A Simple Homeowner’s Guide to Furnace Troubleshooting (22 Common Problems & Fixes)

Square Footage	Furnace BTU
1,000-1,200 sq. ft. Home	40,000 - 60,000 BTU
1,200-1,500 sq. ft. Home	60,000 BTU
1,500-1,800 sq. ft. Home	60,000 - 80,000 BTU
1,800-2,500 sq. ft. Home	80,000 - 100,000 BTU
2,500-3,500 sq. ft. Home	100,000 to 120,000 BTU

Introduction

Let’s face it—you probably don’t think about your furnace until it stops working. And when that happens, it’s usually during a freezing Canadian winter, right when you need it the most.

But don’t panic! Many furnace problems can be fixed easily without calling a professional. All you need is a little knowledge, a careful approach, and this handy guide.

We’ll walk through the 22 most common furnace problems, what might be causing them, and how to fix them—or when it’s better to call a certified HVAC technician.

🛠️ 1. No Heat From the Furnace

If the blower is running but no warm air is coming out, it could be:

Thermostat set to “cool” or fan to “ON”
Faulty igniter or pilot light
Closed gas valve
Empty propane/oil tank

🔧 Fix It:
Check thermostat settings. Set the mode to “heat” and fan to “auto”. Then verify gas supply is active and, if applicable, relight the pilot. Still nothing? Time to get expert help.

🌡️ 2. Uneven Heating Across Rooms

Tired of your basement freezing while your upstairs feels like a sauna?

Causes:

Blocked vents
Dirty air filter
Poor insulation
Duct leaks
Furnace is too small or big

🔧 Fix It:
Open all vents, replace filters, and check for airflow blockages. If you suspect duct issues, consider professional duct sealing or inspection.

🔊 3. Strange Furnace Noises

Banging, clicking, hissing, or buzzing? These can signal:

Loose fan or motor parts
Dirty burners
Ignition issues
Air leaks or clogged filters

🔧 Fix It:
Turn off the unit and check screws, filter, and flame color. If it’s scraping or thumping, shut it down and call a pro.

🔁 4. Furnace Keeps Turning On and Off

This is called short cycling, and it’s bad for your furnace’s health.

Common causes:

Clogged filters
Faulty thermostat
Oversized furnace
Blocked vents

🔧 Fix It:
Change the filter, test thermostat batteries, and ensure airflow isn’t blocked. If it persists, schedule a system check to avoid long-term damage.

🔥 5. Furnace Not Igniting

Clicking but not firing? No flame? That’s likely due to:

Weak gas supply
Dirty igniter
Failing pilot light

🔧 Fix It:
Replace thermostat batteries, clean the igniter carefully, and ensure the gas line is open. If ignition still fails, don’t DIY—it’s a safety risk.

💨 6. Blower Motor Not Working

If your furnace isn’t pushing air, it could be:

Tripped breaker
Bad capacitor
Blocked filter
Worn drive belt

🔧 Fix It:
Reset breakers, change the filter, and inspect belts. If there’s no power to the blower motor, call an HVAC tech.

🧠 7. Faulty Thermostat

Symptoms include:

Furnace won’t start
Inconsistent temperatures
Random shutdowns

🔧 Fix It:
Change batteries, clean the inside, and ensure it’s not near a heat source. Consider upgrading to a smart thermostat for more accurate control.

🧼 8. Dirty or Clogged Filters

Dirty filters = low airflow = high energy bills.

🔧 Fix It:
Replace filters every 1–2 months in winter. If reusable, vacuum + wash it gently and let it dry before reinserting.

📖 Related: How to Replace a Furnace Filter

💧 9. Condensate Drain Problems

Standing water near your furnace? Your condensate drain line may be blocked.

🔧 Fix It:
Vacuum the line, flush it with vinegar, or clear it with a flexible brush. If leaks continue, check for cracked tubing or faulty pumps.

