Nest Thermostat Says Heating but Furnace Not Turning On: Troubleshooting Guide

The Nest thermostat indicating “heating” while the furnace remains off is a common issue homeowners face. This guide explains likely causes, step-by-step checks, wiring and power diagnostics, and when to call a professional. Follow the troubleshooting steps carefully to diagnose and resolve the problem.

Symptom	Possible Cause	Quick Action
Nest Says Heating But Furnace Off	Thermostat command failure, wiring, or power issue	Check display, wiring, and breaker
Furnace Runs Intermittently	Flame sensor, limit switch, or airflow problem	Inspect filter, vents, and error codes
No Response From Furnace	Transformer, control board, or C-wire issue	Test 24V at furnace and thermostat

How The Nest And Furnace Communicate

The Nest thermostat sends a 24-volt control signal to the furnace’s control board to call for heat. When configured for conventional systems, the Nest closes the R (power) to W (heat call) circuit or energizes the relay inside the HVAC control board. If the Nest shows a heating call but the furnace does not respond, the signal fails somewhere between the thermostat and the furnace control board or the furnace is preventing ignition.

Common Causes

Wiring Or Connection Problems: Loose or swapped wires between the Nest base and the furnace can prevent a valid heat call from reaching the system.

Power Or C-Wire Issues: Insufficient power to the thermostat (missing or weak C-wire) may cause ghost commands or failure to energize the relay.

Blown Fuse Or Tripped Breaker: A blown 3A-5A fuse on the furnace control board or a tripped breaker will stop the furnace from responding.

Furnace Control Board Or Transformer Failure: If the transformer that provides 24VAC or the control board is faulty, the furnace won’t react to heat calls.

Safety Or Limit Switches: Open high-limit switches, flame rollout switches, or blocked flues can prevent furnace operation even when a heat call is present.

Software Or Thermostat Fault: Rarely, a Nest firmware issue or internal relay fault can misreport status or fail to send a proper signal.

Quick Checks Before Advanced Troubleshooting

Check the Nest display for error messages or warnings. Note any system or furnace messages shown in the Nest app under Equipment or History.

Confirm the thermostat temperature setting and schedule. Increase setpoint 3–5°F above room temperature to ensure a call for heat is active.

Verify the furnace power switch (near furnace cabinet) is on and the home’s HVAC breaker is not tripped. Check the furnace control board fuse for continuity; replace if blown.

Inspect and replace dirty air filters and ensure all supply registers are open; restricted airflow can trigger safety limits that prevent operation.

Step-By-Step Troubleshooting

1. Observe Thermostat And Furnace Response

Set the Nest to heat and watch the thermostat display. Does it show “Heating” and the flame icon? Next, go to the furnace and listen for a click or hum that indicates the control board or transformer was energized.

2. Check 24VAC At The Furnace

With a multimeter set to AC volts, measure across the furnace control board terminals labeled R and C. A healthy transformer reads approximately 24VAC. If no voltage, transformer or power to furnace is faulty.

3. Check Continuity Of W Wire

Turn power off to the furnace. Remove thermostat wires at the furnace board and use a multimeter to test continuity between the thermostat W terminal and the furnace W terminal. A break indicates a wiring fault.

4. Test Thermostat Output

With power on and thermostat calling for heat, measure 24VAC between the R and W wires at the furnace board. If 24VAC is present, the thermostat is correctly calling for heat and the issue lies in the furnace. If no 24VAC, suspect thermostat wiring, C-wire power, or Nest internal relay.

5. Inspect Furnace Safety Devices

Look for tripped limit switches, open flame sensors, or error lights on the furnace control board. Consult the furnace manual for LED fault codes and clear any temporary faults following safe procedures.

Wiring And Power: Detailed Diagnostics

Check The C-Wire: Nest thermostats often rely on a C-wire for steady power. If the C-wire is absent or weak, the Nest may show commands without properly energizing outputs. Measure the R-C voltage at the thermostat terminal; it should be ~24VAC.

Verify R-W Voltage: When calling for heat, R to W at the thermostat should be ~0VAC when idle and ~24VAC when calling. If the thermostat never outputs 24VAC, check wiring and the Nest’s power status.

