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

As temperatures start to drop in Longview, many homeowners are firing up their furnaces for the first time in months. But what if, instead of the comforting warmth you expect, you’re hit with an unpleasant smell?

If your furnace is producing an unusual odor at startup, you’re not alone. This is one of the most common concerns our team at Carthage Air & Insulation hears every fall as homeowners transition back to heating mode.

To assuage your concerns, we’ve compiled a list of common smells and their causes.

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

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

1. My Furnace Has a Musty or Mildew Smell

If your furnace smells like a damp basement or old gym socks, you might be dealing with mold or mildew growth inside your system. This often happens when moisture builds up in your air ducts or evaporator coils, creating the perfect environment for microbial growth.

What to Do:

• Replace your furnace filter if it’s dirty or damp.
• Schedule a professional duct cleaning to eliminate mold and debris.
• If you use a whole-home humidifier, make sure it’s cleaned and maintained regularly.

2. Burning Smell Coming From My Furance

A light burning odor at startup is common, especially after months of inactivity. Dust naturally settles on the heating elements and burners over the summer. When you turn on the heat, that dust burns off—usually 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 like burning plastic or rubber, turn off your furnace and call a professional. This could indicate electrical issues or an overheating motor.

3. My Furance Smells Like Rotten Eggs

A strong sulfur or “rotten egg” smell is a serious warning sign—it may indicate a natural gas leak. Since natural gas is odorless, utility companies add a sulfur-like scent to help detect leaks.

What to Do:

• Turn off your furnace and leave your home immediately.
• Call your gas company or emergency services from a safe location.
• Do not turn on lights, appliances, or anything that could cause a spark until professionals confirm it’s safe.

4. There's a Chemical or Sweet Smell Near My Furance

A chemical odor, similar to acetone or sweet antifreeze, could indicate a refrigerant leak. Refrigerant is essential for cooling your home, and a leak can not only affect your AC’s performance but also pose health risks if inhaled over time.

What to Do:

• Turn off your AC and avoid prolonged exposure to the odor.
• Call a licensed HVAC technician to inspect and repair any refrigerant leaks.
• Never attempt to handle refrigerant yourself, as it requires specialized training and tools.

5. My Furnace Unit Smells Like Dirty Socks or Sewage

A foul, sewage-like smell coming from your furnace could indicate a clogged condensate drain or sewer gas leak near your HVAC system. Bacteria and stagnant water buildup can produce a strong, unpleasant odor when the furnace starts running.

What to Do:

• Clean the condensate drain line using a mixture of white vinegar and warm water.
• Check for standing water or blockages near your furnace.
• If the odor persists, call a professional to inspect both your HVAC and plumbing systems.

Preventing Future Furance Odors

Avoid unpleasant furnace smells this winter by following these maintenance steps:

• Replace air filters every 1–3 months.
• Schedule annual furnace maintenance before the cold season begins.
• Keep your vents, ducts, and drain lines clean and clear.
• Use UV air purifiers or upgraded filtration systems to reduce bacteria, mold, and airborne contaminants.

When to Call an HVAC Professional

While some furnace odors may be temporary or easy to fix, persistent or strong smells often indicate a more serious issue. If you are experiencing any of these odors and aren’t sure of the cause, it’s best to call a professional HVAC technician at Carthage Air & Insulation. Our team can diagnose the issue, ensure your system is running safely, and get your home back to feeling warm and comfortable.

Don’t let a smelly furance ruin the start of your heating season! If you’re in the Longview, area and need expert AC maintenance or repair, contact Carthage Air & Insulation today to schedule a service appointment. Call 903-693-2999 or request service online.