Output 3 behavior depends on the ETO2 application configuration.
In the standard Electric 1-Zone configuration, the ETO2 uses output relays 1, 2, and 3 for the one heating zone. They operate together when that zone calls for heat.
This does not describe every ETO2 mode. Electric 2-Zone assigns the two zones to relays 1 and 2, while Electric 2-Stage uses a different output-control arrangement. Confirm the configured application before troubleshooting output 3.
A qualified installer must use the matching wiring diagram and verify the circuit and relay ratings. Do not change application mode simply to energize an otherwise unexplained output.
Sources
- Manufacturer instructions (publication 13472), PDF pp. 6–7 and 59.