The best thing you can do for your home is to have your furnace serviced before the biting winter cold sets in. But what if you haven’t been able to do that or have done the annual maintenance, but your furnace just won’t turn on anyway? How do you diagnose a furnace, and how can you fix it?
Your furnace isn’t turning on because it’s not getting power or gas, the furnace’s condensate pan is full, or the furnace is dirty, among other things. Potential fixes include checking for tripped circuit breakers, checking the thermostat, cleaning the furnace, and emptying the condensate pan.
Even if you don’t fix the furnace yourself, knowing how to troubleshoot ignition issues before calling a professional could save you time, money, and unnecessary freezing. This article provides you with a furnace troubleshooting checklist you can refer to when your furnace won’t turn on. I’ll also tell you how each issue is fixed so that you can try to do it yourself or talk to your local HVAC expert about the cost.
The Furnace’s Power Switch Is Turned Off
Have you at one time put an appliance upside down while trying to find out why it won’t start, only to discover later that the power button was off? Well, this could happen with your furnace too!
Whether you have a gas or electric furnace, your heater has a power switch located somewhere above or near the unit, which could easily be mistaken for a light switch and unintentionally turned off.
How To Fix
It’s a fairly simple fix if you’re aware that this switch exists, but many people don’t even know about it as it’s not something you use every day.
The solution is to flip the power switch to ON. You can locate the switch by looking around the furnace or referring to the owner’s manual. Remember where it’s located the next time your furnace stops working.
You Have a Tripped Circuit Breaker
I once went without power for hours and couldn’t understand why everyone around my house except me had power. I discovered later that my circuit breaker had tripped.
A tripped circuit breaker usually happens when there’s an issue with the circuit. It shuts off the flow of electricity to protect the circuit from overheating and causing damage. But a circuit breaker trip also reveals underlying issues that could be one of these three:
- A short circuit in the power system
- An overloaded circuit or power surge
- A ground fault
How To Fix
A tripped circuit breaker is usually in the middle position between OFF and ON, but it can also be in the OFF position. To fix a tripped circuit breaker, follow these steps:
- Check the switches and plugs in the room to ensure that they’re switched off.
- Find and open the circuit breaker box.
- Find the tripped circuit switch. Electricians usually label circuit breakers, so it’ll be easy to find the one for your furnace.
- If your tripped circuit breaker is between ON and OFF, move it to OFF first and then back to ON. If it’s already OFF, push it straight back to ON.
These steps should reset your power system. But don’t force the switch back to ON if it’s resisting or it re-trips. Instead, check your power system for overload or call a professional to check for damaged wiring, switches, or fixtures.
The following video explains how to check your HVAC circuit breakers:
Your Furnace’s Thermostat Has an Issue
A thermostat is one of the common culprits when a furnace isn’t sending heat to your home. It could be a wrong setting or a maintenance issue.
Here are a few common reasons why your thermostat isn’t working:
- Dust buildup
- A dead battery
- Faulty electrical wiring
- A dead fuse
- A timer error
On the extreme side, it could also be that the thermostat is irreparably damaged.
How To Fix
Fixing your thermostat will depend on the issue affecting it:
- Wrong thermostat setting: Ensure the thermostat is set to HEAT (not OFF or COOL) and at least 5°F (2–3°C) above room temperature.
- Dusty thermostat: Open your thermostat and carefully blow the dust and debris off with canned air or wipe it with a dry towel.
- Dead battery: If your thermostat runs on batteries, you need to change them regularly. Depending on the model, some thermostats have a low battery indicator light.
- Faulty electrical wiring: Inspect the wiring and look for loose connections or frayed wires that could be causing a dead electric line and have them replaced or repaired accordingly.
- Dead fuse: Identify and swap out dead fuses.
- Time error: Ensure the time, date, and timer on your thermostat are correct and up to date.
- Damaged thermostat: Consult with an HVAC expert. You’ll most likely have to replace your thermostat.
You Have a Clogged Furnace Filter
If you’ve been neglecting filter change, it’s likely accumulated soot, dirt, and dust that can compromise your furnace’s airflow.
In general, you need to check your furnace filter every 30 days and replace it every 90 days, especially during seasonal transitions.
A clogged furnace filter is often accompanied by a humming noise, which should make you suspect a dirty filter, especially if your furnace stops working after weeks of humming.
How To Fix
If your filter is dirty but otherwise in good condition, remove it, clean it, and put it back in. But a clogged furnace filter often means it’s worn out, so you’re better off replacing your filter.
If your filter has been replaced recently yet it’s full of dirt, you should inspect the furnace to see what’s causing the issue. These include:
- Carbon monoxide
- Damaged vents
The Furnace’s Flame Sensor Is Dirty or Faulty
In a furnace, the ignition/flame sensor is an essential part that senses flames and makes the furnace fire up. If it’s faulty, an ignition sensor can’t sense heat even if the furnace is burning hot and will, therefore, shut it down.
