i replaced my old thermostat with a sensi and now my fan won't turn off

Diagnosis

This issue typically stems from a "bridged" connection or a configuration mismatch between the Sensi ST55 and your HVAC control board. In most cases, a stray strand of copper from the R (power) wire is physically touching the G (fan) terminal, creating a direct electrical path that bypasses the thermostat's internal relay. Because the fan relay on the furnace board is "normally open," any 24VAC signal applied to the G terminal will trigger the blower to run regardless of whether the system is calling for heat or cooling. Another common cause is an incorrect "Fan Control" setting within the Sensi app or device menu. If the thermostat is set to "On" instead of "Auto," the blower will run 24/7. Additionally, if you have a heat pump system but configured the ST55 for a conventional furnace, the logic for the G-wire activation may be inverted or stuck in a continuous-run state during the initialization phase. You can confirm this by removing the Sensi faceplate from the wall base; if the fan stops immediately upon removing the faceplate, the issue is a setting or a short in the baseplate. If the fan continues to run with the faceplate off, you have a physical wiring short or a stuck relay on the HVAC control board.

How to Fix It

Follow these steps to isolate the electrical short and correct the software configuration:

1. Power down your HVAC system at the breaker or the 24V transformer to avoid blowing the fuse on the control board.
2. Pull the Sensi ST55 faceplate off the wall baseplate.
3. Inspect the G terminal. Use a pair of needle-nose pliers to ensure the green wire is inserted fully and that no copper strands are fraying outward toward the R or Rc terminals.
4. If the wire looks loose, strip an additional 1/8 inch of insulation, bend the wire into a sharp 90-degree hook, and firmly seat it into the G terminal, tightening the screw until the wire is immobile.
5. Restore power and snap the faceplate back onto the base.
6. Open the Sensi App on your smartphone. Navigate to the "Fan" section on the main dashboard.
7. Ensure the Fan toggle is set to AUTO. If it is set to ON, the fan will never stop.
8. Go to Settings > Equipment and verify that your system type (Gas/Electric, Heat Pump, etc.) matches your actual HVAC hardware. If you have a conventional furnace, ensure "Conventional" is selected to prevent incorrect fan logic.

If That Didn't Work

• Perform a Hard Reset: Remove the faceplate and disconnect the 24V power for 60 seconds. This clears the volatile memory of the ST55 and forces the internal relays to reset to their default open position.
• Test for a Shorted Wire: Disconnect the G wire from the Sensi baseplate entirely. If the fan stops, the wire is fine, but the thermostat is sending a signal. If the fan continues to run with the G wire disconnected, you have a short-circuit in the wiring between the thermostat and the furnace, or a fused relay on the HVAC control board itself.
• Check the C-Wire Connection: Ensure the C (Common) wire is seated properly. While the ST55 can work without a C-wire in some configurations, an unstable power supply can cause the relays to "chatter" or stick in the closed position, keeping the fan running.
• Inspect the HVAC Control Board: Open your furnace panel and locate the G terminal on the control board. Use a multimeter to check for continuity between the R and G terminals. If there is continuity while the thermostat is disconnected, the relay on the furnace board has failed and needs replacement.

Heads Up

• Avoid over-tightening the terminal screws on the Sensi baseplate; stripping the threads can lead to loose connections that cause intermittent fan cycling.
• Always use a voltage tester to confirm power is off before touching the R and C wires, as a short here can blow the 3-amp or 5-amp fuse on your furnace control board.
• If you are using a "Power Extender Kit" or a third-party adapter, ensure the G-wire is routed through the adapter according to the specific wiring map for the ST55.
• Periodically check for dust buildup around the baseplate, as conductive debris can occasionally cause "ghost" signals in high-humidity environments.