7 Warning Signs Your Furnace Won't Make It Through the Winter

Every furnace that dies in January was sending signals in September. Not all of them, and not always loud ones. But most.

The trouble is that September is when nobody's paying attention to the furnace. It's off. It worked last winter. There's still corn at the farm stands on Chicago Drive. So the signals go unnoticed until the first cold night, when the furnace becomes the only thing in the house that matters and every heating company in Kent and Ottawa County is booked solid.

I've been running furnace calls around here since 1987, and I'd rather you spend twenty minutes in the basement now than have this conversation in November. So this is the list I'd go through if it were my house — seven signs, and for each one, whether it's a "plan for it" or a "shut it off."

1. It's Past 15 With a Repair History

Age by itself doesn't condemn a furnace. I've got customers running 25-year-old equipment that passes inspection every fall, and I'm not taking those out.

But age changes the odds, and age plus history changes them a lot. A furnace over 15 has cycled its heat exchanger through thousands of expansions and contractions. Its igniter, inducer motor, blower motor, and control board are usually the originals. If it's already needed a couple of repairs in the last few winters, those weren't unlucky one-offs. Those were the first parts to reach the end of their design life, and the rest of the parts are the same age.

Look at the data plate on the cabinet. The manufacture date is either printed outright or coded in the serial number. If you're past 15 and you've had a technician out more than once in the past three winters, put a star next to this one and keep reading.

2. Rust, Corrosion, or Water Staining

Walk around the furnace with a flashlight and look for three things: rust on the outside of the cabinet, rust or white chalky streaks inside the burner compartment, and water staining or drips at the flue pipe or the base.

Outside surface rust from a damp basement is mostly cosmetic. It's ugly but not dangerous.

Rust or mineral streaks inside, especially near the burners or the heat exchanger, means combustion moisture is condensing somewhere it shouldn't. On a standard-efficiency furnace that usually points to a venting problem or a furnace that's been running too cool. Either way, that moisture eats the heat exchanger from the inside, and a heat exchanger is the one part of a furnace you can't repair.

Water at the base of a high-efficiency furnace usually means the condensate drain is clogged or the trap is failing. That's a repair, not a replacement, but left alone it backs up into the secondary heat exchanger and becomes something worse.

3. The Flame Is Yellow, Orange, or Dancing

This is the one I want you to actually go look at.

Turn the thermostat up until the furnace fires, then look through the sight glass or the small opening on the burner door. A healthy gas flame is blue, steady, and roughly the same on every burner. It might have a small yellow tip. That's fine.

What you don't want is a flame that's mostly yellow or orange, one that flickers and rolls, or one that leans or lifts off the burner when the blower kicks on. Yellow means incomplete combustion, which means the burner is dirty, the air-to-gas mix is off, or the heat exchanger has a problem. A flame that shifts when the blower starts is a classic sign of a heat exchanger crack, because air from the blower is getting where it shouldn't and pushing the flame around.

Incomplete combustion makes carbon monoxide. If your flame looks like this, shut the furnace off at the switch and call. This is not a "watch it through the season" item.

4. It Short Cycled or Started Slow Last Winter

Think back to last February. Did the furnace fire, run a few minutes, shut off, and repeat all day? Or did you notice it clicking and trying two or three times before it finally lit?

Short cycling on a furnace has a handful of causes, and some are cheap. A clogged filter starves the heat exchanger of airflow, the furnace overheats, the high-limit switch shuts it down, it cools, it tries again. That's a filter. But if the filter is clean and it still cycles, the limit switch itself may be failing, or the heat exchanger is running hot for a reason we need to find. On an older furnace, a limit that trips repeatedly is a furnace that's been running hot for a long time.

Slow starts, where the igniter glows and glows before the burners catch, or it fails to light and retries, are usually a dirty flame sensor or a weakening hot surface igniter. Both are tune-up items. But an igniter that struggled last February will not be stronger this November. It'll be the part that fails on the first cold night.

