When a Nest thermostat will not start heat, the cause is often a mode, schedule, equipment, or furnace safety issue rather than a failed thermostat.
These steps apply to Nest Learning Thermostat, Nest Thermostat 2020, and Nest Thermostat E connected to a furnace or heat pump.
Does this affect you?
Use this if you raise the temperature and no warm air starts, or the thermostat shows heat without the furnace actually firing.
Check Eco Mode, schedules, and Heat mode
- Look at the thermostat display for the Eco leaf.
- If Eco is active, select Heat or choose a specific target temperature.
- Confirm the mode is Heat, not Cool, Off, or another mode.
- Open the Google Home app.
- Tap the thermostat and check the Schedule tab.
- Edit the schedule or set a temporary hold if it keeps lowering the target.
- Raise the target temperature 3 to 5 degrees above the current room temperature.
- Wait a few minutes for the heat call to start.
A tiny one-degree change may not trigger a heating cycle immediately, especially if the thermostat is already close to the target.
Check equipment settings and furnace power
- Open the Google Home app.
- Go to thermostat Settings > Equipment.
- Confirm the heating system type matches your real furnace, heat pump, boiler, or dual-fuel system.
- Check the furnace power switch near the unit or basement stairs.
- Check the breaker labeled furnace or HVAC.
- Look for a diagnostic light on the furnace control board if visible.
- Write down any blinking error code before calling a technician.
If the blower runs but air stays cold, the furnace may have an ignition, gas supply, heat pump, or reversing-valve issue. Nest can request heat, but it cannot make broken equipment fire.
Wiring and safety checks
Some Nest models show an error if a wire is loose or in the wrong terminal. Before checking wiring, turn off power to the furnace or air handler at its switch and breaker if needed.
- Photograph the wire connections before touching anything.
- Do not guess at wire colors; use terminal labels.
- Stop if wires are damaged, loose, unlabeled, or confusing.
- Call a licensed HVAC technician if you are not comfortable with low-voltage thermostat wiring.
If settings and wiring are correct but the furnace still does not respond, the problem is likely downstream from the thermostat and needs HVAC service.
Sources
- Google Nest Help – Nest Thermostat isn’t turning on the heat (2025)
- Google Nest Help – Check your Nest thermostat’s wiring (2025)
- Google Nest Help – Nest Thermostat equipment settings (2025)