F81/F91 Fault — Cylinder charging pump / Storage tank not fully charged

What does F81/F91 mean?

The boiler is faulting and showing both F81 and F91, indicating that the cylinder or storage tank is not fully charging within the expected time. This could be due to air in the pump, a defective pump, or blockages.

• Air in the actoSTOR pump (shift load pump) or pump is defective
• Defective diverter valve
• Blocked secondary heat exchanger
• Faulty pump
• Cylinder charging sensor or cylinder sensor issue

Click steps to track your progress.

1. 1

   Inspect the pump cable harness for damage or loose connections.

2. 2

   Check the impeller sensor and/or limiter in the product (requires specialist tools).

3. 3

   Check the cylinder charging sensor and the main cylinder sensor (requires specialist tools).

4. 4

   Verify that the diverter valve is functioning correctly (requires specialist tools).

5. 5

   Check if the secondary heat exchanger is blocked (requires specialist tools).