If your furnace clicks but doesn’t turn on or ignites and shuts down immediately, the ignition sensor is either dirty or faulty.
How To Fix
If your furnace’s ignition sensor is dirty, it’s easy to remove it, clean it, and put it back into place. If it’s faulty, you have to replace it. To determine if your sensor needs cleaning or replacement, follow these steps:
- Turn off the power for your furnace.
- Close the gas valve.
- Remove the ignition sensor by unscrewing and pulling it out of the furnace.
- Assess the ignition sensor to determine if it’s dirty or damaged. If the sensor is just sooty or corroded, use an emery cloth to clean it. A good quality emery cloth like the 3M Emery Cloth (Amazon.com) will get the job done. The cloth backing ensures no pieces of dirt are left on the sensor.
- If your flame sensor shows signs of damage, such as cracks or missing pieces, you’ll need to replace it.
- After cleaning, reinstall the cleaned sensor, switch on the furnace, open the gas valve, and ignite the furnace.
If the ignition problem isn’t resolved after cleaning and putting the sensor back, you’ll have to replace it. But first, ensure all other parts of the furnace are functional and not responsible for a non-igniting furnace to avoid changing a perfectly good flame sensor.
Your Furnace’s Gas Supply Is Interrupted
If the gas line isn’t sending gas into your furnace, the furnace won’t work. The gas supply for newer furnace models is controlled by an automatic combination gas valve that integrates the gas valve and the pressure regulator into a single unit.
As such, issues with the gas valve also mean an interrupted gas supply. The gas valve opens and closes to allow gas to flow to the furnace burners and pilot light.
How To Fix
The easiest and safest way to check if gas is flowing to your furnace is by turning on other appliances in your home that run on gas. If these appliances work, the gas interruption probably has something to do with the furnace. Here are a couple of things you should check:
Check the Gas Valve and the Gas Line Valve
Check the gas valve of your furnace to ensure it’s not closed. Note that a closed gas valve could indicate issues in the gas line, so you should be careful when handling it. Call a professional if you’re not sure.
If your furnace’s gas valve is open and other gas appliances are working, your furnace has other issues not related to the gas line. But if none of the gas appliances in your home can turn on, check the main gas valve that controls the gas flow into your home to ensure it’s not turned off before calling the gas supply company.
Check the Pilot Light
Most furnaces have a pilot light that lights up when gas flows to your furnace. If this light is switched off, gas isn’t flowing. A pilot light that’s turned off could indicate gas issues such as leaks that should be handled by a professional.
Remember that any signs of a leak (e.g., a foul gas smell or noise of leaking gas) should be handled immediately by closing the gas supply valve and calling the supply company.
Your Furnace’s Condensate Pan Isn’t Draining
The condensate drain pan in your furnace system is a container that collects condensed moisture. It has a drain and pump that removes the moisture and sends it to the wastewater drainage system.
If the condensate pan’s pump isn’t working or the drain is blocked, a float switch will trigger, stopping your furnace from turning on.
How To Fix
You’ll need to determine whether the condensate pan’s drain is clogged or the pump is jammed.
If the Pump Is Jammed
You’ll need to clean the pump if it’s dirty or move it if it’s stuck. Always clean your hands after handling a condensate pan pump as condensed water from the furnace contains acid. If cleaning or repositioning the pump doesn’t work, you might need to replace it.
If the Condensate Pan Drain Is Clogged
Clean the drain and remove the dirt causing the clog. Adding condensate pan tablets is also a good way to keep the drain clear. I recommend the high-quality SimpleAir Clean Flow HVAC Drain Line Treatment Tabs, which are activated by moisture. These tabs will prevent any form of growth in the drain line and can last up to 2 months.
Once both your condensate pan pump and drain are cleaned, you can switch the furnace back on, and it should fire up in a minute unless it has other issues.
Your Furnace’s Blower Motor Is Bad
A furnace blower motor is a component that circulates air from the furnace to the vents and through your home. If it’s not working, your entire heating system is compromised.
Here are a few indicators that your blower motor is overheating:
- A burning smell
- An unusual humming sound
- A furnace that won’t power on
Overheating can also be caused by dust and dirt buildup on the motor, making the blower unable to vent itself.
How To Fix
Troubleshooting a furnace blower motor isn’t exactly easy if you’re not an expert. If your furnace is displaying a blinking green light, it may be an indicator that something’s up with the blower motor.
Although you’ll probably have to replace the blower motor anyway, you can follow this video to troubleshoot it:
So, whether the blower motor is dirty, aged, or worn out, you’ll need to call a reliable HVAC expert to check and fix or replace it.
The Blower Capacitor Is Dead
The blower capacitor is a battery-like component attached to the blower. It sends a signal to the ignitor when the thermostat tells it and provides the steady power the blower needs to spin and keep the fan going.
