t10 pro says 'call for heat' but my furnace isn't kicking on. my old thermostat worked fine.

Diagnosis

When the Honeywell Home T10 Pro displays "Call for Heat" but the furnace remains dormant, the thermostat's internal relay has successfully closed, but the electrical signal is not reaching the furnace control board or the board is refusing to trigger the ignition sequence. Because your previous thermostat worked, the issue is almost certainly centered on the C-wire power delivery, a misconfigured equipment setting in the T10's installer menu, or a loose connection at the RC/RH terminals. In many cases, the T10 Pro requires a stable 24VAC common wire (C-wire) to maintain the connection between the thermostat and the furnace. If the C-wire is not seated properly or if the thermostat is attempting to "power steal," it may show the "Call for Heat" status on the screen while failing to provide enough current to actually trip the furnace's heat relay. Additionally, if the T10 is configured for a Heat Pump but you have a conventional gas/electric furnace, the thermostat will send the signal to the wrong terminal (O/B instead of W), resulting in a "call" that the furnace never sees.

How to Fix It

To resolve this, you must first verify the physical wiring and then calibrate the internal equipment settings to match your specific HVAC hardware.

1. Verify the C-Wire: Pull the T10 Pro faceplate off the wall bracket. Ensure the blue (or black) wire is firmly clamped into the 'C' terminal. Tug gently on the wire to ensure it is not slipping out of the terminal block.
2. Check the R-Terminal Jumpers: Ensure the power wire is in the RC terminal. If you have separate RC and RH wires, ensure both are secure. If you only have one power wire, it must be in RC for the T10 Pro to function correctly.
3. Enter Installer Setup: Press and hold the Menu button for 5 seconds to enter the advanced settings. Navigate to Installation > Equipment Setup.
4. Configure Heat Type: Check the "Heat Pump" setting. If you have a standard furnace, this must be set to OFF. If it is set to ON, the thermostat looks for an O/B wire instead of a W wire, which prevents a standard furnace from firing.
5. Set the Reversing Valve (Heat Pump Only): If you actually have a heat pump, go to Equipment > Heat Pump > O/B Reversing Valve. Toggle this between 'O' and 'B'. Most brands (like Carrier or Trane) use 'O', while Rheem and Ruud typically use 'B'. Save the setting and test.
6. Verify Stage 1 Heating: Ensure the Heat Stage 1 setting is mapped to the 'W' terminal. If it is mapped to an auxiliary terminal you aren't using, the signal will never reach the furnace.

If That Didn't Work

• Test for 24V Output: Use a multimeter to measure the voltage between the R terminal and the W terminal while the thermostat says "Call for Heat." You should see approximately 24VAC. If you see 24V at the thermostat but the furnace isn't running, the break is in the wiring inside your walls or at the furnace control board.
• Check the Furnace Fuse: Locate the control board inside your furnace cabinet. Look for a small automotive-style blade fuse (usually 3A or 5A). If the T10 Pro caused a momentary short during installation, this fuse will blow, cutting all power to the thermostat's switching capability even if the screen stays on.
• Reset the HVAC Control Board: Power down the furnace at the breaker for 60 seconds. This clears any "lockout" codes on the furnace board that may have occurred if the T10 Pro sent rapid-fire signals during the initial setup phase.
• Verify Common Wire Continuity: If you don't have a C-wire and are using a power extender kit, ensure the adapter is installed correctly at the furnace board. The T10 Pro is highly sensitive to voltage drops; if the C-wire connection is weak, the relay may not have enough torque to close the circuit.

Heads Up

• Avoid "Power Stealing": The T10 Pro is not designed to operate without a dedicated C-wire. Attempting to run it without one can lead to intermittent "Call for Heat" failures or cause your furnace to short-cycle.
• Terminal Tightness: The T10 Pro uses push-terminals. Ensure the wire is stripped exactly 1/4 inch; too much exposed copper can cause a short, while too little prevents a solid connection.
• Wait for the Delay: Remember that most furnaces have a built-in 5-minute delay to protect the heat exchanger. After changing settings, wait at least 5 minutes before deciding if the fix worked.