Your boiler must operate within the correct pressure range for your central heating system to function efficiently. If the pressure is too high, the boiler may shut down or lock out. If it keeps losing pressure, it could signal a leak. This guide explains ideal boiler pressure ranges, how to check the gauge, and what to do if the pressure is too high or low.
What Should the Boiler Pressure Be When the Heating Is On?
Your boiler needs to run within the correct pressure range to operate safely and efficiently. This guide covers ideal settings, reading the gauge, and what to do if the pressure is too high or too low.
What Is Normal Boiler Pressure?
Normal operating pressure is typically between 1 and 2.5 bar when the heating system is running. Most manufacturers recommend a cold pressure around 1.3 bar.
The Pressure Gauge
A round dial (0–4 bar) showing green zones for normal pressure and red zones for critical high or low pressure.
Critical Zones
Below 0.5 bar or above 2.5 bar requires immediate attention.
Important Note
Boiler pressure (heating system pressure) is not the same as tap water pressure.
What Should Boiler Pressure Be When Heating Is On vs. Off?
Boiler pressure naturally rises when the system heats up due to water expansion.
Heating OFF (Cold System)
1 to 1.5 bar
Heating ON (Hot System)
1.5 to 2 bar
Why Pressure Rises When Heating Is On
Heating expands the water in radiators and pipework, raising pressure. The increase should not exceed 1 bar from the cold reading.
What Happens If Boiler Pressure Is Too Low?
Low boiler pressure is not dangerous but can cause heating failure.
Consequences
Below 0.5 bar, the boiler may lock out and stop heating.
Action Required
Check pressure monthly. Persistent drops indicate leaks or component faults.
Common Reasons for Low Pressure
- System leaks
- Bleeding radiators
- Open or faulty PRV
- Faulty expansion vessel
How to Increase Boiler Pressure
If pressure is below 1 bar, the system must be repressurised.
Steps to Increase Pressure
- Turn off boiler
- Locate filling loop
- Slowly add water until pressure reaches 1.3 bar (cold system)
What Happens If Boiler Pressure Is Too High?
High pressure triggers boiler safety systems.
Consequences
Above 3 bar, the pressure relief valve releases water and may force a shutdown.
When to Call an Engineer
If pressure rises above 2 bar when heating is on, or jumps more than 1 bar, the expansion vessel or PRV may be faulty.
How to Reduce Boiler Pressure
If pressure was overfilled or the PRV malfunctioned:
Bleed Radiators
Releasing water will lower the system pressure.
Final Boiler Pressure Summary
Heating OFF
1 to 1.5 bar
If below 1 bar → repressurise.
Heating ON
1.5 to 2 bar
If above 2.5 bar → reduce pressure.
Annual Maintenance
Check pressure monthly. Continuous pressure loss means a leak or faulty expansion vessel—engineer required.
Frequently Asked Questions
What is an appropriate pressure level for boilers?
Cold: 1–1.5 bar
Heating on: 1.5–2 bar
What pressure level should boilers work at?
Within the green zone (1–2.5 bar). Outside this range may cause shutdown.
What causes low boiler pressure?
Leaks, bleeding radiators, faulty PRV, or damaged expansion vessel.
Can low boiler pressure stop downstairs radiators from working?
Yes. Low pressure reduces pump force, making it harder for heat to reach lower or distant radiators.
Conclusion
Managing boiler pressure is simple once you know the correct ranges. Keep cold pressure between 1 and 1.5 bar, rising naturally to 1.5–2 bar when the heating runs. If pressure frequently drops or rises excessively, it indicates leaks or faulty components that require a qualified engineer.
