Low pressure is one of the most common reasons a boiler locks out. The good news: in many cases you can restore it yourself in a few minutes.
What is the right pressure for a boiler?
With the system cold, the gauge should generally read between 1 and 1.5 bar. When heating, the pressure rises a little (up to about 2 bar), which is normal. Below roughly 0.8 bar, most boilers lock out and show a fault code (F22 on a Vaillant, for example). Refer to your appliance manual for the exact value.
Topping up the pressure, step by step
- Switch off the boiler and let the system cool down.
- Find the filling tap (or filling loop), usually under the boiler.
- Open it slowly while watching the gauge: you will hear water flowing in.
- Close it as soon as the needle reaches about 1.5 bar.
- Bleed the radiators if necessary, then check the pressure again.
- Switch the boiler back on.
Do not exceed the recommended value: too much pressure (above 3 bar) opens the safety valve, which then starts dripping.
Why does boiler pressure drop?
- Recent bleeding of the radiators: letting air out lowers the pressure. That is normal.
- A small leak on a radiator, fitting or valve. Look for traces of water, rust or limescale.
- The expansion vessel has lost its charge or is punctured: pressure rises sharply when heating, then falls very low when cold.
- The safety valve dripping into the drain.
- The heat exchanger leaking, on older appliances.
What if the pressure is too high?
Pressure climbing above 2.5 to 3 bar when heating usually means an expansion vessel that needs recharging or replacing, or a filling tap that no longer closes properly. Bleeding a radiator brings the pressure down temporarily, but the cause must be dealt with by a technician.
Why not simply top up every time?
Every top-up brings in fresh water, full of oxygen and limescale. In the long run this causes corrosion and sludge in the heating circuit, which damage the pump and the heat exchanger. It is better to treat the cause: our technicians look for it during a repair or the annual service.
Frequently asked questions
What pressure should a boiler show?
Between 1 and 1.5 bar when cold for most wall-hung boilers. When heating, the pressure may rise to about 2 bar.
Why is my boiler losing pressure?
The most common causes are recent bleeding, a small leak in the system, an expansion vessel that has lost its charge or a dripping safety valve.
Is it dangerous to top up the boiler with water?
No, as long as you open the filling tap slowly and close it at about 1.5 bar. Having to do it often means there is a leak to repair.