September is the closest Illinois gets to a break between the furnace and the air conditioner, and even that month isn’t quiet: NOAA’s 30-year averages put it at about 193 combined heating and cooling degree days, with real demand for both. Most Illinois homes stop running the heat sometime in April or May and don’t need it again until October, when the statewide average temperature drops back below 65°F.
The quietest month in Illinois
September is the month with the least total degree-day demand in Illinois, at roughly 193 heating and cooling degree days combined, according to NOAA’s 1991-2020 Climate Normals. Nationally, the quietest month anywhere in the fifty states runs as low as about 70 and as high as about 246, so Illinois sits in the middle of that range, not at either extreme. That’s worth pausing on: a degree day isn’t a temperature reading, it’s a measure of how hard a heating or cooling system has to work to keep a home at a comfort baseline over a full day. A month can log a mild average temperature and still rack up plenty of degree days if the swings between morning and afternoon are wide.
And that’s exactly Illinois’s problem in September. NOAA’s monthly breakdown shows the month carrying about 77 heating degree days and about 116 cooling degree days at the same time, a split that only happens when a house genuinely needs both systems in the same stretch of days, not one or the other. That’s a changeover, not a pause. Nothing in the state’s climate data supports the idea of a month when the thermostat can simply be left alone.
Compare that to May, the only other month that comes close to the same pattern, with about 198 heating degree days and 60 cooling degree days. Both months sit in a narrow band where morning chill and afternoon warmth arrive in the same week, sometimes the same day. The deep winter months tell a different story entirely: January alone carries roughly 1,234 heating degree days and essentially none for cooling, while July sits at the opposite extreme with almost no heating demand and roughly 326 cooling degree days. Illinois runs a real winter and a real summer, connected by two short, mixed-demand months rather than one long stretch where the house asks for nothing.
What that means practically: there’s no calendar window in Illinois where a homeowner can safely walk away from both systems for weeks at a time. The furnace and the air conditioner each get a genuine off-season, but the handoff between them is compressed into a couple of unsettled months, and those are exactly the periods this page is built to help with.
Shutting the heat down for the year
The spring shutdown in Illinois should wait until the freeze risk has genuinely passed. Because heating demand here runs roughly eight months, from October through May, with the statewide average only climbing back above 65°F in June, shutting the system off too early in a state that still sees frost into April is the kind of mistake that leads to a cold morning and a scramble. Once the timing is right, the process itself follows a set order.
- Stop calling for heat at the thermostat first, rather than cutting power to the system. Let it register the change and shut down on its own terms.
- Let the system finish whatever cycle it’s mid-way through before doing anything else to it. A furnace or boiler that’s abruptly killed mid-cycle can trip safety switches that complicate the next start-up.
- Leave gas connections and pilot assemblies alone. What governs a fuel-burning appliance at shutdown is the manufacturer’s own guidance, not a general habit of shutting off the gas supply as routine maintenance.
- Inspect and change the filter. ENERGY STAR’s maintenance guidance is direct: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” That’s a once-a-month standard covering all three pieces of equipment together, not a once-a-season task, and the spring shutdown is a natural moment to catch up if it’s been neglected.
- Shut down the whole-house humidifier if the ductwork carries one. This is the step almost everyone forgets. A humidifier left running into the cooling season adds unwanted moisture right when the air conditioner is trying to remove it, and the bypass damper is easy to overlook because it’s tucked against the ductwork out of sight.
- Do the physical tidy-up: clear registers of anything stacked against them over winter, vacuum visible dust from vents, and note down anything that seemed off this season, so it isn’t forgotten by the time heating weather returns.
None of this involves cutting the system off from its fuel supply for the summer. In a state where the coldest months still stretch into the tail end of spring, a system left fully disconnected is one more thing standing between a homeowner and a frozen pipe if a late cold snap arrives out of season.
The first run of the cooling season
The best day to test the air conditioner in Illinois is a mild one, not the first ninety-degree afternoon of the year. A mild-day test surfaces a failed capacitor or a clogged condensate line while it’s still an inconvenience, rather than the moment described by NOAA’s own data: Illinois averages about 14 days a year at 90°F or hotter, and every one of those days is the wrong time to discover a problem for the first time.
- Clear the outdoor unit and the ground around it. Leaves, mulch, and anything that’s grown up against the cabinet over spring restrict airflow the unit depends on.
