Preparing Your Furnace for Winter

There's nothing worse than discovering your furnace doesn't work on the first cold night of the year. The good news? A little preparation in early fall can prevent that scenario entirely.

Your furnace sat dormant for months while you enjoyed summer. Dust settled on components, spiders may have built webs near the burners, and you probably haven't thought about it since spring. Now's the time to change that.

The best time for furnace prep is September or early October—before HVAC companies get slammed with emergency calls and before you actually need the heat. Spend 30 minutes now, and you'll have peace of mind all winter.

Before You Turn It On#

Start with these checks while the furnace is still off.

Replace the Filter#

This is always step one. Your filter has been sitting there all summer collecting dust even without the system running. Start the heating season with a fresh filter—it's the single most impactful thing you can do for your furnace's health.

If you're not sure what size you need or what MERV rating to choose, check out our complete guide to HVAC filters.

Clear the Area Around Your Furnace#

Look around your furnace and clear everything within three feet. Over summer, it's easy for storage to creep closer—boxes, holiday decorations, cleaning supplies. All of that needs to move.

This isn't just about airflow (though that matters). It's also a fire safety issue. Furnaces generate heat, and combustible materials stored too close create risk.

Test Your Thermostat#

Replace the batteries if it's been a year—most thermostats take AA or AAA batteries. Then switch the system to heat mode and bump the temperature up a few degrees above the current room temperature. You should hear the system kick on within a minute or two.

If your thermostat is blank, unresponsive, or the furnace doesn't respond when you adjust settings, troubleshoot the thermostat before assuming the furnace is broken.

Check Your Carbon Monoxide Detectors#

This is critical. CO detectors save lives, and heating season is when carbon monoxide risk is highest.

Test each detector by pressing the test button—you should hear a loud alarm. Replace batteries if it's been six months. And check the manufacturing date: CO detectors expire after 5-7 years and need to be replaced entirely, not just re-batteried.

The First Run of the Season#

Once you've done the pre-checks, it's time to fire up the furnace.

What's Normal#

When you first turn on the heat after months of inactivity, you'll probably notice a burning dust smell. This is normal—it's dust that accumulated on the heat exchanger burning off. It should dissipate within an hour or two.

You might also hear some clicking and whooshing as the system starts up, followed by the sound of the blower pushing air through your ducts. Some popping from your ductwork is normal too—that's the metal expanding as warm air flows through.

What's Not Normal#

Pay attention during this first run. Problems are easier (and cheaper) to fix now than in January.

Burning smell that persists: If you still smell something burning after a couple of hours, turn off the system and investigate. A persistent burning smell could indicate an electrical problem or something stuck in the system.

Grinding, scraping, or banging: These sounds suggest mechanical problems—a failing blower motor, loose components, or delayed ignition. Turn off the furnace and call a technician.

No heat coming from vents: If the furnace runs but the air isn't warm, something's wrong. Check that the filter isn't clogged, then call for service.

Rotten egg smell: This indicates a gas leak. Turn off the furnace, leave the house immediately, and call your gas company from outside. Do not flip any light switches or create any sparks.

The Flame Test#

If your furnace has a viewing window (most modern furnaces do), look at the burner flame while it's running.

A healthy flame is blue with small yellow tips. This indicates complete combustion—the gas is burning efficiently and safely.

A yellow or orange flame is a problem. It means incomplete combustion, which can produce carbon monoxide. Turn off the furnace and call a technician.

Red, purple, or green flames indicate a serious issue—possibly a contaminated gas supply or cracked heat exchanger. Stop using the furnace immediately.

When to Call a Professional#

Some tasks require a licensed HVAC technician. Schedule a professional tune-up if:

• It's been more than a year since your last service
• Your furnace is 10 years or older
• You heard unusual sounds during your test run
• The flame color is wrong
• You smell anything unusual (other than the brief burning dust)
• The system short cycles (turns on and off frequently)

What a Pro Checks#

A professional tune-up includes things you can't safely do yourself:

• Heat exchanger inspection for cracks (critical for CO safety)
• Combustion analysis measuring CO levels, gas pressure, and temperature rise
• Burner cleaning and adjustment
• Electrical connection tightening
• Blower motor inspection
• Safety control testing

Expect to pay $100-200 for a standard tune-up. It's money well spent—catching a small problem now prevents expensive emergency repairs later.

Book Early#

Here's a pro tip: HVAC companies are slow in September and slammed in November. If you wait until the first cold snap to call, you might wait days for an appointment—or pay emergency rates. Book your tune-up early when scheduling is easy and prices are normal.

Quick Fall Checklist#

Use this as your reference each year:

Before turning on:

• Replace air filter
• Clear 3-foot zone around furnace
• Test thermostat (replace batteries)
• Test CO detectors (replace batteries, check expiration)
• Visually inspect furnace for rust or damage
• Check that vents and registers are open and unblocked

During first run:

• Listen for unusual sounds
• Check for burning smell (brief = normal, persistent = problem)
• Verify warm air at vents
• Check flame color through viewing window (blue = good)

Schedule professional service if:

• Over 12 months since last tune-up
• Furnace is 10+ years old
• Any concerns from your test run