Chicago's normal high in early September is about 80 degrees. This week we're running roughly ten degrees over that, with heat forecast at 100 to 105 — a stretch one Northwestern climate scientist called "a significant and potentially dangerous late-season event."
For your air conditioner — the one in the yard or the one in the window — that means something specific. This is the hardest it will work all year, and it is the last time it will work that hard until June.
That makes right now the most useful week of the year to pay attention to it. A system that struggles under this kind of load is a system you can get looked at in September, while HVAC companies still answer the phone and quote shoulder-season rates. The same weakness discovered on the first hot day next June is a two-week wait and a peak-season invoice.
A system under load tells you the truth. The same system idling in October tells you nothing.
What should I do this week?
Three things. All three apply whether you run central air or window units — but the actual task is different for each, so both are spelled out.
1. Deal with the filter today, not next month.
Under this kind of load a dirty filter starves the blower. Airflow drops, the coil ices over, and the system shuts itself down — usually at the worst possible hour, on the hottest night, when nobody is picking up the phone.
Central air: Replace it. The filter costs a few dollars and takes four minutes. The service call to thaw a frozen coil and tell you the filter did it does not. If you can't remember the last time you changed it, that is your answer. Write the date on the edge of the new one in marker — it removes the guessing next time.
Window units: Don't buy anything. Rinse it. Nearly every window unit uses a washable mesh screen that slides out from behind the front grille — warm water, let it dry completely, put it back. The Department of Energy recommends cleaning these often, and every four to six weeks is the working standard. In a stretch like this one, sooner. That screen is the only thing between city air and the coil, and a clogged one is the single most common reason a window unit stops cooling well.
2. Look at the outdoor coil, and clear whatever is choking it.
Every air conditioner dumps its heat outside through a coil. If that coil can't breathe, the compressor runs hot — and the compressor is the expensive half of any system, in a yard or in a window.
Central air: Go find the big metal box. Clear two feet of space around it — pull the weeds, the grass clippings, the cottonwood fluff, and whatever the landscaper blew against it in July. Then shut the power off at the disconnect box and hose the fins down from the inside out, so you're pushing debris away from the coil rather than deeper into it. If the fins are bent flat across a section, a tech can comb them straight in about ten minutes.
Window units: Same principle, different job — and one hard rule: do not spray water into an installed unit. You will soak electrical components that were never built to get wet. What you can do is lean out and look at the back and sides of the unit, because those side vents are where it breathes. Curtains, a window screen pressed against the housing, storage on the sill, a planter box, a window well full of leaves — all of it quietly strangles the unit. Clear a few inches all the way around. The deep coil cleaning happens at the end of the season, when it comes out of the window.
3. Follow the water. This is where the two systems really diverge.
In humid stretches like this one your air conditioner is pulling gallons of water out of the air every day. Where that water goes — and what happens when it can't — is the biggest difference between the two setups.
Central air: flush the condensate line. Every drop leaves through one narrow PVC pipe near the indoor unit. Clog that line with algae, dust, or sludge and the drain pan overflows. Into your ceiling. Or, if you're in a condo, into your downstairs neighbor's ceiling, which turns a maintenance issue into an insurance claim and a genuinely awkward conversation at the next association meeting. A quarter cup of distilled vinegar down the access port once a month is the entire prevention. If you already see standing water in the pan, the line is clogged right now and you should get on it today.
Window units: there is no line to flush. Go looking for a condensate pipe on a window unit and you won't find one. They handle water two other ways. Many modern units use a slinger ring — a ring on the fan blade that picks the water up out of the base pan and throws it against the hot condenser coil, where it evaporates. That's by design, it improves efficiency, and it's why a perfectly healthy unit spits and gurgles on a humid day. Others drain through small holes in the base pan, underneath the half that hangs outside.
So the window-unit checks are different ones:
- Confirm it's still tilted correctly. Units that drain through holes need to sit slightly lower on the outside — manufacturers generally specify somewhere between a half-degree and four degrees, so check your manual rather than guessing. Units settle over a season. If yours has started dripping onto the floor or running down the interior wall, lost tilt is the first suspect.
- Check the drain holes from outside. Rust flakes and grit block them. A blocked hole means water standing in the pan, which is where the musty smell that kicks in with the unit comes from.
- Never drill your own drain hole. There are refrigerant lines running through that base pan. Puncture one and the unit is finished.
