Furnaces almost never quit without warning, and honestly a lot of the furnace repair calls we run start with somebody saying, “You know, it’s been making this sound for a couple of weeks.” Sometimes it’s a noise and sometimes it’s a smell nobody can place. Other times it’s a back bedroom that never quite gets warm, or a gas bill that jumped for no reason anybody can think of. Catch those early and you’re usually looking at a smaller repair and a smaller bill, and you get to stay in a warm house instead of sitting up waiting on a technician during the coldest night of the year.
We’d pay closer attention than usual this winter if you live in central or north Georgia. There’s a Super El Niño in the forecast, and our winters under El Niño tend to run wet and moody, warm enough for shorts one week and then hit by Arctic cold the next, usually with an ice storm or two mixed in. Your furnace feels every one of those swings, since it ends up starting and stopping far more than in a steady winter, and each start chips away at the parts that keep it going.
And if you’re the type who’d rather wait and see, these numbers might change your mind:
- A furnace that’s struggling costs you money quietly. Heating and air conditioning are the biggest slice of what a home uses, 52% of a typical household’s annual energy use in the U.S. Energy Information Administration’s figures, which is why a furnace that’s only slipped a little still ends up on your bill.
- Heating problems pile up right when it’s coldest. We were honestly surprised by this one, but the National Fire Protection Association averaged 37,365 home heating equipment fires a year from 2020 to 2024, and close to half of them hit between December and February.
- Some of the worst furnace problems are invisible. By the CDC’s count, more than 20,000 emergency room visits a year come from unintentional carbon monoxide poisoning, along with roughly 450 deaths, and a damaged furnace that’s leaking it gives off no smell for you to catch.
We’ve sorted the furnace repair warning signs below by what you’d hear, feel, see and pay, and with each one we’ll tell you what we’d do if it were our house.
Sounds and smells that signal trouble
Chances are you’ll hear or smell a furnace problem well before you see anything wrong. Most furnaces we work on make a soft click when they start, a quiet whoosh as the burners catch and then a steady hum from the blower, and once yours starts doing something louder or sharper than that, or anything that makes you look up from the TV, it’s worth a closer listen and maybe a furnace repair call.
Banging, booming or rattling noises from the furnace
If you hear a bang or a boom right as the furnace kicks on, what’s usually happening is that gas is building up for a split second before it lights. We call that delayed ignition, and most of the time it’s dirty burners or a weak igniter behind it. It’ll make you jump, sure, but the bigger problem is the stress it puts on the heat exchanger, the metal chamber that keeps the combustion gases out of the air you’re breathing.
Rattling is a different animal, and it nearly always means something worked itself loose, like a panel, a screw, a duct connection or a piece inside the blower. The trouble with a little rattle is that the vibration shakes other things loose over time, so either one is a good excuse to have a furnace repair tech come take a look (our post on what strange furnace noises are telling you goes through more of them).
Squealing, screeching or grinding
That high-pitched squeal usually traces back to the blower motor, or on older furnaces, a belt that’s worn thin. Grinding or scraping is the one that worries us more, because it can mean the motor bearings are on their way out, and when bearings go, the blower can stop altogether, which means no warm air reaches the rooms even with the burners lit and working fine.
So if you hear grinding, turn the furnace off and give us a call. Running a motor on bad bearings is a good way to burn it out, and then a furnace repair that would’ve been easy becomes a whole new motor.
Burning, musty or gas smells
Every fall, the first cold day brings a dusty, burning smell for a few minutes, and that’s completely normal, since dust settles on the heat exchanger all summer and burns off once the furnace fires up. If the smell’s still there the next day, though, or it smells more like hot plastic or something electrical, shut the system off and call a professional (we explain why your heat smells funny when you first turn it on in more detail).
A musty smell usually points to moisture or mold in the ductwork or around the furnace, and that’s a lot more likely in a wet El Niño winter. Rotten eggs is something else entirely. Natural gas has that smell added on purpose so a leak can’t sneak up on you, and if you ever catch it, get out of the house right away and call your gas utility once you’re outside.
Heating performance warning signs
Sometimes there’s no noise and no smell, just a house that doesn’t feel right. If the furnace is running and your comfort keeps slipping anyway, that’s the system telling you it isn’t doing its job the way it should be.
Cold air or not enough heat
When the vents are blowing cool or lukewarm air while the thermostat is calling for heat, we’d start with the easy stuff. Check that the thermostat is on heat and not fan only, with the setting above room temperature, and then pull the air filter, because a badly clogged filter can make the furnace overheat until a safety switch shuts the burners off.
If those all look fine, the problem’s probably inside the furnace. A dirty flame sensor, an igniter that’s failing or a hiccup in the gas supply can keep the burners from lighting or from staying lit, and on a heat pump, low refrigerant or a stuck reversing valve can give you the exact same cold-air complaint. Those are all jobs for a trained technician on a proper furnace repair visit.
