my honeywell t9 says it's heating but my furnace isn't kicking on, help?

Diagnosis

This issue occurs when the Honeywell T9 (RCHT9610WFSW2003) registers a "Call for Heat" on the display, but the physical furnace ignition sequence never begins. This indicates a communication break between the thermostat's control board and the HVAC system's control board. Essentially, the T9 is sending a signal to close the circuit, but the 24V AC current is not reaching the furnace's W (heating) terminal, or the furnace is failing to respond to that signal due to an internal safety lockout. To confirm the root cause, you must determine if the failure is at the thermostat, the wiring, or the furnace. If the T9 screen shows the heating icon (usually a flame or a red-tinted screen) and the "Heating" status, the thermostat's internal relay has triggered. If the furnace remains silent—no blower starting and no inducer motor humming—the problem is either a loose wire at the R or W terminals, a tripped float switch in your condensate drain pan, or a blown 3-amp or 5-amp fuse on the furnace's integrated control board.

How to Fix It

Follow these steps to isolate the break and restore heating functionality:

1. Verify the Temperature Differential: Ensure the set point is at least 3 degrees Fahrenheit above the current room temperature. Some T9 models have a built-in "temperature swing" or hysteresis setting that prevents the furnace from cycling too frequently; if you only raise it by 1 degree, the relay may not engage.
2. Inspect the C-Wire Connection: Pull the T9 faceplate gently off the wall bracket. Ensure the Common (C) wire is securely clamped. Without a stable 24V return path, the T9 may show it is heating on the screen, but it cannot actually sustain the current needed to trigger the furnace relay.
3. Perform a Manual Jump Test: With the power off at the breaker, remove the T9 faceplate. Locate the R (Red) and W (White) wires. Carefully strip a small amount of insulation if needed and use a jumper wire (or a small piece of insulated copper) to momentarily bridge the R and W terminals. Turn the power back on. If the furnace kicks on immediately, the T9 thermostat is faulty or incorrectly configured. If it still doesn't start, the issue is in the wiring or the furnace itself.
4. Check the Furnace Fuse: Open the furnace cabinet and locate the control board. Look for a small automotive-style blade fuse (usually purple 3A or orange 5A). If the filament inside is broken, the thermostat cannot send power to the furnace. Replace it with an identical amperage fuse.
5. Clear the Condensate Line: Locate the PVC drain pipe coming out of the furnace. If the line is clogged with algae, the float switch will trip and physically break the R-wire connection to prevent a flood, meaning the T9 will "call" for heat, but the signal is blocked by the safety switch. Vacuum out the drain line from the exterior.

If That Didn't Work

• Check for "Eco" or "Schedule" Overrides: Navigate to the T9 Menu > Schedules. Ensure a "Sleep" or "Away" profile isn't overriding your manual set point. Sometimes the T9 will display the target temperature but will not engage the relay if a priority "Eco" mode is active and the current temperature is within the allowed "Eco" range.
• Inspect the Heat Pump/Conventional Setting: Go to Menu > Equipment > Heat Type. If your system is a standard gas/electric furnace but the T9 is set to "Heat Pump," it will send the signal to the O/B terminal instead of the W terminal. Ensure it is set to "Conventional" for standard furnaces.
• Verify the R-Wire Jumpers: If you have a separate heating and cooling system, ensure the R and Rc terminals are properly jumped or wired. If the T9 isn't receiving power on the specific R terminal connected to the furnace, the W signal will never be energized.
• Test for Low Voltage: Use a multimeter set to AC Volts. Place one probe on the C terminal and the other on the W terminal while the T9 is calling for heat. You should see 24V. If you see 0V, the T9's internal relay has failed.

Heads Up

• Avoid Over-Tensioning Wires: When pushing the T9 faceplate back onto the wall bracket, ensure the wires are tucked neatly. If a wire is pinched or pulled out of the terminal block, it can cause intermittent heating failures.
• Check Air Filters Monthly: A heavily clogged filter can cause the furnace to overheat and trip the "High Limit Switch." This shuts down the heating sequence even if the T9 is calling for heat.
• Power Cycle Warning: If you replace a fuse on the furnace board, always turn off the breaker first. Touching the board while powered can blow the microprocessor, requiring a costly board replacement.
• Battery Backup: While the T9 primarily relies on C-wire power, ensure your WiFi connection is stable; a sudden disconnect during a firmware update can sometimes glitch the relay logic.