Furnace Not Heating House: Troubleshooting Guide

On this page

Quick answer
Confirm the symptom
Tools and prep
Step-by-step troubleshooting
Common causes table
Parts matching
When to call a pro
FAQ
Related guides

Quick answer

Check the thermostat settings and battery, ensure the air filter is clean, and verify the pilot light is lit. If these don't resolve the issue, the furnace may need professional servicing.

Confirm the symptom

Thermostat is set to heat but house remains cold
Furnace fan runs but no warm air is produced
No furnace noise when thermostat calls for heat
House feels colder than thermostat reading

Tools and prep

Thermostat screwdriver
New air filter (if needed)
Flashlight
Screwdrivers (flathead and Phillips)
Work gloves
Turn off furnace power at circuit breaker before inspecting components

Step-by-step troubleshooting

Check Thermostat Settings
Do: Verify thermostat is set to 'Heat' mode and desired temperature is higher than room temperature.
Observe: Thermostat displays correct settings but house remains cold.
Means: Thermostat settings are correct but another issue exists.
Next: Check Thermostat Battery
Check Thermostat Battery
Do: Remove thermostat cover and inspect battery compartment for corrosion or low battery indicator.
Observe: Battery is dead or corroded.
Means: Thermostat requires new battery or cleaning.
Next: Replace or Clean Battery
Replace or Clean Battery
Do: Replace with new battery or clean terminals with a dry cloth. Reassemble thermostat.
Observe: Thermostat powers up and responds to changes.
Means: Battery issue resolved.
Next: Check Air Filter
Check Air Filter
Do: Locate and remove air filter from return duct or furnace cabinet.
Observe: Filter is dirty or clogged with debris.
Means: Restricted airflow is causing heating issues.
Next: Replace Air Filter
Replace Air Filter
Do: Install a new filter with correct airflow direction indicated on filter frame.
Observe: Airflow improves through vents.
Means: Air filter was the cause of restricted airflow.
Next: Check Pilot Light
Check Pilot Light
Do: Look through access panel to see if pilot light is lit. If not, follow manufacturer relighting instructions.
Observe: Pilot light is out or flickering.
Means: Gas supply or ignition issue exists.
Next: Check Gas Supply
Check Gas Supply
Do: Verify gas valve is in 'On' position and gas company has not shut off service.
Observe: Gas valve is off or service is disconnected.
Means: Gas supply interruption prevents heating.
Next: Call Gas Company or Professional
Call Gas Company or Professional
Do: Contact gas provider to restore service or hire an HVAC technician for further diagnosis.
Observe: Gas service is restored or technician resolves issue.
Means: Professional intervention was required.
Next: Monitor Heating Performance

Common causes

Cause	Likelihood	Difficulty	Part often needed
Thermostat Battery Dead Low battery prevents thermostat from signaling furnace to heat.	Common	Easy	Thermostat battery
Dirty Air Filter Clogged filter restricts airflow, causing furnace to overheat or underperform.	Common	Easy	Air filter
Pilot Light Out Unlit pilot light prevents gas ignition in gas furnaces.	Sometimes	Easy–Medium	Igniter or gas valve
Gas Supply Interruption Gas valve turned off or utility service disconnected.	Sometimes	Easy	Gas valve
Faulty Limit Switch Overheating triggers switch to shut off furnace blower.	Sometimes	Medium	Limit switch
Blower Motor Issues Motor failure or belt slippage prevents air circulation.	Sometimes	Medium	Blower motor or belt
Ignition Control Module Failure Failed module cannot ignite gas, even with working pilot.	Rare	Hard	Ignition control module
Heat Exchanger Crack Cracks prevent proper heat transfer and trigger safety shutdown.	Rare	Hard	Heat exchanger replacement

Parts matching

Common parts needed for furnace heating issues include:

Thermostat battery (CR2032 or AA, depending on model)
Standard air filter (1x1x1 inch or custom size)
Igniter assembly for gas furnace
Limit switch with correct voltage rating
Blower motor or belt for your furnace model

Always match replacement parts to your furnace's model number for compatibility.

When to call a professional

Furnace makes loud noises or emits strange odors
Pilot light won't stay lit after multiple attempts
Thermostat shows error codes not listed in manual
House remains cold after replacing filter and checking gas supply
You detect a gas smell near furnace or vents
Furnace is over 15 years old with recurring issues

FAQ

Why is my furnace fan running but not blowing hot air?

This usually means the furnace is not igniting properly or the heat exchanger is malfunctioning. Check for gas supply issues or a faulty ignition system.

How often should I replace my furnace air filter?

Replace every 1–3 months depending on filter type and household conditions (e.g., pets, allergies).

Can a bad thermostat cause no heat?

Yes, a faulty thermostat or dead battery can prevent the furnace from receiving a heat signal. Test with a backup thermostat if possible.

What should I do if I smell gas near the furnace?

Immediately shut off the gas valve, evacuate the home, and call your gas company or emergency services.