Dettson furnace shows E18; unit may not respond to thermostat changes.

Safety first: Turn off power to the furnace before changing control board settings.

• 1. Verify thermostat compatibility: Confirm that the thermostat is rated for gas furnaces and matches the staging (single-stage, two-stage, modulating) of your Dettson unit.
• 2. Check thermostat setup menu: Enter the installer setup on the thermostat and ensure system type, fan control, and number of stages are correctly configured.
• 3. Inspect control board switches/jumpers: Compare dip switch or jumper positions on the Dettson control board to the installation manual for your model and thermostat type.
• 4. Correct any mismatches: Adjust thermostat settings and control board configuration so they agree. Power cycle the furnace afterward.
• 5. Replace incompatible thermostat: If the thermostat cannot be configured correctly, replace it with a Dettson-approved or standard furnace thermostat.