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Uneven temperatures from one room to the next
No house is perfectly even, and a room that runs a degree or so cooler than the rest isn’t anything to lose sleep over. What we’d want to hear about is the living room feeling fine while the kids’ bedrooms stay chilly all evening. That usually means the furnace isn’t moving enough air, and when we go looking, we tend to find a weak blower motor or a dirty blower wheel, or else a duct leaking somewhere you’d never see it.
You’re also paying for it, because the system keeps running and running trying to get those far rooms up to temperature. When we come out for that kind of furnace repair, our tech measures the airflow and checks the blower first, and then climbs into the attic or the crawl space to hunt for leaks or a duct section that’s pulled loose.
Short cycling
Short cycling is what we call it when a furnace fires up, runs a few minutes, quits, and then starts the whole routine over again. Because it never finishes a full heating cycle, the house never quite gets comfortable, and the furnace is beating itself up with all those extra starts in the meantime.
Most of the short cycling we fix comes down to a dirty flame sensor or a furnace overheating from poor airflow. Sometimes it’s the thermostat, mounted too close to a lamp or a sunny window, and every so often it’s a furnace somebody put in that was simply too big for the house. In a winter full of temperature swings, short cycling piles extra wear on the igniter and everything else that works during start-up, which makes it one of the furnace repair warning signs we’d most want checked before the coldest weeks get here.
Safety red flags that mean furnace repair right now
Some warning signs are about comfort and money. The ones in this section are about your family’s safety, though, so if you notice any of them, turn the furnace off and get a professional out right away rather than waiting until it suits your schedule.
A yellow flame or soot around the furnace
A gas furnace should burn with a steady flame that’s mostly blue, and a lot of them have a little window where you can see the burners without opening anything up. A flame that’s gone yellow or orange, or one that flickers, tells you the fuel isn’t burning cleanly, and when we track it down it’s usually dirty burners, not enough air getting to the furnace, or something wrong with the venting.
Dirty burning like that makes more carbon monoxide. Look for black soot, too, since that’s what dirty burning leaves behind, usually around the furnace itself or on the vent pipe, and sometimes near the registers. Soot is the kind of furnace repair we’d want to get to this week, not next, and in the meantime please keep your hands off the burners and the gas valve.
Carbon monoxide alarms or symptoms nobody can explain
Carbon monoxide can get into the house through a cracked heat exchanger or a flue that’s blocked, and there’s nothing to see or smell when it does, so honestly your detector is about the only thing standing guard. When that alarm sounds, don’t stop to investigate. Get everybody out into the fresh air first, then call 911 or your gas utility, and keep everyone outside until they say it’s clear.
Watch how your family’s feeling too. Headaches, dizziness, nausea or flu-like symptoms that get better when you leave the house and come back when you return can point to carbon monoxide exposure, and pets may act sluggish as well. Any of that calls for medical attention right away and a professional inspection before the furnace runs again (here’s why we’d check your carbon monoxide detectors before winter).
Ignition problems and tripped breakers
Newer furnaces light with an electronic igniter instead of a standing pilot light. If you hear clicking over and over and the burners never light, or the furnace tries to start a few times and then locks itself out, the igniter, the flame sensor or the gas valve could be going, and on an older furnace, a pilot light that keeps going out is the same kind of warning.
Then there’s the furnace that keeps tripping its breaker, which we’d never brush off, since it can be a short circuit, a motor on its way out or a wiring problem. Resetting it once is fine. If it trips a second time, though, leave it off and call for furnace repair, because a breaker that won’t stay on is sometimes an electrical hazard waiting to happen.
Hidden furnace repair warning signs in your bills and records
Not every furnace repair warning sign shows up in the furnace closet. You’ll find some of them on the utility bill and some in that drawer full of repair receipts, and a few are really just about how many years the furnace has been in the house. Paying attention to them is how you tell whether one more furnace repair is money well spent, or whether you ought to start planning for what’s next.
Energy bills that keep creeping up on you
Pull up this winter’s gas or electric bills and set them next to the same months from last year. Sure, rates change, and a colder month is always going to cost more. But if the bills keep climbing and you can’t think of a reason, the furnace is probably losing efficiency somewhere. We usually find dirty burners or a blower that’s struggling, though leaky ducts do it too, and so does a heat pump compressor that’s starting to fail.
When heating and cooling make up more than half of what your home uses, even small losses get expensive in a hurry. Part of a good furnace repair visit is figuring out where the efficiency is going, and then telling you honestly whether a repair, a deep cleaning or some duct work is what’ll bring those bills back down.
An aging furnace
Taken care of, most gas furnaces last roughly 15 to 20 years, and heat pumps often a little less, since they’re working all year round. The manufacture date is usually on the label of your indoor unit, or in the paperwork from when you bought the house.
Being old doesn’t automatically mean a furnace needs replacing, and we work on plenty of older systems that still run great because somebody kept up the maintenance. As one gets close to the end of its expected life, though, the parts wear faster, the efficiency drops and the heat exchanger gets more likely to crack, and that’s when these warning signs deserve a closer look.
Frequent repairs and rising repair costs
One furnace repair every few years is normal. Several repair calls over the last two winters is a pattern, and when the same furnace needs a new igniter one year, a blower motor the next and a control board after that, those costs stack up fast.
