This guide is for Gas Safe Registered Engineers only.
Incorrect fan pressure readings can result in unsafe combustion, nuisance fault codes, or long-term boiler damage.
What Is a Fan Pressure Test?
The fan pressure test is a diagnostic procedure used by Gas Safe-registered engineers to assess boiler fan pressure readings and identify combustion airflow issues. As part of wider Worcester boiler diagnostics, this test helps confirm whether poor performance is linked to heat exchanger blockage or general airflow restriction.
The test is primarily used to identify restricted airflow caused by a partially blocked heat exchanger, which is common on older or poorly maintained appliances.
How to Perform the Test
- Connect a calibrated digital manometer to the fan pressure test point
- Force the boiler to maximum output using the manufacturer’s service mode
- Record the fan suction pressure and compare it with the reference values
Red zone
Indicates restricted airflow. The heat exchanger should be removed and cleaned.
Grey zone
Operating within acceptable limits. No corrective action required.
Fan Pressure Reference Chart
Abnormal boiler fan pressure readings often indicate restricted airflow and should be investigated before replacing components.
| Boiler Model | Fan Suction (mbar) | Action Required |
|---|---|---|
| 30Si | -4.5 | Clean heat exchanger |
| 25Si | -3.5 | Clean heat exchanger |
| 28i JR | -3.9 | Clean heat exchanger |
| 24i JR | -3.1 | Clean heat exchanger |
| 24i S | -3.1 | Clean heat exchanger |
| 18i S | -7.4 | No action required |
| 15i S | -5.5 | No action required |
| 12i S | -3.6 | Clean heat exchanger |
| 24Ri | -3.1 | Clean heat exchanger |
| 18Ri | -7.4 | No action required |
| 15Ri | -5.5 | No action required |
| 12Ri | -3.6 | Clean heat exchanger |
Important Technical Notes
- Readings must always be taken at maximum boiler output
- Fan suction values between -3.5 and -4.5 mbar on most JR and Si models typically indicate airflow restriction
- This test should be carried out before replacing components such as fans, gas valves, or PCBs
- In many cases, failed readings are caused by heat exchanger blockage rather than fan or gas valve failure.
- Accurate Worcester boiler diagnostics rely on confirming airflow performance before progressing to combustion analysis or part replacement.
Worcester Boiler Support – Molesey, Surrey & London
Certificates4UK Ltd provides technical support, diagnostics, and specialist servicing for Worcester boilers across Surrey and London. If you encounter repeated faults, borderline readings, or combustion-related issues, professional assistance is available.
More information can be found on :
worcester-bosch guarantees-terms-and-conditions page
CHECK OUR BLOG FOR MORE USEFUL INFORMATION:
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Contact Certificates4UK Ltd
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