Why does combi boiler pressure drop, and what should you do?

What should the pressure be?

Look at the gauge on the front of the boiler while it is cold. On most boilers, the reading should be between 1 and 1.5 bar. As the boiler runs and heats up, the water expands and the pressure rises a little; it is normal for it to reach around 2 bar. If the pressure gets close to 3 bar, the pressure relief valve on the boiler opens and lets out water, which is a sign of a problem.

Why does the pressure drop?

• Bleeding radiators: When you bleed the radiators, some water also comes out of the system, so the pressure drops. This is normal.
• Water leak: A small leak at the radiator valves, the fittings or the pipes inside the walls lowers the pressure little by little. Look for stains under the radiators, drips under the boiler or damp on the walls.
• Expansion vessel: When the air in this vessel inside the boiler runs low, the pressure rises too high as the boiler heats up, the pressure relief valve lets out water, and then the pressure drops when the boiler cools down. This is the most common cause of pressure that keeps rising and falling.
• Pressure relief valve: If the valve does not close fully, it keeps letting out a small amount of water. You will see traces of water at the end of the valve pipe under the boiler.
• Leak inside the boiler: There may be a leak in the heat exchanger or the internal connections. Only a technician can see this, by opening the cover.

How do you add water?

1. Switch off the boiler and wait for it to cool down.
2. Find the filling tap (filling loop). It is usually a small tap or valve under the boiler; the user manual shows where it is.
3. Open the tap slowly and watch the gauge. When the gauge shows between 1 and 1.5 bar, close the tap firmly.
4. Switch the boiler back on. If a warning is still on the display, press the reset button once.

If you have added too much, you can let out a little water by slowly opening the bleed valve on a radiator, with a container underneath.

When should you contact a technician?

• If you need to add water more than once a week,
• if the pressure rises to 2.5–3 bar when the boiler heats up and then drops,
• if you see water under the boiler or in the pressure relief valve pipe,
• if the pressure drops again right after you add water.

Adding water again and again does not solve the problem; the fresh water entering the system also builds up limescale and sludge over time. The leak needs to be found and fixed.

Dereli Teknik · 25 September 2026