radiators. Letting air out also lets out a little water, so the pressure falls. Topping up afterwards is normal.
• A small leak. Look for drips or damp patches under the boiler and around radiator valves and visible pipework.
• A worn or faulty component inside the boiler or system, such as the pressure relief valve or the expansion vessel. These need an engineer to diagnose.

How to top up the pressure

Many boilers can be re-pressurised using the filling loop, usually a braided hose or a lever under the boiler. Vaillant’s own guidance is that the safest option is to have an engineer do it, so only try this if you’re comfortable and your manual covers it:

1. Turn the boiler off and let the system cool.
2. Find the filling loop and check both ends are connected, if it’s the detachable type.
3. Slowly open the valve or valves and watch the gauge rise.
4. Close the valves when it reaches about 1 to 1.5 bar.
5. Remove the filling loop if it’s designed to be removed, turn the boiler back on and reset it if needed.
6. Check the gauge again once the heating has warmed up.

Never take the boiler’s casing off. Anything inside it is work for a Gas Safe registered engineer.

What if the pressure is too high?

Overfilling while topping up is the usual cause. Vaillant advises that if the gauge goes above 2.5 bar you can bleed a radiator to let some water out and bring it back down. If the pressure climbs high on its own without anyone touching the filling loop, something isn’t working as it should, so get it checked.

When to call an engineer

Book a repair if you find yourself topping up regularly, if you can see water leaking, if the pressure drops again soon after a top-up, or if the boiler still locks out after re-pressurising. Repeated pressure loss is a symptom of something else, and the longer it’s left the more it tends to cost.

We diagnose and repair heating faults across Bromley, South London and North Kent. See our heating & plumbing repairs page, or our Prices page for call-out and hourly rates.