The failure mode is different too, and worth knowing. A central system that backs up takes out a ceiling. A window unit that backs up drips inside, stains the wall below the sill, and grows mold in a pan nobody can see. Less catastrophic, more insidious.
If you did a gut rehab: ductless mini-splits are the third category, and on this point they behave like central air. Each indoor head drains through a condensate line, often assisted by a small pump. Those lines clog — and because the head is mounted high on the wall, an overflow runs down the wall behind it before you ever see a drop. If you have mini-splits, treat item 3 as written for central air.
What else should I ask for while I've got someone out?
If you're calling for service anyway, this is the single highest-leverage phone call of your year — because you can book two seasons at once. Most of what follows assumes central air or mini-splits. But if you cool with window units and heat with a furnace, which describes a lot of this city, the furnace paragraph is still very much yours.
Book the furnace tune-up on the same call. Chicago's heating season starts in earnest in six to eight weeks, and every HVAC company in the city gets buried the first genuinely cold week of October. Scheduling now means you pick the appointment window instead of waiting for one.
Ask them to check the capacitor and contactor. These are inexpensive parts that fail with age and take the whole system down when they go. Replacing a weak one during a scheduled visit costs a fraction of an emergency call for the same part.
Ask for the refrigerant charge reading, and write it down. A system that's low is leaking. Low refrigerant doesn't cause the leak, it reveals it — and knowing that now is much better than discovering it in July.
Get the equipment's age on record. Ask what year the condenser and furnace were manufactured. If either is pushing 15 to 20 years, you are on borrowed time and you should be budgeting rather than reacting. This also matters if you're anywhere near selling — HVAC age is one of the first things a buyer's inspector writes down.
What if I'm in a vintage Chicago building?
A lot of this city's housing stock predates central air, and the advice changes.
Window units, end of season. Late September is when to pull them, not November. With the unit out you can finally do the deep clean you can't do while it's installed — coil, base pan, drain holes — then check the foam side seals for gaps and store it upright and dry. A unit left in the window all winter is a hole in your envelope that you pay for every month in heating.
Radiators and steam heat. Before the first cold snap, walk the unit and check that every radiator valve turns and that nothing is being used as a shelf. If you had rooms that never got warm last winter, that's usually trapped air, and bleeding them takes a key and a towel. Do it before you need the heat, not during the first 20-degree night.
Condo owners, one more time on water. In a stacked building, water damage doesn't stay yours. Whether it's a central system's condensate line or a window unit dripping down the inside of an exterior wall, this is the most common preventable claim we see between neighbors — and it is nearly always a five-minute maintenance task that nobody did.
Two-flats and older frame homes. If your condenser is on the roof or in a shared gangway, check that access hasn't been blocked by the neighbor's fence, storage, or new landscaping. Techs charge for the trouble, and some will simply reschedule.
The rest of the September list
The heat is the urgent part. These are the ones that get expensive if you wait for October.
- Gutters, before the leaves fall. Clearing them now takes one pass. Clearing them in November takes three, in the cold — and a clogged gutter through a Chicago freeze-thaw cycle is how you get ice dams and interior water stains.
- Outdoor spigots. Locate your shutoff valves now, while it's still pleasant to be in the crawlspace. You don't have to close them yet. You have to know where they are before the night you do.
- Weatherstripping and door sweeps. Run your hand around every exterior door frame. Daylight means air. This is the cheapest heating-bill fix that exists.
- Ceiling fans. Once the heat breaks, flip them to clockwise. It pushes warm air down off the ceiling instead of creating a draft, and it makes a real difference in rooms with high ceilings.
- Smoke and CO detectors. Fresh batteries before heating season, not after. Carbon monoxide risk goes up the moment the furnace starts running, and Illinois requires CO alarms within 15 feet of every sleeping area.
- Dryer vent. Pull the line and clear the lint. It's a fire-prevention task that also cuts your drying time roughly in half.
The point
None of this is glamorous, and none of it is optional if you'd like to spend the winter thinking about something else.
Chicago houses are honest. They tell you what's wrong, but only under load — and load, for anything that cools, is happening right now and won't happen again for nine months. Twenty minutes this week buys you information you cannot get any other time of year.
Do the three. Book the furnace tune-up. Then enjoy the last hot weekend of 2026 with a clear conscience.
Vesta Preferred Realty is a Chicago team of ~40 agents led by Grigory Pekarsky who have learned this city block by block. If you're weighing a move — this fall, next spring, or someday — we're happy to be a resource long before you need one.
Last updated: September 2, 2026
