Why Fall Prep Matters in Chicago
Chicago's heating season runs roughly November through March — five months of consistent furnace use. The city averages around 6,500 heating degree days per year, which puts it in the top tier of U.S. cities for furnace demand. A furnace that's borderline in October will fail in January.
The goal of fall prep is simple: find problems before you need heat, not after. A $150 tune-up in October is far better than a $300 emergency service call at 11 PM in February.
Step 1 — Test the Furnace in September
Don't wait until the first cold night to find out if your furnace works. In September, set the thermostat to HEAT and raise the set point above room temperature. Confirm the furnace fires, runs through a complete cycle, and produces warm air at the vents.
If it doesn't fire, check the basics: circuit breaker, furnace power switch, thermostat batteries, and gas supply valve. If those are all fine and it still won't run, call for service while technicians are less busy.
Step 2 — Replace the Air Filter
Install a fresh filter at the start of heating season. A clean filter at the beginning of the season means the furnace starts with optimal airflow. Check it again in December and January — during heavy use, 1" filters can clog in 4–6 weeks.
If you're upgrading to a higher MERV filter (MERV 11–13), confirm your furnace can handle the increased static pressure. High-MERV filters restrict airflow more than standard filters. Ask your technician if you're unsure.
Step 3 — Schedule a Professional Tune-Up
An annual furnace tune-up by a licensed technician is the most important thing you can do. September and October are the best months — technicians are available, and you have time to address anything found before you need heat.
A thorough tune-up includes cleaning the burners, inspecting the heat exchanger, testing the igniter and flame sensor, checking gas pressure, inspecting the flue or PVC vent, and verifying all safety controls. See our Furnace Maintenance Checklist for the full list.
Step 4 — Inspect the Vent System
High-efficiency furnaces (PVC vents): Check both PVC pipes — the intake and exhaust — where they exit through the exterior wall. Look for cracks, disconnections, or debris blocking the openings. Birds and insects sometimes nest in these pipes over summer. A blocked vent will shut the furnace down.
Standard furnaces (metal flue): Inspect the flue pipe from the furnace to where it exits the home. Look for rust, holes, disconnected sections, or bird nests. The flue carries carbon monoxide — any damage is a safety issue, not a cosmetic one.
Step 5 — Check the Condensate Drain (High-Efficiency Only)
High-efficiency furnaces produce condensate (water) that drains through a PVC line. Over summer, algae can grow in the drain line and cause a clog. Pour a cup of diluted bleach (1 part bleach to 10 parts water) down the drain to clear any buildup. Confirm the drain flows freely by pouring a cup of plain water into the drain pan.
Step 6 — Test CO Detectors
Carbon monoxide detectors should be on every level of your home and within 15 feet of sleeping areas. Test each one and replace batteries. CO detectors have a lifespan of 5–7 years — check the manufacture date on the back. If yours are older than 7 years, replace them.
A CO detector is your last line of defense against a cracked heat exchanger or flue problem. Don't skip this step.
Step 7 — Clear the Area Around the Furnace
Furnaces need clearance for airflow and safety. Remove any stored items, boxes, or flammable materials from within 3 feet of the furnace. Check that the return air intake isn't blocked — a common issue in finished basements where furniture gets rearranged.
Step 8 — Check the Humidifier (If You Have One)
Whole-home humidifiers connected to the furnace need annual maintenance. Replace the humidifier pad (water panel) at the start of each heating season — mineral deposits from Chicago's hard water build up over the season and reduce efficiency. Clean the water distribution tray and check the water supply line for leaks.
Step 9 — Open All Vents and Check Airflow
Walk through the house and confirm all supply and return vents are open and unobstructed. Closing vents in unused rooms doesn't save energy — it increases static pressure in the duct system and can cause the furnace to overheat. Leave all vents open.
Step 10 — Program Your Thermostat
If you have a programmable or smart thermostat, set your heating schedule for the season. A setback of 7–10°F during sleeping hours and when the house is empty can reduce heating costs by 10% or more. Don't set back more than 10°F — it takes longer to recover and can cause the furnace to run inefficiently trying to catch up.
Chicago-Specific: Snow and Vent Terminations
After major snowfalls, check the exterior vent terminations on high-efficiency furnaces. Heavy snow can bury the PVC pipes that exit through the side of the house. A blocked intake or exhaust will shut the furnace down — and it's a common service call after Chicago blizzards. Keep a path clear to the vent terminations, and check them after any significant snowfall.