Getting your furnace ready for winter comes down to five jobs: a fresh filter, a thermostat test, clear vents and exhaust pipes, working carbon monoxide alarms, and a professional tune-up before the first hard freeze. Most homeowners can handle the first four in an afternoon. A fall tune-up covers the parts you can’t safely reach, like the burners, flame sensor, and heat exchanger.

Missouri weather rarely eases into winter, so a furnace that sat idle all summer gets no warm-up period.

Start With a Clean Furnace Filter

A dirty filter is the most common reason a furnace struggles in its first cold week. Dust builds up all summer because the furnace blower also moves air for your AC. When the filter clogs, airflow drops, and the furnace can overheat and shut itself off.

Pull the filter now and replace it if you can’t see light through it. One-inch filters usually need changing every one to three months during heating season, while thicker media filters last longer.

Test the Thermostat Before You Need It

Don’t wait for the first frosty morning to find out whether your heat works. Switch the thermostat to heat and set it a few degrees above room temperature. The furnace should start within a few minutes.

A faint burning smell during the first run is normal. Dust settles on the burners and heat exchanger over the summer and burns off within the first hour or so. If the smell is sharp, smells like gas, or doesn’t fade, shut the furnace off and have it checked.

Clear the Space Around the Furnace and Its Vents

Furnaces need open air to burn fuel safely and push heat through the house. Move storage boxes, paint cans, and laundry away from the unit, leaving a few feet of space on every side.

High-efficiency gas furnaces vent through two white PVC pipes that usually exit a side wall or the roof. Leaves, nests, or drifting snow can block them. Older furnaces vent through a metal flue, which should connect tightly to the chimney with no gaps or rust holes.

Electric furnaces don’t burn fuel and skip the exhaust check. Their heating elements still depend on clean airflow.

Make Sure Your Carbon Monoxide Alarms Work

Any furnace that burns natural gas or propane can produce carbon monoxide if a part cracks or the venting fails. You can’t see or smell the gas, so a working alarm is your only warning. Press the test button on each alarm and swap in fresh batteries.

Alarms belong on every level of the home and near bedrooms. Each one also has an expiration date printed on the back.

Why a Fall Tune-Up Belongs on the List

The first four steps keep air moving and alarms working. They don’t reach inside the furnace, where most winter breakdowns start. During a furnace tune-up, a technician cleans the burners, tests the flame sensor and igniter, checks gas pressure, inspects the heat exchanger for cracks, and confirms the furnace vents safely.

Fall timing matters because weak parts show up before the furnace runs around the clock. A tired igniter might light fine on a 50-degree night and fail on a 10-degree one. Booking furnace maintenance in early fall also beats the first cold-snap rush.

Many manufacturers require yearly service to keep the warranty valid. A maintenance plan with two visits a year covers both the fall heating check and the spring AC check.

When Your Furnace Needs More Than a Tune-Up

Some problems need more than cleaning. Watch for these warning signs:

• Short cycling: The furnace turns on and off every few minutes without warming the house.
• Yellow or flickering flames: A healthy gas flame burns mostly blue, so a lazy yellow flame needs a professional look.
• Banging, squealing, or rattling: Loud new noises usually point to a failing motor, belt, or ignition part.
• Rooms that won’t warm up: Cold spots can mean blocked airflow, leaky ductwork, or a furnace that’s too small for the house.

Any of these is a reason to schedule furnace repair before winter settles in. For an older furnace, the $5,000 rule helps frame the next step. Multiply the furnace’s age by the repair estimate, and if the result tops $5,000, replacement deserves a serious look.

Schedule Your Fall Furnace Tune-Up

Level 9 Heating & Cooling handles fall furnace tune-ups across the St. Louis area, with local HVAC roots that go back to 1975. Our technicians inspect, clean, and test your furnace, then leave a hand-written summary of what they found and your options. Ask about current seasonal rebates and financing when you schedule a furnace tune-up before the cold arrives.

Frequently Asked Questions About Furnace Winter Prep

How often should my furnace kick on in winter?

Most furnaces cycle on two to three times an hour in cold weather, running about 10 to 15 minutes each time. On the coldest nights, they run longer with shorter breaks. A furnace that starts every few minutes, or runs nonstop without reaching the set temperature, needs a technician’s look.

What temperature should you set your furnace to in the winter?

Many homeowners start around 68 degrees while they’re home and awake, then lower it several degrees overnight or while they’re away. Longer setbacks save more energy, and a programmable thermostat handles them automatically. If you have a heat pump, keep setbacks small, since big swings can trigger less efficient backup heat.

How much does it cost to get a furnace ready for winter?

The cost of a pre-winter tune-up depends on the furnace’s age, its fuel type, and whether the technician finds parts that need replacing. A visit that only needs cleaning and testing costs far less than one that turns up a failing igniter or blower motor. Maintenance memberships that include fall visits can also lower what you pay per visit over time.