- Check the condensate path for blockage. A line that backed up over winter with dust or algae growth will show it the first time the system runs long enough to produce real condensation.
- Change or wash the filter before the first real run, not after. ENERGY STAR is specific about what a dirty filter costs: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” That’s a meaningful loss from one overlooked, inexpensive part.
- Run the system in COOL for a full cycle on a mild day, watching for it to actually shut off rather than run continuously trying to satisfy a setpoint it can’t reach.
- Feel the air at a register far from the equipment, not the nearest one. A weak or lukewarm airflow at the far end of the house often points to a duct or airflow problem the near register won’t reveal.
How this list applies depends on what’s actually installed. EIA’s state-level survey data shows 96% of Illinois homes run some form of air conditioning, with 74% on a central system and the remaining share, about 28%, relying on individual equipment such as a window unit, a wall unit, a ductless Mini-Split, or a portable. For that individual-equipment share, the outdoor-unit and condensate steps above don’t apply the same way; the relevant task is cleaning or replacing the unit’s own filter and confirming it seals properly against the window frame or sleeve before the first hot stretch. Illinois also has ceiling fans in 73% of homes, which is worth remembering on the mild days between the shutdown and the first real cooling need: a fan can stretch comfort a few extra days without running the compressor at all.
The days that ask for both
A house that calls for heat at six in the morning and for cooling by mid-afternoon isn’t malfunctioning, it’s living through exactly what NOAA’s monthly figures describe for May and September in Illinois. Using a simple convention stated here, not a published NOAA category, a month counts as asking for both when it carries at least 50 heating degree days and at least 50 cooling degree days in the same thirty days. May, with about 198 heating and 60 cooling degree days, and September, with about 77 heating and 116 cooling degree days, both clear that bar. Nothing about that is a contradiction; it’s a house genuinely needing warmth at dawn and relief by afternoon, in the same week.
The practical question is what to actually do with the thermostat during those weeks, and the choices trade off differently.
| Approach | What it does | Trade-off |
|---|---|---|
| Manual switching between HEAT and COOL | Runs only the mode that’s actually needed at that hour | Requires remembering to switch it back, easy to forget overnight |
| AUTO changeover mode | Lets the thermostat pick heat or cool automatically as the indoor temperature drifts | Costs more to run if the gap between setpoints is narrow, since the system can flip back and forth |
| Wide deadband between setpoints | Sets a bigger gap, say several degrees, between the heating and cooling targets | Lets the house drift naturally through a comfortable middle range without either system firing |
| Windows open instead of either system | Uses outdoor air directly during the mild hours of the day | Only works when outdoor humidity and pollen are tolerable, and someone has to remember to close them again |
The expensive mistake in these shoulder months is a narrow gap between the heating and cooling setpoints, something close to a single degree apart. With AUTO changeover mode and that kind of tight deadband, a house heats the morning air, then a few hours later spends energy cooling that very same air back down, paying twice for a comfort swing that a wider deadband, or simply opening a window for an hour, would have handled for nothing.
And the changeover in the other direction
Illinois’s heating demand starts building again in October, the first month when the statewide average temperature drops below the 65°F baseline NOAA uses to count heating degree days, and it climbs sharply from there, with November already averaging around 713 heating degree days on its own. That’s the point to run the autumn version of the checks covered here: clear filters, a listen for unusual furnace noise on the first cold-weather run, and a look at anything flagged during the spring shutdown.
What that fall checklist actually looks like depends heavily on what’s installed. EIA’s state data shows 76% of Illinois homes heat primarily with a furnace and 78% run on natural gas, so for most of the state the fall routine is the furnace-and-filter version described earlier in this guide, reversed. But 11% of homes run a steam or hot-water boiler, and that’s a different appliance with its own pre-season needs, bleeding radiators, checking the circulator pump, confirming the expansion tank pressure, none of which apply to a forced-air furnace. A small share, 4%, heats with propane, and EIA’s Illinois data doesn’t report a usable figure for fuel oil or kerosene heating in the state; that’s a suppressed or too-small sample, not evidence that no Illinois home uses it. The full step-by-step for that fall restart, including what changes for boiler households specifically, belongs in this site’s heating-season guide, where it’s covered in the depth it deserves.