Look For Shared Equipment Or Heat Pump Configuration: If the system is a heat pump or multi-stage system, verify the Nest is configured correctly in Equipment settings. Incorrect configuration can send the wrong signals to the furnace or air handler.

Common Furnace Failures That Block Ignition

Faulty Transformer: The transformer stepping down to 24VAC can fail, eliminating the control voltage that thermostats use to call for heat.

Control Board Problems: A damaged relay or control board IC may not accept or process the call even if 24VAC is present.

Ignition And Flame Sensors: A faulty ignitor, flame sensor, or gas valve problem prevents startup. Furnaces often lock out after failed ignition attempts.

High Limit And Safety Switches: Dirty burners or blocked exhaust can trip safety switches. Reset procedures vary and should be performed per the furnace manual.

Thermostat Settings And Software Checks

Open the Nest app or the thermostat settings to confirm system type is set to “forced air” or the correct furnace mode. Incorrect selection such as “electric” vs “gas” can change behavior.

Check the Nest’s software for pending updates. Reboot the thermostat from the settings menu if it behaves erratically. Rebooting can clear temporary software glitches.

If the Nest shows a persistent error like “No Power” or “Heat Link not connected,” follow Google Nest’s troubleshooting steps for that message. Document any error codes shown by the Nest for technician reference.

When To Call A Professional HVAC Technician

If the homeowner finds no 24VAC at the furnace, a blown control board fuse, a failed transformer, or complex ignition faults, a licensed HVAC technician should be contacted. Handling gas valves, burners, or internal control board repairs requires training and tools.

Call a technician if the furnace displays fault codes that the homeowner cannot clear, if there is a gas smell, or if any inspections reveal damaged wiring or corrosion. Technicians can safely test, replace, and calibrate components to restore reliable operation.

Temporary Workarounds And Safety Notes

A temporary workaround includes switching to a backup thermostat if available, but only after verifying compatibility and wiring. Avoid bypassing safety interlocks or shorting control terminals.

Always switch off electrical power before accessing furnace internals. If a gas leak is suspected, evacuate the area and call the gas company immediately. Never attempt to repair gas valves or burners without proper certification.

Preventive Maintenance To Avoid Future Failures

Schedule annual HVAC inspections to catch failing transformers, control boards, or ignition components early. Replace air filters every 1–3 months according to system recommendations.

Ensure thermostat wiring is secure and uses a C-wire where possible. Consider installing a Nest Power Connector or an external common wire adapter if the C-wire is not available.

Keep the furnace area clean, vents unobstructed, and combustion air intake clear. Regular professional cleaning of burners and flame sensors reduces ignition failures and limit trips.

FAQ

Why Does the Nest Say Heating But Nothing Happens?

Because the Nest can display a heating call while the physical 24VAC output is not reaching or being accepted by the furnace. Causes include wiring faults, missing C-wire power, blown fuse, failed transformer, or furnace safety lockouts.

Can A Nest Thermostat Break A Furnace?

It is uncommon for a thermostat to damage a furnace. Most issues are wiring-related or pre-existing furnace failures. Incorrect wiring or voltage spikes may stress control components, so proper installation is essential.

How Much Does It Cost To Fix This?

Costs vary widely: simple issues like replacing a fuse or correcting wiring may be low cost, while replacing a transformer or control board ranges from a couple hundred to several hundred dollars including labor. Diagnostics fees may apply.

Is A C-Wire Always Required?

A C-wire is strongly recommended for Nest thermostats to ensure stable power. Some Nest models can work without a C-wire using power stealing, but this can cause unreliable behavior on some HVAC systems.

Resources And References

Consult the Nest Help Center for model-specific guidance and error message explanations. Review the furnace manufacturer’s service manual for control board LED codes and safe reset procedures.

For electrical tests, a basic digital multimeter and knowledge of 24VAC circuits are required. If unsure about testing, hire a licensed HVAC technician or electrician.

Key Takeaway: When the Nest says heating but the furnace does not run, methodically check wiring, 24VAC presence, control board fuses, and furnace safety switches. Document findings and escalate to a professional for electrical, gas, or control board repairs.