That Old Furnace Kicked On Last Night. Did You Hold Your Breath?
It’s the first cold night of fall. The furnace clicks, rumbles, and a small voice in your head asks: Is this thing still safe?
If your gas furnace is more than 10 years old, that question is fair. It’s unsettling when the machine keeping your family warm is also burning gas a few feet from where they sleep.
Here’s the good news. Most furnace dangers give off warning signs long before they turn serious. Good furnace maintenance is how you catch them. In this guide, you’ll learn what actually goes wrong inside an older furnace, the signs you can spot yourself, what a real safety inspection should include, and when it’s time to stop repairing and start replacing.
What’s Really Happening Inside a 10-Year-Old Furnace?
A gas furnace burns fuel inside a sealed metal chamber called the heat exchanger. Your blower pushes house air across the outside of that hot metal. Meanwhile, the exhaust gases, including carbon monoxide (CO), stay inside and vent out through the flue. That thin wall of metal is the only thing keeping combustion gases out of your air.
Why Metal Gets Tired
Every time your furnace runs, the heat exchanger heats up and expands. When it shuts off, it cools and shrinks. That happens thousands of times each winter.
After 10 or 15 years, that constant flexing can cause stress cracks. They usually start small, at seams, welds, and bends. Rust speeds things up.
Here’s the thing most homeowners don’t know: age alone isn’t the biggest cause. How the furnace has been treated matters more.
The Three Habits That Crack Heat Exchangers
The first is a dirty filter. Low airflow means the metal can’t shed heat, so it runs hotter than it was designed to. The second is closed vents. Shutting off unused rooms seems smart, but it chokes airflow the same way. The third is an oversized furnace. A unit that’s too big for your home heats fast, shuts off, and restarts again and again. That short cycling adds wear with every start.
Sound familiar? The fix for all three is simple, and it’s cheaper than a new furnace.
The Coastal Virginia Twist Nobody Talks About
Most furnace articles are written for dry, cold places. Hampton Roads isn’t one of them.
Our humid summers create a problem you won’t find on most checklists. In many homes, the AC coil sits right on top of the furnace. All summer long, that coil drips water. If the drain pan is cracked, clogged, or rusted, that water can drip down onto the furnace below.
Look, a heat exchanger that sits wet all summer rusts from the outside in. It can look fine from the front panel while rust flakes off inside.
Furnaces in crawl spaces and attics face the same risk, since damp air is hard on metal. That’s why crawl space encapsulation and insulation can actually protect your heating equipment, not just your floors.
When a Tighter House Changes the Rules
There’s one more whole-home detail worth knowing. Older 80% efficiency furnaces pull the air they burn from inside your home. If you’ve added new windows, sealed air leaks, or upgraded your attic insulation, your house is tighter than it used to be. That’s great for your energy bill.
But a very tight house can starve an older furnace of combustion air. When that happens, exhaust gases may not vent the way they should. A good technician checks draft and combustion air after any big upgrade to your home. It’s a simple test that’s easy to skip.
Warning Signs You Can Check Yourself
You don’t need tools to spot trouble. You just need to know where to look. Here’s a quick guide.
| What You Notice | What It Might Mean | How Urgent |
|---|---|---|
| CO alarm sounds | CO in your home from any fuel-burning source | Emergency. Get everyone out and call 911 |
| Headaches, nausea, or dizziness that ease when you leave the house | Possible low-level CO exposure | Urgent. Get fresh air and call a pro today |
| Burner flame is yellow, lazy, or flickers | Poor combustion or a venting problem | Call for service soon. Don’t ignore it |
| Flame jumps, bends, or “dances” when the blower starts | Air pushing through a possible crack | Shut the furnace off and call for service |
| Black soot around the furnace door or vents | Incomplete burning of gas | Call for service soon |
| Rust or water stains on or under the furnace | Leaking drain pan or condensate | Schedule an inspection |
| Furnace turns on and off every few minutes | Short cycling from airflow, sizing, or sensor issues | Schedule service |
| Heavy moisture on windows during heating season | Possible venting issue | Have it checked |
This list doesn’t replace a professional look, since many cracks hide deep inside the unit. But if you notice any of these, don’t wait for your yearly tune-up.
The CO Alarm Fact That Surprises Most People
Standard home CO alarms are built to avoid false alarms. Under the UL 2034 safety standard, an alarm doesn’t have to sound at 70 parts per million until CO has been present for one to four hours.
What does that mean for you? A small, slow leak might never set it off. Yet low levels over weeks can still cause headaches and fatigue. This is especially true for older adults, babies, and people with heart or lung problems.
So keep working CO alarms on every level of your home and near every sleeping area. Replace them based on the date printed on the back, usually every 5 to 10 years. If you want more peace of mind, a low-level CO monitor shows readings well below the standard alarm point.
