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Sounds and smells, decoded

Furnace Noises and Smells: What’s Normal and What Isn’t

Some sounds and smells are part of a furnace’s normal life.

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24/7 · Licensed & insured · Pueblo & Pueblo West

Carbon monoxide alarm on a hallway wall outside a bedroom, with a night light below it

Some mean turn it off and call us. One means get everyone out of the house — that’s first.

Gas smell or CO alarm: leave first, then call 911

If you smell rotten eggs or sulfur

  1. Get everyone out of the house right away.
  2. Don’t flip light switches, use the garage door opener, or make a call while you’re still inside — a spark can ignite gas.
  3. From outside and away from the house, call 911 or Xcel Energy’s 24-hour gas emergency line, 1-800-895-2999. Xcel supplies gas inside Pueblo city limits and in part of Pueblo West. If your gas bill comes from Colorado Natural Gas (part of Pueblo West), call 911 and the emergency number printed on that bill.
  4. Stay out until the gas company or fire department tells you it’s safe.

If a carbon monoxide alarm goes off

  1. Get everyone outside into fresh air immediately. Don’t stop to look for the source.
  2. Call 911 from outside.
  3. Don’t go back in until responders say it’s safe, and have every fuel-burning appliance checked before you use it again.

Carbon monoxide symptoms often feel like the flu without a fever: headache, dizziness, weakness, upset stomach, vomiting, chest pain or confusion. People who are asleep can be overcome before they notice anything.

We’re a repair company, not an emergency responder. For a gas smell or a CO alarm, call 911 or the gas company — not us. Once they’ve made the house safe, we can repair the furnace.

Smells

Leave nowRotten eggs or sulfur

Natural gas has no smell of its own; utilities add an odorant so leaks smell like rotten eggs. Follow the gas steps above. Xcel notes some people can’t smell it reliably — allergies, smoking and “odor fatigue” all dull it — so a hissing sound near a gas line counts too.

Usually normalDusty, burning smell on the first cold night

Dust burning off the heat exchanger. It typically fades within a few hours. In Pueblo that first run can come early: the coldest October reading on record here was −6°F, on October 30, 2019.

Turn it off, call usBurning plastic or hot electrical smell

That points to something electrical overheating. Turn the furnace off at the thermostat and the furnace switch, then call.

Turn it off, call usStrong metallic smell, especially with soot or a yellow flame

Possible signs of a cracked heat exchanger. Don’t run it until it’s been inspected.

Noises

Call us nowA boom or bang right as the burners light

Delayed ignition: gas collects for a moment before it lights. Turn the furnace off and call — it points to a burner or ignition problem, and it shouldn’t be ignored.

Usually normalPopping or ticking in the ducts

Sheet-metal ducts expand as they heat and contract as they cool.

Schedule a repairHigh-pitched squeal

Usually the blower motor — often worn bearings, or a worn belt on older belt-driven blowers.

Typical U.S. range to replace a blower motor: $300–$900, per HomeGuide, Nov 2026. That’s a national range, not a Pueblo quote — you get the exact price after diagnosis.

Schedule a repairRattling

A loose panel, loose parts or debris in the blower compartment.

Call usClicking over and over, no flame

An ignition or gas-supply problem. See furnace won’t turn on.

Carbon monoxide and Pueblo homes

A furnace makes carbon monoxide as it burns gas; the heat exchanger and venting keep it out of your air. That’s why the warning signs of a cracked heat exchanger matter — a yellow or flickering flame, soot, short cycling, a strong metallic smell, or a CO alarm that trips again and again.

Colorado law requires a working CO alarm within 15 feet of the entrance to every bedroom in homes with a fuel-burning furnace or appliance, a fireplace or an attached garage — on sale, on rental turnover, and when permitted work like a furnace replacement is done. The CDC recommends battery-backed alarms on every level, too. With so many older houses in Pueblo — the city puts the average home at more than 55 years old — it’s worth checking yours tonight.

And during a cold snap, never heat the house with a gas oven or stovetop. Both the CDC and the CPSC warn it can produce dangerous carbon monoxide.

Questions we hear

Why does my furnace smell like burning when I first turn it on?

Dust settles on the heat exchanger and burners over the summer and burns off the first time the furnace runs. Carrier says that smell typically lasts from about 30 minutes to a few hours. If it doesn’t fade, or it smells like burning plastic or hot wiring instead of dust, turn the furnace off and call us.

Is a loud bang when the furnace starts dangerous?

A boom right as the burners light can mean delayed ignition — gas builds up for a moment before it catches. Trane says that needs immediate attention. Turn the furnace off and call us. (Lighter popping or ticking from the ducts as they heat and cool is normal metal expansion.)

What does a cracked heat exchanger smell like?

Trane lists a strong metallic odor among the warning signs, along with soot around the furnace or flue, a yellow or flickering flame instead of a steady blue one, short cycling, and a CO alarm that keeps going off. A CO alarm is a last line of defense, not a diagnostic tool — if it sounds, get outside and call 911.

Does Colorado require carbon monoxide alarms?

Yes. Colorado law (C.R.S. 38-45-101 and following) requires a working carbon monoxide alarm within 15 feet of the entrance to each bedroom in homes with a fuel-burning furnace or appliance, a fireplace, or an attached garage. It applies when a home is sold, when permitted work such as replacing a furnace is done, and when rentals change tenants. The CDC also recommends an alarm on every level of the home.

I smell gas but only near the furnace. Can I just turn it off?

No — treat any rotten-egg or sulfur smell as a leak. Get everyone outside without flipping switches, then call 911 or Xcel Energy at 1-800-895-2999 from outside. If your gas comes from Colorado Natural Gas (part of Pueblo West), call 911 and the emergency number on that bill. Don’t call us first.

Strange noise or smell? Call us.

For anything other than a gas smell or CO alarm, call us 24/7. Our licensed & insured technicians track down the cause and tell you the price before any work starts.

Call (719) 670-8043
Where the Pueblo facts on this page come from

Checked against these public sources on November 3, 2026. Weather figures are National Weather Service records for Pueblo Memorial Airport.

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