Every January we get the same call: “It was working fine, then it just stopped.” It was never working fine. Furnaces almost always telegraph their death for weeks — homeowners just don't speak the language yet. Here are the five signs we see every fall on Aurora furnaces that don't survive the winter, ranked by how much time each one buys you.
On this page
- 1. It's 15+ years old and never had a real tune-up
- 2. Your heating bills climbed with no rate change
- 3. Short cycling: on-off-on-off every few minutes
- 4. Rooms that used to heat fine now don't
- 5. New smells: musty, metallic, or (worst) formaldehyde-sweet
- What to do this week
- What the January emergency actually looks like
1. It's 15+ years old and never had a real tune-up
Age alone doesn't kill a furnace — neglected age does. A 15-year-old furnace with annual tune-ups has plenty of life left. A 15-year-old furnace that's never been opened? The heat exchanger has been thermally cycling twice a day for a decade and a half with nobody checking it for cracks.
In 80011's housing stock, original-equipment furnaces from the 2005–2010 builder wave are hitting this window right now. If you don't know the last service date, assume there wasn't one and book the tune-up. This is the single highest-leverage thing on this list.
2. Your heating bills climbed with no rate change
Pull last winter's gas bills and compare therms used, not dollars — rates move, usage tells the truth. A steady 10–20% climb in gas usage for the same thermostat setting means the furnace is working harder for the same heat: dirty blower wheel, weak airflow, burners out of tune, or a heat exchanger losing efficiency.
Efficiency doesn't plateau and then cliff-dive; it erodes. The winter it finally can't keep up is the winter the inducer motor or the exchanger gives out under the extra strain. Rising bills are the furnace asking for maintenance in the only language it has.
3. Short cycling: on-off-on-off every few minutes
A healthy furnace runs 10–15 minute cycles. If yours fires up, runs three minutes, shuts off, and repeats, something is wrong — overheating on a clogged filter or restricted duct, a flame sensor dropping the call, or an oversized unit that's always been wrong for the house.
Short cycling is brutal on components: the ignitor and gas valve take 5× the normal cycles per day. Every week of short cycling burns through ignitor life that should have lasted the season. This sign buys you the least time — weeks, not months.
4. Rooms that used to heat fine now don't
When the back bedroom goes cold while the thermostat says 70, homeowners blame the room. It's usually the furnace: weakening blower output, a blower wheel caked in dust, or duct connections that have separated in the attic over years of thermal movement.
The danger isn't the cold room — it's what the cold room indicates about airflow. Low airflow overheats the heat exchanger, and overheated exchangers crack. Cold spots in October are a tune-up item. Cold spots in January are an emergency call.
5. New smells: musty, metallic, or (worst) formaldehyde-sweet
Musty smell on first fall startup is usually dust burning off — normal for a day. A persistent metallic or electrical smell is overheating components. And a sweet, almost formaldehyde-like odor can indicate a cracked heat exchanger leaking combustion byproducts — that's a shut-it-down-and-call situation, especially if anyone has headaches or nausea when the heat runs.
Your nose is a legitimate diagnostic tool. New smells from the vents are never “just the furnace getting old.” They're the furnace filing a complaint.
What to do this week
If you recognized one sign: book the fall tune-up and mention the symptom when you call — it tells the tech where to look first. Two or more signs, or a furnace over 15: ask for the tune-up plus a heat-exchanger-focused safety check. If the furnace is 18+ with multiple symptoms, ask us for the honest repair-vs-replace math before you spend repair money on a unit with one winter left.
The theme of all five: October problems are cheap, January problems are expensive, and the difference is just timing.
What the January emergency actually looks like
To make the stakes concrete, here's the anatomy of the call we run hundreds of times each January:
11:40 PM: furnace stops. House is 68°F and falling about 2–3°F per hour (faster in wind). You call; we're honest that the overnight tech is on another no-heat call and yours is next.
2:15 AM: tech arrives, diagnoses a failed ignitor — a part that costs little and rides on every truck. Twenty minutes later: heat. Total cost: after-hours diagnostic plus the part.
The alternative timeline: October tune-up finds the ignitor testing weak. Replaced in daylight, scheduled, at standard rates, with the diagnostic waived into the tune-up visit. Same part, same tech, roughly half the cost, zero frozen-pipe anxiety at 2 AM.
Multiply that by the failures that aren't a simple ignitor — the cracked exchanger found at midnight means no heat until a replacement is installed, which means space heaters and possibly a hotel. October you is the only person who can prevent January you's worst week. Be October you.
Seeing any of these five? Don't wait for January
Fall tune-up with full safety check — we'll tell you straight how many winters yours has left.
Frequently asked questions
My furnace is 20 years old but runs fine. Should I replace it preemptively?
Not necessarily — but get the safety inspection. If the heat exchanger is sound and efficiency is reasonable, keep it maintained. If it's showing two or more signs above, price the replacement now so January doesn't price it for you.
How long do furnaces last in Colorado?
15–20 years is typical for gas furnaces here. Dry air is actually kind to equipment; what kills them is neglected maintenance and oversized short-cycling units from the builder era.
Is a tune-up worth it on a furnace I'm planning to replace?
If replacement is happening this fall, skip it. If it's “maybe next year,” the tune-up is cheap insurance — and the written report tells you exactly how urgent “next year” is.
What does a cracked heat exchanger actually risk?
Carbon monoxide entering your home's air supply. It's odorless and colorless, which is why the annual check and a working CO detector are non-negotiable on older furnaces.
Do you offer free second opinions on replacement quotes?
Yes — if another company told you the furnace is dead, we'll verify the diagnosis before you spend thousands. Bell's old offer was a good idea; we just aim it at everyone.
If my furnace dies at 2 AM, what do I do while waiting?
Don't use the oven or grill for heat (carbon monoxide risk). Space heaters are okay attended and away from flammables. Drip faucets to prevent frozen pipes if the house drops below 50°F. Then call — our after-hours line is for exactly this.
PROUDLY SERVING AURORA, CO 80011