What Real Furnace Maintenance Looks Like
It’s frustrating to pay for a “tune-up” that turns out to be a filter swap and a quick glance. With an older furnace, here’s what a real safety visit should include.
The Heat Exchanger Inspection
A technician should remove the burners or use a camera to look inside the heat exchanger. A flashlight from the front doesn’t cut it. They should watch the flame with the blower on and off, too. A flame that changes when the blower starts is a red flag.
Combustion and Draft Testing
This is the step many companies skip. A trained tech uses a combustion analyzer to measure CO right in the flue gases, as well as oxygen levels and draft. They should also check the air around the furnace and in your living space. Numbers beat guessing every time.
The Safety Controls
Your furnace has built-in guards: a flame sensor, a high-limit switch, a rollout switch, and a pressure switch. Each one shuts the furnace down when something’s wrong. Each one gets tested. Your tech should also check the gas pressure and look for leaks at every fitting.
Airflow and the Whole System
Filters, blower, ducts, and the AC drain pan above the furnace all get checked, because airflow problems are what cook heat exchangers. Even your thermostat settings can cause short cycling.
At Fusion Home Services, our NATE-certified techs look at the furnace as part of your whole home, not as a lone box in the closet. Because we started in insulation and air sealing, we check how your ducts, crawl space, and house tightness affect the way your furnace breathes. That’s how we find the real cause the first time.
So You Found a Crack. Now What?
Take a breath. A cracked heat exchanger is serious, but you have options. Stop using the furnace until it’s fixed or replaced, and ask to see photos or camera footage. You have every right to see the proof.
Repair or Replace?
On a furnace older than 10 years, replacing the heat exchanger rarely makes sense. Labor is heavy, and the rest of the furnace is just as old. Check your paperwork, though. Many manufacturers offer long heat exchanger warranties to the original owner, so the part itself may be covered. Labor usually isn’t.
The exception is a furnace that’s only a few years past 10 with a strong warranty and no other issues. Then a repair might be worth pricing out.
For most older units, a new furnace installation is the safer long-term choice. It’s also a chance to right-size the system. If your home got tighter over the years, you may need a smaller furnace than the one you have now. And a modern sealed-combustion furnace pulls its air from outside, which removes the combustion air problem entirely.
Worried about cost? That’s normal. Fusion Home Services offers financing options so a safety fix doesn’t have to wait for a better month. New installs also come with free first-year maintenance and a one-year satisfaction guarantee.
When a Repair Is Enough
Not every scary sound means a crack. Dirty flame sensors, worn igniters, and failed inducer motors are common in older furnaces and are very fixable. Our heating repair team will tell you plainly which problem you have.
How Often Should an Older Furnace Be Checked?
Once a year, ideally in early fall before the first cold snap. Between visits, change your filter every one to three months. Keep supply and return vents open. Keep boxes, paint cans, and laundry away from the furnace. And test your CO alarms monthly.
A maintenance plan takes the remembering off your plate. Your yearly visit gets scheduled for you, so the safety check actually happens.
Frequently Asked Questions
How much does furnace maintenance help an older furnace last?
Regular furnace maintenance won’t stop metal from aging. But it removes the biggest causes of early cracks: poor airflow, short cycling, and moisture. Many well-kept gas furnaces reach 15 to 20 years, while neglected ones can fail much sooner.
Can a cracked heat exchanger leak carbon monoxide?
Yes, it can. A crack can let combustion gases mix with the air your blower sends through your home. Not every crack leaks CO right away, but it’s an unsafe condition that should be addressed before you run the furnace again.
Will my CO alarm catch a cracked heat exchanger?
Not always. Standard alarms are designed to ignore low levels for hours. That’s why a combustion test during a maintenance visit, plus a low-level CO monitor, gives better protection than an alarm alone.
Why does the location of my furnace matter in coastal Virginia?
Humidity. Furnaces in crawl spaces, garages, and attics face damp air, and units under an AC coil can get dripped on all summer. That moisture rusts parts from the outside. Checking the drain pan and controlling crawl space moisture protects the furnace.
Peace of Mind Is Worth a Yearly Visit
Older gas furnaces crack from heat stress, poor airflow, and oversizing. In Hampton Roads, summer moisture adds rust to the mix. You can spot some signs yourself. The hidden ones take a camera and a combustion analyzer.
Your CO alarm is a last line of defense, not your first.
You shouldn’t have to lie awake listening to the furnace kick on. Let Fusion Home Services give your system a true safety inspection, with photos, test readings, and straight answers. Learn more about our furnace services, or contact us to book your visit. Need help tonight? We answer 24/7 at (757) 938-6709.
Have a question about your own furnace? Leave it in the comments below. And when the heat’s not right, Call Fusion First.
