Quick Summary A Nest thermostat that displays the correct setpoint but does not turn on the furnace or heat pump is experiencing one of four problems: the Nest…
Quick Summary
A Nest thermostat that displays the correct setpoint but does not turn on the furnace or heat pump is experiencing one of four problems: the Nest is in an incorrect heating mode (e.g., set to Cool or Off instead of Heat), the Rh or Rc wire has lost continuity, the furnace itself has a fault (tripped limit switch, clogged filter, or ignition failure), or the Nest's wiring configuration does not match the HVAC system. This guide covers the Nest Learning Thermostat (3rd gen), Nest Thermostat E, and Nest Thermostat (2020) for both gas furnace and heat pump systems.
How Nest Controls Heating
The most common reason Nest does not turn on heat is that the mode is set to Cool, Eco, or Off. Check this first before any wiring or hardware troubleshooting.
Step 1: Verify Heating Mode and Setpoint On the Nest display: check the mode shown below the temperature. It should say "Heating to [X]°" when actively calling for heat, or "Heat" when in heating mode but the setpoint has been reached. If it says "Cooling", "Eco", or "Off": rotate the ring to the mode you want, or use the Nest app → tap the thermostat → Mode → select Heat. Ensure the setpoint is higher than the current room temperature. If the setpoint is 20°C and the room is already 21°C, the Nest will not call for heat. Check for a "Delayed" message on the Nest display. This means the Nest is waiting before calling for heat to protect the compressor (for heat pumps) or to prevent short-cycling. Wait 5 minutes.
Step 2: Check the Nest App for Error Messages
The Nest app often shows error messages that are not visible on the thermostat display. Nest app → tap the thermostat → check for any alerts or error banners at the top of the screen. Common errors that prevent heating: "No power to Rh wire": the heating power wire has no voltage. Check the furnace power switch and circuit breaker. "Furnace issue detected": the Nest detected the furnace did not respond to a heat call. The furnace has a fault. "Wiring issue": a wire has come loose or is incorrectly connected. If no errors appear: proceed to Step 3.
Step 3: Check the Furnace Power and Reset
Before checking wiring, verify the furnace itself has power and is not in a fault state. Locate the furnace power switch (usually a wall switch near the furnace that looks like a light switch). Ensure it is ON. Check the circuit breaker for the furnace. A tripped breaker will cut power to both the furnace and the Nest (if the Nest is powered by the furnace's 24V transformer). Check the furnace's built-in reset button (usually a red button on the burner assembly). If the furnace has overheated or had an ignition failure, the limit switch trips and the reset button pops out. Press it firmly to reset. Check the furnace filter. A severely clogged filter causes the furnace to overheat and trip the high-limit switch, which shuts the furnace down. Replace the filter if it has not been changed in the last 1–3 months. After any furnace reset: wait 5 minutes for the Nest to detect the restored power and attempt a new heat call.
Step 4: Check Nest Wiring
Loose or incorrectly connected wires are a common cause of heating failure, especially after a recent Nest installation or after the thermostat has been removed and reinstalled. Remove the Nest display from the base by pulling it straight off the wall. On the Nest base, check that all wires are firmly seated in their connectors. Each connector has a small button — press it and re-insert the wire if it looks loose. For heating to work, the minimum required wires are: Rh (or R): 24V power from the heating transformer W (or W1): heat call wire to the furnace C (common): recommended for stable power; required for Nest Thermostat (2020) If the W wire is missing or not seated: the Nest cannot send a heat call to the furnace. Re-insert it firmly into the W terminal. Nest app → Settings → Equipment → Wiring → confirm the wiring diagram matches your physical connections.
Step 5: Check Heat Pump Configuration
If your system is a heat pump (not a gas furnace), the wiring and mode configuration are different. Heat pumps use the O/B wire to control the reversing valve. If this wire is connected to the wrong terminal or configured incorrectly in the Nest settings, the heat pump will run in cooling mode even when Heat is selected. Nest app → Settings → Equipment → check Heat Pump settings. Ensure O/B wire is set correctly for your heat pump brand (most use O for cooling, but some brands like Rheem use B for cooling). If the heat pump runs but blows cold air in Heat mode: the O/B setting is reversed. Change it in the Nest app → Settings → Equipment → Heat Pump → O/B wire activates on.
Step 6: Test the Heat Call with a Jumper Wire
This step confirms whether the Nest or the furnace is the source of the problem. Turn off the circuit breaker for the furnace before touching any wires. Remove the Nest from the base and locate the Rh and W terminals on the base. Cut a short piece of wir