11 Signs Your Furnace is in Need of a Repair

Well, it’s that time of year again - time to crank the heat! Snow needs shovelling, cars need clearing, and there’s no relief quite like a cozy, warm home after being out on a winter’s day.

But if Jack Frost settles in and your furnace kicks out, it can be mighty inconvenient. Keep your home warm and comfortable over the holidays by knowing how to tell your furnace is in need of repair.

Note: If your furnace has not been checked over by a professional in over 2 years, it should undergo a maintenance inspection. This way, you can be sure that your furnace won’t break down unexpectedly when you need it the most. Ideally, furnace maintenance should be scheduled once a year.

Higher Than Usual Energy Bill

It’s common for energy bills to go up during the cold season, but a severe spike in costs could mean that your furnace isn’t at its best. However, it could also mean that your home is not properly insulated. 

What It Means:

In a nutshell, your furnace is running inefficiently - which is obvious by your soaring utility bill. But what’s causing the inefficiency? It could be as simple as needing to change your furnace filter, but if that doesn’t help, here are a few other things to try.


Check your thermostat and make sure it’s set correctly. The issue could be that it has been wired incorrectly. If your home is empty during the day, be sure your furnace isn’t going full-blast while nobody’s around by checking the timer presets. If you have an older furnace, updating the thermostat with the right replacement could help. 

It’s also a good idea to go through your home and check for any drafts. There could be a new air leak that wasn’t noticeable over the summer and is letting all of your heat out, causing your furnace to run more often.

Unfortunately, this issue can be hard to diagnose if your average troubleshooting methods don’t succeed. Many different systemic issues could result in your furnace burning more fuel than it needs to.

Odd Smells

After a few months of sitting dormant, it isn’t unusual for a furnace to have a bit of a musty smell. But if strange odours persist while your furnace is running, you should investigate. Depending on the type of smell you’re noticing, there could be a serious issue at hand - one that may even result in a dangerous situation. 

If you are smelling something burning when your furnace comes on and the smell does not quickly subside, shut off your furnace right away.

What It Means:

If you notice an odd smell when you first turn your furnace on for the season, this is fairly typical and should dissipate quickly - it’s likely just the dust that has settled through your vents and furnace over the summer while it wasn’t in use. 

However, if you notice smokey or chemical smells that are lingering, this is a sure sign that something is wrong. You should turn off your furnace immediately and contact a professional if this is the case.

If you smell rotten eggs, it is likely an issue with your natural gas supply - suppliers will often treat the gas to smell this way so that leaks are more easily detected. If this is the case, you should follow the above advice as well as contact your natural gas company. 


A musty smell, or a smell akin to dirty socks, is a simple indicator that there is moisture or bacteria trapped in your furnace. It will likely clear within a day or so. If it doesn’t, you may have an infestation of mould on your hands. Having your ducts cleaned can help to mitigate these types of issues.

If you’re smelling burning oil, this probably means that you need to change your furnace filter. If that doesn’t stop the smell, you should contact a pro to determine whether there is an oil leak.

Strange Noises

Older furnaces will occasionally protest their work with some grunts and groans, and even newer models may experience the occasional hiccup. The materials of your home expanding with the heat could result in strange creaks and knocking.

But if you hear more than the odd disturbance here and there - or you are noticing the same strange noise over and over again - it’s best to call a professional to have a look. 

What It Means:

Different problems will cause different noises. The first step is to do your best to find out where exactly the noise is coming from.

If you’re noticing a strange whistling noise when the furnace starts blowing, it’s likely an issue with your ductwork. If it’s a rattling or vibrating noise, it could be that there is a screw or bolt loose somewhere. 

A banging noise could just be your ducts moving with air pressure and temperature changes, but it could also be ignition roll-out - a small explosion that can cause big trouble.


Figure out where the noise is coming from. If it’s simply loose ductwork or another minor hardware problem, you could likely fix the issue with some DIY handiwork. But if you’re not sure about how serious the disturbance is, call a technician to be safe.

Inconsistent Heat Flow

A sure sign that your furnace needs repair is severe fluctuations in temperature. If your home’s heat is constantly fluctuating, or you can’t seem to get heating in a specific part of your house, it’s likely something is off with your furnace.

What It Means:

Unfortunately, there are many different reasons that your furnace isn’t maintaining the preferred temperature. In this situation, it’s especially important to understand the problem before looking for a solution. After all, what good is a furnace if it’s not keeping your home comfortable?

Be sure to get as specific as possible in what type of temperature problem you’re experiencing. Is your home getting very hot then very cold, or just leaning to one extreme? Is the furnace running but blowing cold air? Or is it heating one part of the house but not the rest? 


If your furnace is blowing cold air, check your fan switch. (See the troubleshooting section of number 8.) 