Either symptom is a reason to get it looked at before heating season, and if you had both, that's two starred items.

5. Your Gas Bill Crept Up With Nothing Else Changing

Pull up last winter's gas bills against the winter before, or if you've been in the house long enough, a few winters back. Weather varies year to year, so this isn't a precise measurement. But if your usage went up noticeably and you didn't add square footage, change the thermostat habits, or lose a bunch of insulation, the furnace is a suspect.

Furnaces lose efficiency as they age — burners foul, heat exchangers scale up, blowers weaken, and the furnace runs longer to deliver the same heat. It's gradual, which is why nobody notices it month to month. Over five or six winters it adds up to a real number.

This one won't tell you the furnace is about to die. It tells you it's costing you more each year to keep alive, which is a variable that belongs in the repair-or-replace math. On a furnace that's also over 15 and needed repairs, the bills are the tiebreaker.

6. New Noises

Furnaces make noise. The question is whether it's making a noise it didn't make three years ago.

A bang or boom when it lights. That's delayed ignition, where gas builds up in the burner compartment before the igniter catches it. Dirty burners, a weak igniter, or a gas pressure problem. Each little explosion stresses the heat exchanger. Get it looked at.

Squealing or screeching. Blower motor bearings or a belt on an older belt-drive blower. It's a wear item and a repair, but a blower motor that seizes mid-January is a no-heat call.

Rattling or vibration. Sometimes a loose panel or a screw. Sometimes a blower wheel that's out of balance or has a cracked mounting. Take the panel off, tighten what you can, and if it persists it's ours.

Scraping metal on metal. Stop running it and call. That's usually a blower wheel dragging on the housing, and it will destroy the motor.

I wrote a longer guide to what each HVAC sound means, and the furnace section applies here. The short version is that a new sound means a part is changing, and parts don't change back on their own.

7. Last Winter's Repair Was "Good for One More Year"

If a technician stood in your basement last winter, fixed something, and said some version of "this'll get you through the season," you already have your answer about this fall. He was telling you the furnace was on borrowed time and giving you a way to put off the decision. Fair enough. But the season he was talking about is over, and now the question is whether you borrow again.

I do the same thing, for the record. I was in a basement in Wyoming a couple winters back with a 20-year-old furnace and a failed inducer motor. Homeowner didn't have the money for a replacement in January, so I put the motor in and told her plainly it would probably get her to spring and I wouldn't bet on the following winter. She called me the next August, before it got cold, and we replaced it on a schedule that worked for her instead of an emergency one. That's the version of this story that goes well.

If you got that speech last winter, the plan-ahead window is right now.

Scoring Yourself

Go back through the list. If you starred one item, and it isn't the flame or the CO items, get a fall tune-up and let the technician tell you what he sees. One sign by itself doesn't add up to a diagnosis.

If you starred two or three, especially if age is one of them, get the tune-up and ask directly for an honest read on how many more winters the furnace has in it. Then sit down with the repair-or-replace math while it's still warm out and you can think clearly. Knowing what a replacement would run in this market helps; our guide to furnace cost in Grand Rapids gives the honest ranges.

If you starred four or more, or if you starred the flame or a CO item, don't wait for a tune-up appointment to open up. Call now.

Why September, Specifically

None of the seven signs gets better on its own, and none of them cares what month it is. The reason to check now is about you, not the furnace.

In late August and September, we can get to you on a day that works, parts are on the shelf, and if the answer turns out to be a new furnace, you have room to get a second opinion, compare quotes, and schedule the install for a mild week. That's the calm version.

The other version is the first genuinely cold night in October or November, when the furnace that was showing three of these signs finally quits and you're deciding between a repair and a replacement at 11 p.m. with a space heater running. Everybody in that situation makes a worse decision than they would have in September, and I include myself.

Twenty minutes with a flashlight now. Book the visit online if you find anything on this list, or call and tell us what you saw.