my heat won't turn on, the thermostat just says 'heating' but i hear nothing from the furnace.

Diagnosis

When your Emerson Sensi Lite ST26 displays "Heating" but the furnace remains dormant, it indicates a "Call for Heat" has been successfully initiated by the thermostat's internal relay. The ST26 is completing the circuit between the R (power) and W (heat) terminals, sending 24VAC to your furnace control board. Because the screen shows the heating status, the thermostat's software and interface are functioning correctly; the breakdown is occurring in the physical communication between the thermostat and the furnace, or within the furnace's own safety sequence.

The most common cause is a triggered safety lockout. Modern furnaces are designed to kill all power to the burners if a specific fault is detected, even if the thermostat is demanding heat. This is often caused by a tripped condensate overflow switch, a blown 3-amp or 5-amp fuse on the furnace control board, or a door switch that isn't fully depressed. To confirm, you can use a multimeter to test for 24V AC at the W terminal of the furnace control board while the Sensi Lite is in "Heating" mode; if voltage is present but the furnace is silent, the issue is internal to the HVAC unit.

How to Fix It

Follow these steps in order to isolate and resolve the power gap between the Sensi Lite and your heating equipment:

1. Verify the Furnace Door Panel: Open the furnace cabinet and ensure the blower door is seated perfectly. Most furnaces have a "door kill switch" (a small spring-loaded button). If the panel is loose by even a quarter-inch, the switch won't engage, and the furnace will not fire despite the thermostat's signal.
2. Inspect the Condensate Drain: If you have a high-efficiency furnace (90% AFUE or higher), locate the PVC condensate drain lines. Look for a plastic "float switch" or "trap." If the drain is clogged with slime or debris, the water level rises, lifts the float, and breaks the R-wire connection to prevent a flood. Vacuum out the drain line from the exterior of the house using a wet/dry vac.
3. Check the Furnace Fuse: Locate the integrated control board inside the furnace. Look for a small automotive-style blade fuse (usually 3A or 5A, purple or amber). If the filament inside the fuse is broken, the thermostat cannot trigger the furnace. Replace it with an identical amperage fuse.
4. Reset the Service Switch: Locate the 120V toggle switch (which looks like a standard light switch) usually mounted on the side of the furnace or a nearby wall. Flip it OFF for 30 seconds and then back ON to power-cycle the control board and clear any temporary logic errors.
5. Verify Gas Valve: Ensure the gas shut-off valve on the pipe leading into the furnace is turned parallel to the pipe (Open position).

If That Didn't Work

• Test the Thermostat Wiring: Remove the Sensi Lite ST26 from its wall plate. Take a small jumper wire and momentarily touch the R terminal to the W terminal. If the furnace kicks on immediately, the issue is a faulty relay inside the ST26 or a loose wire connection. If it still doesn't start, the problem is definitely in the furnace wiring or the control board.
• Inspect the Flame Sensor: If you hear the inducer motor start and the igniter glow, but the burners never light (or light for 2 seconds and then shut off), your flame sensor is likely coated in carbon. Use a piece of fine emery cloth or a toothbrush to gently clean the small metal rod located in the heat exchanger.
• Check the Air Filter: A severely clogged filter can cause the furnace to overheat rapidly during the startup sequence, triggering the "High Limit Switch." This shuts down the system to prevent a fire. Replace your 16x25 or 20x20 filter and try restarting.
• Verify the C-Wire Connection: While the Sensi Lite is designed to work without a C-wire in some configurations, power fluctuations can cause the relay to "chatter" without actually closing the circuit. Ensure the C-wire is securely fastened to the common terminal on both the thermostat and the furnace board.

Heads Up

• Voltage Warning: Always shut off the 120V breaker before touching the furnace control board to avoid frying the sensitive electronics or risking electrical shock.
• Fuse Replacement: Never replace a 3A fuse with a 5A or higher fuse; doing so can lead to the control board burning out completely if there is a short circuit.
• Short-Cycling: Avoid repeatedly turning the heat on and off in rapid succession to "test" it. This can trigger a "lockout mode" on many furnace brands, requiring a full hard reset of the power.
• Drain Maintenance: If you live in a freezing climate, ensure your condensate lines are properly sloped and insulated to prevent ice blockages during the first cold snap of the season.