September is the calmest month Indiana gets, but “calm” here still means about 200 combined heating and cooling degree days of demand. That’s not a dead month, and it’s not one either, which is exactly why the state needs two separate changeover routines instead of one long off-season, and why the real question isn’t “when do I shut it off” but “when do I do each of the two switches.”
The quietest month in Indiana
Degree days measure demand, not temperature, each one represents how far the day’s average ran from the 65-degree baseline NOAA uses to track heating and cooling need, added up over the month. September, at the reference station in Indianapolis, comes in around 200 combined heating and cooling degree days, according to NOAA’s 1991-2020 Climate Normals. Compared against the rest of the country, where the lowest quietest-month figure runs about 70 and the highest about 246, Indiana’s September sits closer to the busy end of that range than the quiet one.
That matters for what this page can honestly tell you. A state with a quietest-month total near 70 gets real weeks where neither the furnace nor the air conditioner has any reason to run. Indiana isn’t that state. At roughly 200, September is a month where the house still asks for something most days, just less of it, and the asking comes from both directions at once rather than settling into one steady mode.
Picture the shape of the year in words rather than in a monthly chart: a long, heavy stretch of cold-weather demand that dominates most of the calendar, easing gradually rather than snapping off, followed by a shorter but intense run of hot-weather demand that ramps up fast and holds. Between those two stretches there isn’t a flat, quiet plateau, there’s a narrow hinge where the direction of demand is still flipping back and forth day to day. September falls right in that hinge, not past it.
So if you came here looking for a single date to flip the system off and walk away until fall, the honest answer is that Indiana doesn’t offer one. What it offers instead are two changeover moments, each with its own short list of things worth doing properly, plus a stretch of shoulder weather where the smart move is managing both systems rather than retiring either one. Those are covered next.
Shutting the heat down for the year
The heating shutdown that matters is the one done at the thermostat, not at the breaker. Do it in this order:
- Turn the thermostat out of heat mode, or set it low enough that the system stops being called, rather than cutting power at the switch or breaker.
- Let any cycle already running finish on its own, don’t interrupt a furnace or boiler mid-run.
- If you heat with a furnace or boiler, book the manufacturer-recommended annual inspection now, while the technician’s schedule is lighter. Anything involving the gas supply or a pilot on modern equipment is governed by that manufacturer’s guidance and belongs to that visit, not to a DIY step.
- Inspect, clean, or replace the filter. As ENERGY STAR puts it: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” That advice covers all three pieces of equipment together, so it applies whether the house runs a furnace, a heat pump, or both.
- If the ductwork carries a whole-house humidifier, shut it down and drain it. It’s the step almost everyone skips, and a humidifier still adding moisture once the cooling season starts just gives the air conditioner extra work.
- Clear registers and return vents of anything stacked against them over winter, and vacuum the dust that’s built up on visible grilles.
- Leave the thermostat live rather than disabling the system entirely. Indiana’s heating season runs roughly eight months, and the area’s normal daily average doesn’t climb back above the 65-degree baseline until June, so a cold night in April or even May is ordinary, not freak weather. A house with no heat available on a bad night risks frozen plumbing, so keep the system able to answer if called.
The first run of the cooling season
Do this on a mild day, not the first hot one. A mild afternoon gives you time to catch a fault and fix it before you’re actually depending on the system to keep the house livable.
- Clear the outdoor unit and the ground around it of leaves, mulch, or plant growth built up over winter.
- Check the condensate drain line and pan for clogs before the system runs under load.
- Change or wash the filter, the same monthly habit ENERGY STAR recommends for the furnace applies here too.
- Run the system in COOL for a full cycle on a mild day, not a scorching one.
- Feel the air at a register far from the unit, not the nearest one, to catch weak airflow before it becomes a hot-day problem.
That fifth step matters more than it sounds. As ENERGY STAR notes, “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system can be mechanically sound and still underperform badly if a filter or a blocked return is starving it of air.
Most of that checklist assumes a central system, and in Indiana that’s the fair assumption: 81% of homes run central air conditioning, out of the 94% that have any air conditioning at all, per the EIA’s Residential Energy Consumption Survey. But 20% run some form of individual unit, a window, wall, Mini-Split, or portable, and for those homes the outdoor-unit and condensate steps don’t apply the same way; the relevant checks are cleaning or reinstalling the unit itself and confirming its own drainage runs clear. With an average 14.6 days a year hitting 90 degrees or higher, cooling equipment here does get real work to do, which is one more reason to test it on a calm day rather than the first hot one.
The days that ask for both
This is the week the thermostat calls for heat at six in the morning and for cooling by three in the afternoon, and no single seasonal switch explains it, because it isn’t one season, it’s the overlap of two. Using a straightforward convention (a month counted as asking for both when it carries at least 50 heating degree days and at least 50 cooling degree days in the same month — a line drawn here, not a published NOAA category), Indiana’s data shows May and September both qualify. May runs 131 heating and 86 cooling degree days; September runs 58 heating and 142 cooling. Neither number is a contradiction, it’s a month where mornings still want warmth and afternoons already want cooling.
| Approach | What it does |
|---|---|
| Manual mode switching daily | Works fine, but relies on remembering to flip modes as the day’s temperature swings, and it’s easy to forget overnight. |
| AUTO / changeover mode | The thermostat decides automatically. Convenient, but a system that calls for cooling shortly after finishing a heating cycle adds extra runtime and wear. |
| A wide deadband between setpoints | Setting several degrees of gap between the heat and cool targets lets the house float through the middle of the day without either system engaging, that gap is the entire point. |
| Open windows | The free option on days when the outdoor temperature sits between the two setpoints, skip the equipment altogether. |
The expensive mistake is a narrow gap between the two setpoints. Set heat and cool just a degree or two apart and the house heats in the morning, then spends the afternoon cooling the very air it just paid to warm.
And the changeover in the other direction
Indiana’s heating season begins in October, the first month whose normal average temperature drops below the 65-degree baseline NOAA uses to track heating demand, and it runs about eight months before the average climbs back above that line in June. Demand peaks hard in January, at 1,131 heating degree days for that month alone. In practice, most households start noticing the reverse changeover, needing more heat than cooling, sometime in October.
What that pre-season check should focus on depends on what the house actually runs. In Indiana, 82% of homes heat with a furnace and 65% run on natural gas, according to the EIA’s state-level survey, so the checklist that matters most locally is furnace-centered: filter, ignition system, combustion safety, all handled at the manufacturer-recommended service visit. Boilers account for only 3% of homes here, so radiator-bleeding and boiler-specific steps apply to a much smaller share of households than they would in a state where hot-water heat is the norm. Electric heat covers 26% of homes and central heat pumps 7%; figures for fuel oil or kerosene weren’t reported in the survey, which reflects a suppressed estimate rather than zero households using it.
The full pre-season routine, done in the right order, belongs to the heating-season guide for this state rather than repeated here, this page’s job is simply to flag when that changeover starts, which for Indiana means treating October as the month to stop thinking about the air conditioner and start thinking about the furnace again.