🔥 10. Pilot Light Keeps Going Out

Could be:

Drafty room
Dirty intake
Malfunctioning thermocouple
Weak flame (low oxygen)

🔧 Fix It:
Relight using the manual’s instructions. If it won’t stay lit or burns yellow/orange, shut down the system and call a licensed tech.

⚠️ 11. Faulty Limit Switch

A bad limit switch can shut your furnace down for safety, especially if it’s overheating.

🔧 Fix It:
Turn off power and test it with a multimeter. If you’re not sure, leave this one to the pros.

🛑 12. Carbon Monoxide Leak

This is extremely serious.

Signs:

Headaches or dizziness indoors
Pilot flame is yellow
Soot stains or rust
CO detectors going off

🚨 Emergency:
Evacuate immediately. Call 911. Install CO detectors near the furnace and bedrooms. Don’t wait!

🌡️ 13. Thermocouple Problems

The thermocouple is a small safety device that shuts off gas if the flame isn’t burning.

🔧 Fix It:
Use a multimeter to test output (should be 25–30 mV). If below, replace it. Clean any corrosion on the tip.

⚡ 14. Electronic Ignition Failure

Modern furnaces often use electronic igniters which can fail due to:

Surge damage
Wrong part installed
Dirty filter causing shutdown

🔧 Fix It:
Call a pro to match correct voltage and replace if needed.

🌬️ 15. Air Duct Issues

Leaky or blocked ducts reduce airflow and comfort.

🔧 Fix It:
Inspect visible ducts and seal joints with UL 181 metal foil tape. Avoid cloth duct tape. Consider duct inspection services for older homes.

💦 16. Furnace Leaking Water

Main culprits:

Cracked condensate line
Clogged trap or pump
Evaporator coil drainage failure

🔧 Fix It:
Check for visible leaks and clean drainage areas. Replace faulty pump if necessary.

🔌 17. Furnace Not Turning On

No sound, no lights?

Check:

Thermostat batteries
Tripped breakers
Safety switches
Gas supply

Still nothing? Call a technician.

🌬️ 18. Furnace Blowing Cold Air

Fan on “ON” mode
Pilot light out
Overheated heat exchanger
Low gas supply

🔧 Fix It:
Set fan to AUTO, check pilot, and change filters. If the air is still cold, your system may be overheating or misfiring.

🔥 19. Burning Smells

Types:

Dusty: normal on first use of season
Plastic or metallic: melting wires
Rotten: dead animal in duct

🔧 Fix It:
Turn off furnace, clean filters, and inspect ducts. Call a pro if smell persists.

📖 Read more: Burning Smell from Furnace – Causes & Fixes

⚙️ 20. Pressure Switch Issues

If stuck open, furnace won’t fire up. Could be:

Blocked vent
Bad inducer motor
Defective switch

🔧 Fix It:
Clear vents, inspect the motor. Replace switch if needed (or call a tech).

🌀 21. Furnace Fan Issues

Not working: check breaker, filter, wiring
Always on: thermostat set to ON, or faulty fan limit switch

🔧 Fix It:
Switch to AUTO mode. Replace faulty parts if necessary.

🧊 22. Furnace Shuts Off Before Reaching Temp

This usually points to:

Overheating
Oversized unit
Dirty filter
Thermostat issues

🔧 Fix It:
Replace filter, test thermostat, and check airflow. Still happening? Call a pro for diagnostics.

When to Call a Professional

Call an HVAC tech if:

You smell gas or CO
You see water damage
The furnace shuts off repeatedly
Thermostat or ignition fails
You hear loud or strange sounds

🔧 Pro Tip: Book an annual tune-up before winter hits.

🧰 Furnace Maintenance Checklist

Replace filters monthly
Test smoke & CO detectors
Inspect thermostat
Clear around furnace vents
Seal window/door drafts
Schedule annual inspection

Conclusion

Furnace acting up? Don’t sweat. With this guide, you now have the knowledge to handle the most common issues confidently. And remember, when in doubt—play it safe and call a trusted HVAC pro.

🏡 Keep your home warm, safe, and comfortable all winter long.

👉 Compare quotes from certified local HVAC experts now