Understanding boiler pressure (sealed systems)
Quick summary
Boiler pressure is the water pressure inside a sealed heating system. Recording the pressure when cold and again after heating has run gives useful clues when diagnosing lockouts, leaks, or discharge problems.
Safety first
- Do not remove boiler covers or attempt internal repairs.
- If you smell gas or suspect carbon monoxide, leave the property and call 0800 111 999.
- Do not repeatedly top up or discharge water until you understand the pressure pattern.
Full checklist: Safety guidance
What to check (in order)
- Check and record the pressure when the system is cold.
- Run the heating, then check and record the pressure again when the system is hot.
- Look outside for any discharge from the pressure relief pipe.
- Compare your readings with the manufacturer’s guidance for your appliance if available.
What the result means
- Around 1–1.5 bar when cold is common on many sealed systems, but always follow appliance guidance.
- Pressure that rises sharply when hot suggests expansion-related behaviour.
- Pressure that falls steadily when cold suggests a leak, discharge, or filling loop issue.
What you can safely do
- Record the pressure when the system is cold.
- Check pressure again after the heating has been on.
- Note any discharge from the outside PRV pipe.
- Use those readings to judge whether the pressure problem is steady, hot-only, or intermittent.
When to call a professional
- Pressure behaves unpredictably or changes rapidly.
- There is water discharging outside from the PRV pipe.
- You need frequent topping up or the boiler is locking out.
Engineer notes
Use cold and hot readings to distinguish steady loss, heat-related pressure rise, and discharge patterns. Correlate with PRV evidence, vessel charge, and filling loop behaviour.
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