Why does a boiler lose pressure?

Why does pressure drop?

Boiler pressure can drop for several reasons:

Normal usage:

• Bleeding radiators - Releasing air from radiators also releases water, reducing pressure
• Small leaks - Tiny leaks in pipes, radiators, or valves can cause gradual pressure loss
• Evaporation - Very small amounts of water can evaporate from the system over time

Problems:

• Leaks in the system - More significant leaks in pipes, radiators, or the boiler itself
• Faulty pressure relief valve - A valve that's stuck open can release pressure
• Expansion vessel issues - Problems with the expansion vessel can cause pressure fluctuations
• Faulty pressure gauge - The gauge itself may be faulty, showing incorrect readings

How do I check the pressure?

To check your boiler pressure:

1. Look at the pressure gauge - Usually on the front of the boiler or on the control panel
2. Check the reading - It should show pressure in "bar" (typically 1-1.5 bar when cold)
3. Check when cold - Pressure is most accurate when the system is cold (not heating)

The gauge usually has a green zone (normal pressure) and a red zone (too high or too low). If the needle is in the red or below 0.5 bar, you need to repressurise.

How do I repressurise my boiler?

To repressurise your boiler:

1. Turn off the boiler - Switch it off and let it cool down
2. Find the filling loop - This is usually a flexible hose with two valves (or a key-operated valve)
3. Open the valves - Open both valves (or turn the key) to let water into the system
4. Watch the pressure gauge - As water enters, the pressure will rise
5. Stop at 1-1.5 bar - Close the valves when the pressure reaches the green zone
6. Turn the boiler back on - Restart the boiler and check it's working

Important: Only repressurise if you're confident doing so. If you're unsure, call a qualified engineer.

What if pressure keeps dropping?

If your boiler keeps losing pressure after repressurising:

Check for leaks:

• Look for water around radiators, pipes, and the boiler
• Check for damp patches on floors or walls
• Look under radiators for signs of leaks

Check the pressure relief valve:

• This valve (usually outside) may be dripping
• If it's constantly dripping, it may need replacing

Check the expansion vessel:

• This may need recharging or replacing
• A qualified engineer should check this

If you can't find a visible leak but pressure keeps dropping, you likely have a hidden leak that needs professional attention.

What if pressure is too high?

If your boiler pressure is too high (above 2 bar):

• Bleed a radiator - Release some water to reduce pressure
• Check the filling loop - Make sure the valves are fully closed
• Check the expansion vessel - This may need attention

High pressure can cause the pressure relief valve to open, releasing water. If this happens frequently, get an engineer to check it.

When should I call an engineer?

Call a qualified heating engineer if:

• Pressure drops repeatedly - After repressurising, it keeps falling
• You can't find the filling loop - Different boilers have different systems
• You're not confident - It's better to get professional help than risk damage
• There are visible leaks - These need to be fixed properly
• The boiler won't work - Even after repressurising

Regular boiler servicing can help identify and prevent pressure problems before they become serious.

Summary

• Boiler pressure should be between 1 and 1.5 bar for normal operation
• Pressure can drop due to bleeding radiators, leaks, or system problems
• Check the pressure gauge on your boiler to see current pressure
• Repressurise by opening the filling loop valves until pressure reaches 1-1.5 bar
• If pressure keeps dropping, check for leaks or call an engineer
• High pressure can also be a problem and may need bleeding
• Call a qualified engineer if you're unsure or if problems persist
• Regular servicing can help prevent pressure-related issues