A rule a lot of techs go by is to compare the repair cost against what a new system is worth, with the furnace’s age figured in. When a big repair on an older furnace starts eating up a large share of what replacement would cost, it can be smarter to put that money toward a new, more efficient system, and a technician you trust can lay both options out for you (we go into it more in when to repair vs replace your furnace).
What to do when you spot a furnace repair warning sign
Spotting the sign is the easy part. Whatever you do next is what decides if this stays a quick furnace repair or turns into a cold and expensive night, so this is the plan we’d follow in your shoes.
Quick checks before you call
There are a few problems with simple causes you can rule out by yourself. Start at the thermostat, since a dead battery or a setting stuck on cool fools more people than you’d believe. We’ve also shown up to more than a few furnaces that were just switched off, usually because somebody mistook the service switch next to the unit for a regular light switch, so take a look at that, and if the breaker for the furnace has tripped in the panel, reset it once before you go any further.
The filter is the next suspect, and if yours has turned gray and clogged, it’s due. Air has to get out of the vents and back into the returns for any of this to work, and over a year the furniture has a way of drifting, so go see if a couch, a toy bin or a rug has wandered over one of them since last winter. Got a high-efficiency furnace? Then finish up outside by the intake and exhaust pipes, where leaves and debris like to collect and where ice can close them off after a storm, and that one simple check has saved plenty of folks a furnace repair bill. Did the heat come back and stay on? Then just keep an eye on it. If it didn’t, that’s when we’d want you to call for furnace repair.
When to call right away
Some signs can’t wait for a convenient time. A gas smell or a carbon monoxide alarm means everybody goes outside first and then you call. For soot, a yellow flame, a burning electrical smell or grinding from the blower, shut the furnace down and call for furnace repair right away instead of waiting to see if it gets better.
A furnace that won’t heat in the middle of a cold snap is urgent too, especially in a house with young kids, older adults or anybody with health concerns. With the forecasters warning about ice storms and sharp cold this winter, we’d have a heating company you trust picked out ahead of time, and our emergency service line is there for exactly those nights.
What happens during a furnace repair visit
A good furnace repair visit starts with a conversation. The first thing our furnace repair tech will ask is what you noticed and when it started, and that’s where any notes you kept really pay off. After that they’ll test their way through the whole system, starting at the thermostat and the electrical controls and working through the burners and flame sensor to the blower and the venting.
When they find the cause, you should hear about it in plain English, with a clear price, before anybody picks up a wrench. And on an older furnace, or one that’s needed a lot of furnace repair already, a good tech will tell you straight whether fixing it or replacing it makes more sense for your house and budget, and then leave that call to you.
Frequently asked questions about furnace repair warning signs
What are the most common signs my furnace needs repair?
If you asked our furnace repair techs, they’d probably put odd noises at the top, banging and grinding especially. Cool air from the vents comes up a lot too, and so do rooms that never warm up, a furnace that keeps turning itself on and off, and a bill that climbs when nothing about your routine has changed.
Is a burning smell from my furnace normal?
On the first cold day, for a few minutes, yes, because that’s just a summer’s worth of dust burning off the heat exchanger. If it’s still there the next day, or it smells like hot plastic or wiring, we’d want a furnace repair tech to take a look.
What should I do if I smell gas near my furnace?
Get out of the house first, and don’t flip any light switches on your way, then call your gas utility from outside and stay out there until they tell you it’s safe to go back in.
Why does my furnace keep turning on and off?
What you’re describing is short cycling. In our experience it’s most often a dirty flame sensor or a clogged filter that’s making the furnace overheat, and less often a thermostat in a bad spot or a furnace that’s just too big for the house.
What color should my furnace flame be?
A healthy flame burns steady and mostly blue. Yellow, orange or flickering is a different story, and so is soot around the furnace, since those can point to incomplete combustion and more carbon monoxide, which is a good reason to call for furnace repair.
How long does a furnace usually last?
With somebody keeping up the maintenance, a gas furnace usually gets about 15 to 20 years before furnace repair calls start getting frequent. Heat pumps often come in a little under that, because they’re heating and cooling all year long.
Should I repair or replace an older furnace?
We’d look at how old it is, how much furnace repair it’s needed lately and what this particular repair costs next to a new system. Any furnace repair tech worth calling should lay both options out side by side for you, so you can pick whatever fits your home and budget.
Fast, honest furnace repair when you need it
We like to think of these warning signs as your furnace asking for help while there’s still time. Act on them early and you’ll usually keep the house warm and the bills reasonable, and keep your family safe, which counts for even more in an unpredictable Super El Niño winter.
Our furnace repair techs at The Air Conditioning Specialist work in homes all over Covington, Milledgeville, Augusta, Athens and the surrounding Georgia communities, tracking down what’s wrong and fixing it right the first time. They’ll walk you through it without the jargon, and you’ll have your options and the price in front of you before any furnace repair work starts. If some of these warning signs are sounding a little too familiar, give us a call, and we’ll get somebody out to your place before the next cold snap does.