Here are a few symptoms of a bad capacitor:
- Overheating causes a safety switch to trip and leads to the furnace shutting down and not igniting when it should.
- A burning smell when starting the furnace indicates frayed or corroded capacitor wires and a faulty power supply.
- A slow or overheating furnace blower means the capacitor doesn’t hold the charge needed to run the blower motor.
A faulty capacitor makes the blower malfunction, thereby compromising the furnace’s functionality by preventing it from turning on.
How To Fix
Capacitor failure is a common yet easy fix. That’s, of course, assuming you’re sure it’s the capacitor that’s broken, not something else.
Here’s what you have to do:
- Shut off the furnace.
- Open the panel and check what capacitor your furnace uses.
- Get an identical replacement capacitor.
- Unplug the old capacitor and plug the new one in. You may have to unscrew a few screws first.
- Close the panel and test the furnace.
You can watch this video just to see how easy it is:
Having the capacitor checked during the annual maintenance is the best way to ensure it always works.
There’s a Block in the Airflow
To run efficiently, your furnace needs good air inflow and outflow. So, if you’ve checked the issues above and there’s no problem with the ignition, you should shift your focus to the vents and pipes.
A foreign body could be blocking the PVC pipe, and if the fumes from the furnace cannot exit, it’ll stop working. Also, furniture blocking warm air blowing from the vents will prevent it from entering the room.
Instead, the warm air will be concentrated in and around your furnace, causing it to shut down from overheating.
How To Fix
Checking your vents and warm air outlets to ensure nothing is blocking them will prevent any issues related to poor ventilation. Ensure indoor vents are clear and check the outdoor heat vents to see if any objects are sitting in front of the PVC pipe.
Your Furnace Is Burning Too Much Gas
Furnace manufacturers have specifications for how much gas your furnace can burn at a given moment. When your home is frigid, you might try to regulate the furnace settings to get more heat, which means burning more gas than recommended.
If your furnace is overconsuming gas, it’ll overheat and automatically activate a safety mechanism that won’t let you turn it back on. Also, more gas burned means more dollars burned too.
How To Fix
Other than playing with the thermostat, there’s not much you can do here.
Ensure you have the annual furnace maintenance done, as this will include a combustion analysis to check if your furnace is burning the recommended amount of gas.
If you had adjusted the furnace to higher heat levels causing higher gas consumption, turn it back down to the recommended level. Seek the advice of an HVAC professional if you’re not sure about this issue.
Your Furnace Has a Damaged or Cracked Heat Exchanger
The heat exchanger is a furnace component that separates the combustion from the air exchange. It has metal chambers and passageways starting at the burner assembly and going to the chimney vent. Watch this video to learn what heat exchangers are and how they work:
If the heat exchanger fails, the furnace won’t work as it should, and it may fail to fire up altogether. There are three issues that signify a bad heat exchanger:
- Cracks: especially along the heat exchanger’s seams.
- Discoloration: mostly from soot that has escaped from cracks on the heat exchanger.
- A “jumping” flame: happens when cracks on the heat exchange allow air in the furnace to mix with the air outside.
A bad heat exchanger will also cause furnace ignition failure due to dangerous carbon monoxide leaks. Besides, cracked heat exchangers may also point to other furnace problems, such as low airflow.
How To Fix
As you can tell, a damaged heat exchanger can be a source of danger not just to your heating system but also to your health. The best remedy for a damaged or cracked heat exchanger is to replace it.
If you suspect that the damage on your furnace’s heat exchanger is because your entire furnace is aged and overworked, you should replace the entire furnace.
Now that we know all the possible causes behind a furnace not turning on, we still have an important question to answer: What if none of these troubleshooting steps worked, or you’re not sure what exactly you should do? Is it time to call an HVAC Technician?
When to Call a Certified HVAC Technician
I’ve already hinted that some furnace ignition problems will require you to call an HVAC specialist right away. But to give you more precise guidelines, I’ve listed five furnace ignition problems that call for a certified HVAC technician:
- If you’ve checked all the possible furnace ignition issues listed above and your furnace still won’t turn on, you need the expertise of a professional HVAC technician.
- If you smell gas or propane, switch off the gas supply and call a professional immediately.
- Don’t attempt to resolve the issue yourself if you suspect that your furnace has wiring issues.
- If your furnace’s ignition issue points to a bad motor, seek the services of a professional. Troubleshooting a motor without the technical know-how can be dangerous.
- If your furnace’s circuit breaker keeps tripping each time you reset it, this is an indicator that there’s a more serious underlying issue that only an HVAC expert can resolve.
Lack of maintenance is the primary reason your furnace won’t turn on. If it’s not serviced annually, your furnace is dirty or has faulty, damaged, or even dead components. This means it’ll have limited airflow, emit toxic gases like carbon monoxide, and fail to ignite and sustain the flames.
So, what’s the ultimate solution?
Ensure your furnace is cleaned and serviced by an HVAC professional before turning it on to keep you warm throughout winter.