A well-functioning furnace is a home’s main source of warmth and comfort during cold weather. But when a furnace stops working properly, it not only causes discomfort, but can also affect safety and energy bills.
Thousands of homes experience a variety of common HVAC system problems each year – including furnaces that won’t turn on, don’t blow warm air, make noises, or waste energy.
In this guide, we’ve selected the 10 most common furnace problems that homeowners experience frequently. Each problem is explained in three sections:
- Problem and its cause – so that you can find out why the problem is occurring.
- Repair solution – so you can make some basic repairs yourself or talk to a professional about it.
- Maintenance tips – so you can prevent the same problem from recurring in the future.
This guide will not only increase your knowledge, but also help you keep your home safe, warm and energy-efficient.
10 Most Common Furnace Problems
- Furnace Won’t Turn On
- The Furnace is Blowing Air But Not Heating it
- Furnace is turning on and off repeatedly
- Strange noises coming from the furnace
- Weak airflow
- Strange or Foul Odors from the Furnace
- Pilot Light Goes Out Frequently
- Blower Fan Runs Continuously
- High Energy Bills
- Thermostat Not Working Properly
Easy Furnace Repairs for Homeowners
1. Furnace Won’t Turn On
This is one of the most common complaints, especially at the beginning of winter. If your furnace won’t turn on, no sound or activity in furnace, even when thermostat is turned up, there could be several reasons.
Reasons Furnace Won’t Turn On:
- Incorrect thermostat setting or malfunction
- Tripped circuit breaker
- Closed power switch
- Pilot light blown out (in older models)
Repair Solution:
- Check thermostat – is it in “HEAT” mode and the temperature set higher than room temperature?
- Replace thermostat batteries.
- Check switch near furnace unit – is it on?
- Open circuit breaker panel – reset furnace breaker if it is tripped.
- If pilot light is blown out, relight it according to safety instructions.
Tips to avoid turn on problem:
- Check the thermostat, breaker and switch before the season starts.
- Replace the thermostat battery every 6 months.
- Have the pilot system of an older furnace serviced.
2. The furnace is blowing air but not heating it
If the furnace is running but is unable to heat the room, it means there is a problem with the heating element or gas supply.
Signs: Cold air in the room, no difference even after increasing the thermostat temperature, furnace running continuously but no heat.
What cause furnace turns on but no heat?
- Clogged or dirty air filter
- Bad ignition system
- Interruption in the gas supply
- Not getting the right signal from the thermostat
Repair furnace not heating:
- Remove the air filter – if it is dirty, install a new one.
- Check the ignition system (spark igniter or hot surface ignitor) – if it has failed, it will have to be replaced.
- Check the gas supply valve – is it open?
- If everything looks okay, have the thermostat checked for wiring or sensing errors.
Maintenance Tips to avoid Furnace with no heat:
- Change the air filter every 1-2 months.
- Have the furnace inspected by an HVAC professional annually.
- It is helpful to have the thermostat calibrated.
3. Furnace is turning on and off repeatedly (Short Cycling)
Short cycling means the furnace is turning on and off repeatedly in just a few minutes. This not only puts a strain on the unit, but also the home is not being heated properly.
Signs: Frequent furnace turning on and off, incomplete heating, high energy bills.
What Causes Furnace turning on and off repeatedly?
- Clogged air filter
- Incorrect thermostat location
- Overheating
- Blower motor failure
Repair Furnace turning on and off repeatedly:
- Replace the air filter immediately.
- Check the thermostat – it will give incorrect readings if it is installed near a heater or something hot.
- Check the blower motor and heat exchanger – the unit may be overheating.
- Have a professional check the unit’s complete service and safety sensors.
Tips to avoid Furnace turning on and off repeatedly:
- Install the thermostat in a location where the temperature remains stable.
- Regularly clean/change the filter.
- Annual furnace tune-up is a must.
4. Strange noises coming from the furnace
If there is a metallic rattling, whirring or rattling noise coming from the furnace, it could be a sign of a serious mechanical problem.
Signs: Noises like grinding, banging, whining, or squealing.
What causes strange noises coming from the furnace?
- Loose parts
- Bad belt
- Defective blower wheel or motor
- Heat exchanger crack
Repair furnace strange noises:
- Turn off the furnace and check for any loose parts.
- Tighten or replace the belt.
- Have the blower motor and wheel inspected.
- If the noise is loud, contact an HVAC technician immediately.
Tips to avoid strange noises from furnace:
- Lubricate all moving parts at the beginning of each season.
- Repair immediately if you hear noises – don’t delay.
5. Weak airflow
Weak airflow means your furnace isn’t moving enough air into the room — which doesn’t raise the temperature in the home. This often indicates a ventilation or duct problem.
