What’s That Smell Coming From My Furnace at Startup?

As the temperatures begin to drop in Waxhaw, many homeowners are turning off their air conditioners and firing up their heating systems for the first time in months. But sometimes, instead of that cozy rush of warm air, you’re met with something far less pleasant—a strange or even alarming smell.

If your HVAC system is producing an unusual odor at startup, you’re not alone. This is one of the most common concerns our team at Kodiak Heating & Cooling hears each fall as homeowners make the seasonal switch from cooling to heating.

To help you understand what’s going on, we’ve compiled a list of common HVAC odors and what they might mean.

What’s That Smell Coming From My AC on Startup?

Let’s break down what might be causing these smells and what you can do about them.

1. My Furnace or AC Has a Musty or Mildew Smell

If you notice a damp, musty odor (similar to old gym socks), you may be dealing with mold or mildew growth inside your system. This often happens after a humid summer, when moisture builds up in your air ducts, drain pan, or evaporator coils.

What to Do:

• Check and replace your air filter if it’s clogged with dust and moisture.
• Schedule a professional coil cleaning or HVAC tune-up to remove built-up mold and bacteria.
• Consider a whole-home dehumidifier if high humidity is a recurring issue in your home.

2. Burning Smell Coming From My Furnace

A faint burning odor is one of the most common smells you’ll notice when you first switch your system to heat. After months of sitting idle, dust settles on the heating elements and burns off when the furnace starts up—this is normal and should fade within a few minutes.

What to Do:

• If the smell goes away quickly, it’s likely just dust burning off.
• If it lingers or smells electrical (like melting plastic or wiring), turn off your system immediately and call a professional to inspect for overheating or electrical issues.

3. My HVAC Smells Like Rotten Eggs

A rotten egg or sulfur smell is a serious warning sign—it could mean a natural gas leak near your furnace or gas line. While your AC doesn’t use gas, your heating system or nearby appliances might, and your HVAC system can circulate that odor throughout your home.

What to Do:

• Turn off your system and leave your home immediately.
• Call your gas company or emergency services to inspect for a leak.
• Avoid turning on lights or appliances until professionals confirm it’s safe.

4. There Is a Chemical or Sweet Smell

If you notice a sharp, chemical-like odor (similar to nail polish remover or sweet antifreeze), it may indicate a refrigerant leak in your AC system. Even though you’re not using your air conditioner much this time of year, residual leaks can still release that smell.

What to Do:

• Turn off your system and avoid prolonged exposure.
• Contact a licensed HVAC technician to locate and repair the leak.
• Never try to handle refrigerant yourself—it requires special equipment and certification.

5. My System Smells Like Dirty Socks or Sewage

A foul, sewage-like odor could mean your condensate drain line is clogged, allowing stagnant water and bacteria to build up. It might also point to a plumbing issue allowing sewer gas to back up into your HVAC system.

What to Do:

• Clean the condensate drain line using a mix of white vinegar and warm water.
• Make sure there’s no standing water in the drain pan.
• If the smell continues, call a professional to inspect your plumbing and HVAC system.

Preventing Fall HVAC Odors

To keep your home smelling clean and comfortable this season, follow these preventive steps::

• Replace your air filters regularly (every 1–3 months, depending on usage and pets in the home).
• Schedule annual HVAC maintenance to clean your coils and drain line and to have it inspected for potential issues.
• Ensure proper ventilation to prevent moisture buildup and microbial growth inside your system.
• Use UV air purifiers or high-quality air filtration systems to reduce bacteria, mold, and airborne contaminants.

When to Call an HVAC Professional

Some smells are temporary and harmless, but persistent or strong odors often mean something deeper is wrong. If you’re not sure what’s causing the smell, play it safe—call Kodiak Heating & Cooling. We’ll inspect your system, identify the source, and make sure your home stays warm, safe, and comfortable all season long.

Don’t let strange smells spoil your cozy fall nights!! If you’re in the Waxhaw, area and need expert HVAC maintenance or repair, contact Kodiak Heating & Cooling today to schedule a service appointment. Call 704-266-3621 or request service online.