honeywell t9 installed but my heat won't turn on. system mode is set to heat and temp is higher than room temp, but nothing happens.

Diagnosis

The Honeywell Home T9 (RCHT9610WFSW2003) is a power-hungry smart thermostat that relies on a constant 24VAC supply. When the screen is active and the temperature is set above the room ambient, but the heat fails to engage, the issue is typically a break in the "call for heat" circuit. This happens when the T9 sends a signal via the R (Power) and W (Heat) wires, but the signal is interrupted before it reaches the furnace control board or the furnace fails to respond due to a safety lockout. You can confirm if the T9 is actually sending the signal by checking the "Equipment Status" in the app or on the screen. If the T9 shows "Heating" but no air is blowing, the problem is downstream (furnace side). If the T9 is set to heat but the status remains "Idle," the issue is either a wiring fault at the backplate or a failure in the internal relay of the thermostat. A common culprit is a loose common wire (C-wire) causing the thermostat to reboot or glitch exactly when it tries to pull current to trigger the heat relay.

How to Fix It

Follow these steps to isolate the break in the circuit and restore heating:

1. Verify Power Sources: Locate your electrical panel and find the breaker labeled "Furnace" or "HVAC." Even if it looks on, flip it completely to OFF and then back to ON. Then, locate the service switch (which looks like a standard light switch) mounted on the side of your furnace or on a nearby wall; ensure it is flipped UP/ON.
2. Inspect the Furnace Door Switch: Open the furnace cabinet and ensure the blower door is seated perfectly. Most units have a "door kill switch" (a small plunger button). If the door is slightly ajar, the furnace will have power to the thermostat but will refuse to ignite the burners for safety. Push the door firmly into place.
3. Validate Backplate Wiring: Pull the T9 faceplate off the wall. Ensure the Red wire is securely clamped in the R terminal and the White wire is firmly in the W terminal. Tug lightly on each wire; if a wire slips out, re-strip 1/4 inch of insulation and re-insert it.
4. Test the Furnace Directly: With the power OFF, remove the R and W wires from the T9 backplate. Strip a small amount of insulation and manually touch (jump) the R and W wires together using a small piece of insulated copper wire. Turn the power back on. If the heat kicks on immediately, the T9 unit is faulty or configured incorrectly. If it still doesn't start, the problem is a blown fuse on the furnace control board or a failed igniter.
5. Check the C-Wire Connection: Ensure the Blue (or designated) C-wire is connected. The T9 cannot reliably trigger the W-wire without a stable return path for power. If you are using a Power Extender Kit (PEK), verify that the wires at the furnace end are matched exactly to the PEK labels.

If That Didn't Work

• Check for a Blown Furnace Fuse: Open the furnace panel and locate the control board. Look for a small automotive-style blade fuse (usually 3A or 5A, purple or amber). If the filament inside is broken, the thermostat will have power (from the transformer) but the board cannot trigger the gas valve. Replace the fuse and check for short circuits in the thermostat wiring.
• Reset the T9 Factory Settings: Go to the thermostat menu, select "Advanced Setup," and perform a factory reset. Re-run the equipment setup wizard to ensure the T9 knows you have a "Gas/Electric" or "Oil" furnace. If it is accidentally set to "Heat Pump" without an auxiliary heat configuration, it will not trigger the W terminal.
• Inspect the Flame Sensor: If the furnace starts (you hear the inducer motor and igniter click) but then shuts off after 30 seconds without blowing heat, your flame sensor is likely dirty. Use a piece of fine steel wool or a clean Scotch-Brite pad to gently scrub the small metal rod inside the burner assembly.
• Verify the C-Wire Voltage: Use a multimeter to measure AC voltage between the R and C wires. You should see between 24V and 28V. If the voltage is below 22V, the T9 may stay powered on but lack the "oomph" to close the relay for the heat call.

Heads Up

• Avoid "C-Wire Adapters" if possible: The T9 is sensitive to voltage drops. Using a dedicated C-wire is always superior to using a power-stealing adapter or the PEK if a physical wire is available in the wall.
• Check Air Filters: A completely clogged filter can cause the furnace to overheat (Limit Switch trip), which shuts down the heat call to protect the heat exchanger. Replace filters every 30-90 days.
• Avoid Over-Tensioning Wires: When pushing the T9 faceplate onto the wall, ensure no wires are pinched between the backplate and the drywall, as this can cause intermittent shorts that disable the heating circuit.