Signs: No air reaching rooms, air is too slow when it is coming, furnace runs too long.
What causes weak airflow:
- Dirty or clogged air filter
- Leaky or blocked ducts
- Bad blower motor or blower fan
- Clogged or partially clogged vents
Repair furnace weak airflow:
- Remove the air filter first – if dirty, replace it with a new one.
- Open all vents and make sure they are not blocked.
- Check the ductwork – are there any leaks, holes or blockages?
- If the problem persists, have an HVAC technician check the blower motor.
Tips to avoid furnace weak airflow:
- Change filters every 2-3 months.
- Have duct cleaning done every 1-2 years.
- Don’t block vents with furniture or curtains.
6. Strange or Foul Odors from the Furnace
Furnace odors are often caused by dust, gas leakage, or mold in the duct system.
Signs: Burning plastic smell, gas smell, or foul smell.
What causes foul odors from the furnace?
- Burning dust at the beginning of the season
- Gas leaks
- Mold or dead animal remains in the duct
- Overheating in the blower
Repair foul odors from the furnace:
- If the furnace is turned on for the first time and the smell is felt for a short time, it may be normal.
- If there is a gas-like smell, turn off the furnace immediately and call the gas company – this is an emergency.
- Get the ducts checked – if there is a foul smell, get professional cleaning done.
- If there is an electrical or burning plastic smell, turn off the power of the furnace and call a service engineer.
Tips to avoid foul odors from the furnace:
- Install a CO (carbon monoxide) detector.
- Have ducts and vents cleaned annually.
- Don’t ignore any strange odors.
7. Pilot Light Goes Out Frequently
In older furnaces, the pilot light often fails to stay lit, causing the unit to shut down frequently.
Signs: Furnace shuts off frequently, pilot light goes out, no warm air.
What causes pilot light goes out frequently:
- Thermocouple failure
- Dirt in pilot orifice
- Draft or air
- Lack of gas pressure
Repair pilot light which goes out frequently:
- Check thermocouple – if bad, it must be replaced.
- Clean pilot light assembly – remove dust/carbon.
- Make sure there are no open windows or vents blowing air into the unit.
- If problems persist, upgrade to an electronic ignition system.
Tips to avoid pilot light goes out frequently:
- Have the pilot system serviced before each season.
- Have the thermocouple cleaned once a year.
- Look for electronic ignition options when purchasing a new furnace.
8. Blower Fan Runs Continuously
If the furnace fan runs continuously, even when no heating is needed, it may be a problem with the thermostat or limit switch.
Signs: The furnace motor runs continuously, but no heat is coming.
What causes blower fan runs continuously
- Thermostat FAN mode set to “ON”
- Bad limit switch
- Signaling error
Repair blower fan when runs continuously:
- Check the thermostat – is the FAN mode set to “AUTO”? If not, set it immediately.
- If the fan still doesn’t stop, have the limit switch replaced by a professional.
- Reset the thermostat and have the wiring checked.
Tips to avoid blower fan runs continuously:
- Test the thermostat regularly.
- Have the furnace control panel serviced annually.
9. High Energy Bills
If your electric or gas bills have suddenly increased, the furnace may not be working to its full potential or consuming too much power.
Signs: High bills even without extra use, furnace running longer.
Why furnace causes high energy bills?
- Dirty air filters
- Leaks or lack of insulation
- Old or low-efficiency furnace
- Incorrect thermostat settings
Repair solution to avoid high energy bills:
- Replace air filters.
- Check duct and window insulation.
- Set the ECO setting on the thermostat.
- If the furnace is 15+ years old, consider a high-efficiency model.
Tips to avoid high energy bills:
- Use a smart thermostat.
- Have duct sealing and a home energy audit done annually.
- Having a professional tune-up of the HVAC system saves costs.
A well maintained furnace don’t waste energy and save you money on energy bills.
10. Thermostat Not Working Properly
The thermostat is the control center of the furnace. If it is giving incorrect readings or not responding, heating is affected.
Signs: Furnace does not respond to set temperature, screen is off, strange behavior.
What causes thermostat stop working:
- Dead battery
- Wiring problems
- Sensing fault
- Thermostat failure
Repair Thermostat:
- Replace the batteries first.
- Open the thermostat and check the wiring – no loose connections.
- If the digital screen is blank, reset it.
- Install a new smart thermostat if the unit is old or faulty.
Avoid thermostat not working problem:
- Check the thermostat calibration every year.
- Replace batteries every 6 months.
- Check the thermostat along with furnace servicing.
Final Advice:
Annual professional servicing of your HVAC system is the best way to prevent all these problems. Timely maintenance can save you from cold discomfort, costly repairs and high energy bills.