Your furnace isn't heating — what to check first

This article is for educational purposes only and is not a substitute for professional advice. Local codes, regulations, and best practices vary by region.

The furnace isn’t heating. The house is getting cold. You’re trying to figure out if this is a simple fix or if you need to call the HVAC company. There are a few things you can check yourself before calling. Some of them might fix the problem. At minimum, you’ll know more about what’s actually going wrong.

Furnace not heating has several common causes. Some are simple fixes. Some require professional service. Quick troubleshooting helps identify which applies to your situation.

Check the Thermostat

Verify it’s set to heat mode. If it’s set to cool or off, heating won’t run. Set it to heat and a temperature above current room temperature. The furnace should start.

Check thermostat batteries if it has them. Replace low batteries and test again.

Is the display readable or is it blank? If blank, batteries might be dead or the thermostat needs power. Check the breaker if it’s hardwired.

Check the Filter

A clogged furnace filter restricts airflow and reduces heating output. Check the filter location in your furnace or air handler. If it’s visibly dirty, replace it. Most filters are inexpensive ($10-25) and easy to swap.

A very dirty filter can completely block airflow. This single change might restore heating.

Check the Breaker

Your furnace has a dedicated breaker. Check the electrical panel. If the breaker has tripped (middle position or off), reset it. This might be all that’s needed.

If it immediately trips again, you have an electrical problem requiring professional help.

Listen to the Furnace

Is the furnace making any noise? A functioning furnace hums and has blower sounds. Complete silence suggests it’s not running.

Does it make clicking sounds before ignition? This is normal. The furnace attempts to light repeatedly. If it keeps clicking without igniting, the igniter or gas valve might be faulty.

Check Fuel Supply

If you have oil heat, make sure there’s fuel in the tank. If the tank is empty, no heat. If fuel is present but the furnace won’t start, a line blockage or fuel pump failure might be the issue.

For gas heating, verify gas is on. If you smell gas or suspect a gas leak, leave the house and call the gas utility immediately. Do not attempt to troubleshoot.

Air Vents

Are the heating vents open? Closed vents reduce heat distribution. Open them.

Is cold air coming from vents? If so, the furnace isn’t heating. It’s cycling air that’s already cold.

Carbon Monoxide Detector

Does your carbon monoxide detector alarm? If yes, leave the house immediately. This is an emergency. Do not attempt to fix anything. A furnace malfunction might be leaking CO. Call 911 or the fire department from outside.

When to Call an HVAC Technician

Call if the furnace won’t start after checking thermostat and filter. Call if you hear unusual sounds (grinding, squealing) or smell gas. Call if the breaker keeps tripping. Call if the CO detector alarms.

HVAC service calls cost $150-300. Repairs depend on the problem. Blower motor failure might cost $300-600. Igniter failure typically costs $200-400. Heat exchanger replacement is expensive ($1,000+).

Temporary Solutions

Until the furnace is repaired, seal air leaks around doors and windows. Close off unused rooms and move to smaller, warmer spaces. Use portable heaters carefully in rooms you’re using (follow safety guidelines—never leave unattended, keep away from flammables).

These are temporary measures only. A non-functioning furnace needs professional service in cold climates.

Emergency Heat

If you’re in dangerously cold conditions and the furnace can’t be repaired immediately, consider temporarily staying elsewhere or using alternative heat sources carefully.

Space heaters can supplement heat but require caution. Never use them unattended. Keep flammables away. Ensure room ventilation to prevent CO buildup.

Prevention

Annual furnace maintenance (cleaning, inspection, tune-up) prevents most failures. Professional technicians check all components and catch problems early. Maintenance costs $100-200 but extends furnace life and ensures reliable operation.

The Bottom Line

Furnace not heating might be thermostat, filter, breaker, or fuel issue. Check these before calling. If these don’t restore heat, professional service is needed. Don’t delay—cold houses become dangerous. Call an HVAC technician if your troubleshooting doesn’t work. Most furnace failures require professional repair.

© The Whole Home Guide