7 Signs Your Furnace Is Dying (Before It Quits in January)

Here’s the rule in Iowa: never wait until January to figure out your furnace is on borrowed time. Once we hit a -15°F stretch, HVAC companies across Polk County get booked days out, parts get backordered, and you’re sleeping in your living room next to a space heater for the weekend.

These are the 7 things we look for on a service call that tell us the furnace is on its way out. Catch any of these in May, June, July, August — that’s when we have the time to do it right.

If your furnace is making any of the noises below right now, call (515) 494-5483 and we’ll come look. Quotes are free. (Our furnace repair and replacement page has typical prices.)

1. Cold Air Coming Out of the Vents

When a furnace is healthy and the thermostat calls for heat, you get warm air within about 60 seconds. When it’s dying, you get a minute of cold air, then maybe lukewarm, then the system shuts off before the house ever gets warm.

What we check: ignitor (the $40 part that lights the gas — these go out constantly on furnaces 8+ years old), flame sensor (a $30 rod that tells the board “yes, there’s a flame, keep the gas open”), and the gas valve itself. Most “cold air” calls turn out to be an ignitor or flame sensor. The rest are a dying gas valve, a control board that’s losing its mind, or a heat exchanger that’s cracked badly enough to be tripping the high-limit switch. The first two are cheap. The last one is replacement time.

2. Short Cycling

Short cycling means the furnace fires up, runs 90 seconds, shuts off, fires back up 2 minutes later, runs 90 seconds, shuts off. Over and over.

This is the furnace overheating and tripping its high-limit safety — usually because of a restricted airflow problem (filter, blower wheel, closed registers) or because the heat exchanger is cracked and the safety is reading temps that shouldn’t exist there. We start with the easy fixes: filter, blower wheel cleaning, duct check. If those are clean and it’s still short cycling, the conversation shifts to heat exchanger and whether the furnace is worth saving. On a 12-year-old 80% AFUE unit, usually no — you’re better off putting that money toward a new 96% high-efficiency unit that will noticeably cut your gas bill.

3. Yellow or Flickering Flame

Flip open the inspection panel while the burners are firing (most have a sight glass). What you want to see: tight blue flames, steady, no flickering, no yellow tips.

What’s bad: yellow flames, lazy orange flames, flames that flutter or roll out. Yellow means incomplete combustion. Incomplete combustion means the furnace is making carbon monoxide. The fix is usually a burner clean, sometimes a draft pressure adjustment, sometimes it’s the heat exchanger cracking and pulling room air into the combustion chamber. None of these are DIY. If you see yellow flames, get a CO detector battery checked tonight and call us in the morning.

4. Soot, Rust, or Black Streaks Around Vents

Walk by your supply vents in a few rooms. Pull a register off if you’re up to it. Black streaks on the wall above the vent, rust crumbs in the duct, soot on the filter? Those are signs of bad combustion happening somewhere upstream.

We see this most in 20+ year old furnaces in older Des Moines homes (Beaverdale, Drake, the South Side), often paired with an oversized furnace that’s been undersizing the heat exchanger for two decades. Once you’ve got soot in the ducts, it’s never just “clean the ducts.” It’s: clean the ducts AND figure out why combustion is dirty AND fix the source. Source is usually furnace replacement time.

5. Weird Smells

Three smells that mean something specific:

• Burning electrical smell at startup. Blower motor windings overheating or a cracked capacitor. The $200 capacitor is the easy fix. The $700 blower motor is the medium fix. The dying control board feeding bad voltage is the bad fix.
• Dirty sock smell. This is bacterial growth on the evaporator coil (yes, in winter — the coil sits there year-round). It rides through the ducts when the blower kicks on. Coil cleaning fixes it. UV light prevents it from coming back.
• Sulfur or rotten egg smell. Stop. That’s natural gas. Leave the house, call 911 or your gas utility’s emergency line from outside, then call us. Do not flip switches, do not light anything, do not even open the garage door with an opener.

6. Banging, Rattling, or Booming on Startup

Furnaces should start quietly. A click, a whoosh of the inducer fan, then the burners light. That’s it.

Sounds that mean something:

• Boom on ignition. Delayed ignition. Gas is pooling in the burner chamber, then lighting all at once. Caused by dirty burners, weak ignitor, or a partially blocked flue. This is a safety issue. The “boom” is pressure cracking the heat exchanger over time.
• Rattle that comes and goes with the blower. Loose blower wheel or a failing blower motor bearing.
• High-pitched whine. Inducer motor going out — usually $300–$450 to replace, and an easy diagnose.
• Squealing belt. You have an old belt-drive blower (1990s or earlier). Belt replacement is $80, but it’s also a sign your furnace is geriatric.

7. Gas Bill Climbing With No Lifestyle Change

You haven’t added square footage. You haven’t dropped the thermostat any colder. The kids are still home, the dog is still grumpy. But your January MidAmerican bill went from $180 last year to $260 this year.

That’s efficiency loss. As furnaces age, the heat exchanger gets sooty (insulating the heat transfer), the burners run dirty, the blower motor windings lose efficiency, and you burn more gas to produce the same heat. In a Des Moines climate, where heating season runs about six months, the gas savings from a new 96% AFUE unit add up fast.

Iowa Reality Check

Don’t wait. A furnace that’s giving you 2 or 3 of these warning signs in May will absolutely die in January, and when it does, every HVAC shop is buried.

We schedule replacements all summer at a relaxed pace. That means better pricing, the install crew isn’t rushed, you pick the unit you actually want, and you’re never the family camping in the living room when the house is 38°F.

Carrier and Trane are our recommended brands for the metro — both have good local parts availability. We’d rather install a mid-tier Carrier than a top-tier off-brand any day. And while you’re getting ready for winter, it’s worth reading up on preventing frozen pipes too.

If you noticed something on this list, call (515) 494-5483 — we’ll come out, look at the furnace honestly, and tell you whether you’ve got 2 years left or 10. Quotes are free.