Heating guide
Furnace Trouble? 8 Warning Signs, Quick Fixes, and When to Replace
What to check before you call for service, the signs your furnace is wearing out, and the safety warnings you should never ignore.
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The short version
- If you smell gas or a carbon monoxide alarm goes off, leave the house first and call for help from outside.
- Before calling for service, check the thermostat, the air filter, the furnace power switch and the breaker. These solve a surprising number of 'no heat' calls.
- Most gas furnaces last roughly 15 to 20 years. An older furnace that needs a major repair is usually worth replacing.
- Rising gas bills, uneven heat and frequent repairs are early signs a furnace is wearing out.
The first cold night of the year is when most furnace problems show up. A system that sat idle all summer gets switched on, and small issues that went unnoticed suddenly become a cold house.
This guide covers what to check yourself, the signs that your furnace is near the end of its life, and how to decide between a repair and a replacement.
Safety first: two warnings you should never ignore
If you smell gas (a rotten-egg smell), don’t touch light switches, the thermostat or any appliances, and don’t use your phone inside. Get everyone out of the house, then call your gas company or 911 from outside.
If a carbon monoxide (CO) alarm sounds, get everyone outside into fresh air and call 911. Carbon monoxide has no smell, and symptoms like headache, dizziness, nausea and confusion can feel like the flu. Don’t go back in until responders say it’s safe.
If you have a gas furnace and don’t already have working CO detectors, install them near bedrooms and on every level of your home. Test them when you turn the heat on for the season.
Check these before you call for service
Many “no heat” calls turn out to be something simple. Run through this list first:
- The thermostat. Make sure it’s set to Heat, not Cool or Off, and set a few degrees above the room temperature. If the screen is blank, replace the batteries.
- The air filter. A clogged filter chokes airflow, and many furnaces shut themselves off to avoid overheating. If you can’t see light through it, replace it.
- The power switch. Most furnaces have a switch that looks like a regular light switch, mounted on or near the unit. It’s easy to flip off by accident. Make sure it’s on.
- The breaker. Check your electrical panel for a tripped breaker labeled furnace or HVAC. Reset it once. If it trips again, stop and call a technician.
- The furnace door. Most furnaces have a safety switch that keeps the system off if the front panel isn’t fully seated. Make sure it’s snapped firmly in place.
- The gas supply. Make sure the gas valve to the furnace is open (the handle runs in line with the pipe) and that other gas appliances, like the stove, are working.
- The drain line (high-efficiency furnaces). If your furnace has white plastic vent pipes, it makes water as it runs. A clogged drain line or a full condensate pump can shut it down.
If none of these get the heat back on, it’s time to call a professional. Don’t try to repair gas valves, burners or wiring yourself.
8 signs your furnace is wearing out
1. It’s getting old
Most gas furnaces last roughly 15 to 20 years, and well-maintained units sometimes go longer. Check the data plate inside the furnace door for the manufacture date, or search your brand plus “serial number date” to decode it.
2. Your heating bills keep rising
If your gas or electric bills climb year after year and your rates and habits haven’t changed much, the furnace may be losing efficiency as parts wear out.
3. Some rooms are always cold
Uneven heat can come from duct problems or poor insulation, but it can also mean the furnace or its blower can no longer move enough warm air through the house.
4. It turns on and off constantly
Short bursts of heat followed by shutting off, over and over, is called short cycling. It can be caused by a dirty filter, a faulty sensor, a thermostat problem or a furnace that’s too big for the house. Whatever the cause, it wears the system out faster.
5. New or louder noises
Some noise is normal. Watch for banging or booming when the burners light, squealing or scraping from the blower, or rattling that wasn’t there before. These usually point to wear that will get worse.
6. The burner flame looks yellow
On furnaces where you can see the burners, the flames should be mostly blue. A lazy, yellow or orange flame can mean incomplete burning, which can produce carbon monoxide. Have it inspected promptly.
7. You’re calling for repairs every winter
One repair in a few years is normal. Repairs two winters in a row, especially to major parts, are a sign the system is near the end.
8. The air feels dry or the house feels dusty
More dust, soot near vents, or noticeably stale air can point to a failing furnace, dirty ducts, or a cracked part that needs inspection.
Repair or replace?
The same rule of thumb that works for air conditioners works for furnaces:
Age of the furnace × cost of the repair. If the answer is more than $5,000, replacement is usually the smarter long-term choice.
| Situation | What usually makes sense |
|---|---|
| Under 10 years old, minor repair (igniter, flame sensor, thermostat) | Repair it |
| 10 to 15 years old, mid-size repair | Compare the repair against replacement quotes |
| 15+ years old and a major repair (heat exchanger, blower motor, control board) | Get replacement quotes |
| A cracked heat exchanger on an older furnace | Replacement is almost always the right call |
A cracked heat exchanger deserves special mention. It’s the part that keeps combustion gases separate from the air you breathe, so a crack is a safety issue, not just a performance one. If a technician reports one, ask to see photos or evidence, and consider a second opinion before spending thousands.
Your replacement options
If you do replace, you’ll usually choose between:
- A new gas furnace. The most common choice in colder climates and homes that already have gas. High-efficiency models cost more up front but use less fuel.
- A heat pump. It heats and cools with electricity, by moving heat rather than burning fuel. Modern cold-climate models work in much lower temperatures than older heat pumps.
- A dual-fuel system. A heat pump that handles most of the heating, with a gas furnace that takes over on the coldest days.
The best choice depends on your climate, fuel prices where you live, your existing ductwork and whether you need a new air conditioner too. A good contractor will walk you through the trade-offs and size the system with a proper load calculation.
Keep your furnace healthy
- Change the filter every one to three months during heating season, more often with pets.
- Schedule a tune-up once a year, ideally in early fall before the first cold snap.
- Keep the area around the furnace clear. Don’t store paint, gas cans or boxes against it.
- Keep vents and returns open and unblocked by furniture or rugs.
- Test your CO detectors monthly and replace batteries as needed.
Before you hire anyone
Get at least two or three quotes, in writing, with model numbers. Read our checklist of questions to ask before hiring an HVAC contractor. If your air conditioner is also aging, our guide on whether to repair or replace your AC can help you decide whether to replace both at once.