If the temperature keeps going up and down without finding a balance, it might not be your furnace that’s the issue. Your furnace or ductwork could have been improperly sized to your home when installed. Alternatively, your home’s insulation may be lacking - the heat could be leaking out, or too much is rising to the top of the home. 

It could also be that you have the wrong thermostat system for your home. The thermostat may not be detecting the temperature in certain parts of your home correctly, resulting in the wrong temperature.

The Thermostat Isn’t Working

We usually recommend that you check your thermostat for mishaps before scheduling a repair call. Sometimes, the settings or timer may need to be reset for your home to reach the desired temperature. Other times, it simply hasn’t been turned on!

But if fiddling around with your thermostat doesn’t do anything to change the temperature, the issue probably lies with the furnace itself.

What It Means:

The thermostat is faulty, or wiring between the thermostat and furnace needs to be fixed or replaced. 


Check your thermostat settings to ensure that there isn’t a timer or preset turned on and overriding things. Reset your thermostat and furnace (with guidance from the manual or a professional if needed). 

Hard to Start, or Keeps Turning Off

Is your furnace giving you a hard time when it’s time to start it up? Is it taking more time than normal to produce heat, or does it keep turning off unexpectedly?

If you’re thinking “yes” in response to any of the above, it’s likely your furnace requires repair. It’s best to call a licensed technician before the problem gets worse and results in a total break down.

What It Means:

There are a few reasons that this could be happening. If your furnace is not getting enough airflow because of a dirty filter or blockage, then your furnace might automatically shut off.

Alternatively, it could be a faulty thermostat sending the wrong signals to the furnace. 


Check your vents for blockages, including the exhaust pipe. If debris such as sticks and leaves or an animal nest is found, clear it out to see if this helps. Check inside your home around the vents to be sure that there isn’t furniture blocking them. If you suspect something has fallen down the vent that is causing the blockage, you may need to call a professional to investigate.

Checking Breakers & Flipping Switches 

We often get calls that are quickly resolved with a trip to the circuit box. Sometimes flipping a breaker back on is all it takes to get a furnace going again. A tripped switch such as the blower door safety will prevent your furnace from running.

But if you’ve tried flipping all the switches and checked that all the safeties are secure, and your furnace still isn’t running properly - you’re probably in need of a repair. 

What It Means:

Unfortunately, if you’ve checked all of the breakers, safeties, and switches, it’s likely that whatever the issue is, you’ll need a pro to fix it.


Go through your owners manual to be sure you haven’t missed any presets or safeties. There could be a safety or setting that you weren’t aware of, which is affecting your furnace’s performance.

Changing the Filter

So you’ve noticed that your furnace isn’t pushing as much hot air as it should be. Sometimes a clogged furnace filter is the reason, plain and simple. If you have recently undergone renovations or you have pets, your filter may need to be changed more frequently. 

However, if you change the filter but notice all still is not right - it’s time to call a technician.

What It Means:

If you’ve changed the furnace filter, but it hasn’t helped with the problem, it could be one of many issues. Your thermocouple, pilot light, or fan switch could be the cause.


Check your fan settings to ensure the fan is not blowing cold air - if the fan switch is set to “ON” or “Manual Overdrive,” then the fan will blow regardless.

Also, check your pilot light to be sure that it’s working properly, as we touch on in the next point.

The Pilot Light is the Wrong Colour

This is a sure sign that your furnace needs to be fixed. Your pilot light should be bright blue - not yellow, red, or orange. A more “normal” looking flame with these colours is not normal for a furnace pilot light. 

What It Means:

The pilot light is burning inefficiently due to a lack of oxygen. This could even mean that the flame is leaking carbon monoxide into the air, which is extremely dangerous. Alternatively, it could just be that there is a build-up of dirt on the gas intake valve.

In other instances, it may be that the thermocouple is malfunctioning.


Taking care of a faulty thermocouple or gas intake valve is best left to a professional as it can be dangerous if not done properly.

 It Is Constantly Running

If your furnace is always cycling on and off, or it seems to be constantly running, there is a good chance that your furnace needs a repair. This issue will likely result in higher energy bills, whether your furnace is pumping out heat or not.

What It Means:

  1. It could be that the fan limit switch is faulty and needs a repair or replacement.
  2. There is faulty wiring in the thermostat or furnace itself.


  1. Locate your fan limit switch and determine whether it is on the right setting. If it’s in manual override, change it to the correct setting as directed by your manual.
  2. Make sure that your thermostat set correctly.

Lack of Regular Furnace Maintenance

Have you been keeping up with your furnace maintenance? It’s recommended across the board that your furnace should undergo a maintenance check about once a year, before the cold season hits. 

If a licensed technician recommends a repair be made during a maintenance check, it’s best to get it taken care of. Regular maintenance and repairs keep your family cozy and help prevent costly breakdowns or larger repairs.