September gives Utah homeowners the closest thing to a break between the furnace and the air conditioner, but calling it a shutdown window overstates what actually happens. The house still asks for something most days that month, just less of it than in January or July. Knowing exactly how little, and when the two real changeovers should happen instead, is what this page settles.
The quietest month in Utah
The calmest stretch of the year at Salt Lake City International Airport, the reference station for the state, falls in September, when heating and cooling demand together come to about 222 degree days. Set that against the range across all fifty states: the lowest quietest-month total anywhere is around 70, the highest around 246. A total of 222 sits much closer to that upper edge than to the lower one.
A degree day is not a temperature. It is a measure of how much work a heating or cooling system had to do on a given day, tallied up across the month. A high monthly total means the equipment, one system or the other, was earning its keep on most days. A low total means long stretches where neither system needed to run at all.
Utah’s number lands on the demanding side of that scale, and the honest answer follows from it directly: there is no month in this state when the house asks for nothing. September softens the load, it does not remove it. Some mornings still call for warmth before the sun clears the mountains, and some afternoons still pull enough heat through the windows to want the compressor running. What changes is not whether the system works, but which one does the work and how often.
Think of the year in Utah less as a line with a flat, quiet middle and more as a long climb up to winter, a long climb down into summer, and a short landing at the top of each slope where the two systems trade places. September is that landing on the way down from summer. It is real, and it is worth planning around, but it is a changeover, not silence.
Shutting the heat down for the year
Because Utah’s heating season runs roughly eight months, from October through May, the safe window for a full shutdown sits in the warmer stretch that’s left, mainly June through September. Outside that stretch, don’t treat the furnace as optional; overnight lows in the shoulder months can still dip enough to threaten a pipe run through an unheated space. Here is the order that actually protects the equipment when the real shutdown day comes:
- Set the thermostat so it stops calling for heat, rather than cutting power to the system at the breaker or the switch.
- Let the furnace or heat pump finish whatever cycle it’s mid-run on before you touch anything else.
- For gas-fired equipment, follow the manufacturer’s guidance for the off-season, and stop there. This is not a step to improvise around a pilot light or a gas valve on modern equipment.
- Inspect, clean, or change the air filter now rather than leaving it for spring. As the U.S. Department of Energy’s ENERGY STAR program puts it: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.”
- Shut down a whole-house humidifier attached to the ductwork. This is the step almost everyone forgets, and a humidifier left running into a dry summer just adds moisture the house doesn’t need.
- Clear vents, registers, and the area around the furnace of anything stacked against them over the winter.
- Note anything unusual from the season, a noise, a smell, a room that never quite warmed up, so it’s not forgotten by the time heat is needed again.
That ENERGY STAR filter instruction is not just a furnace note. It covers central air conditioners and heat pumps in the same sentence, which matters in a state where a share of homes run one or the other.
The first run of the cooling season
Do this on a mild day, not the first 90-degree afternoon. A fault that shows up when it’s 68 degrees outside is one you can fix on your own schedule. The same fault discovered during the first heat wave means living with a hot house while it gets sorted out, and that’s the entire argument for running this check early.
- Clear leaves, grass clippings, and anything else piled around the outdoor condenser unit.
- Check that the condensate line drains freely and isn’t blocked or pooling near the indoor unit.
- Change or wash the filter, following the same once-a-month standard ENERGY STAR sets for the whole heating and cooling season.
- Run the system in COOL for a full cycle on a mild day, not just a few minutes.
- Feel the air at the register farthest from the unit, not the closest one. That’s where a weak system shows itself first.
Utah’s housing stock leans heavily toward central systems: 92 percent of homes here run some form of air conditioning, and 79 percentage points of that comes from central units. The gap, about 14 percent, runs on individual equipment such as window units, wall units, ductless mini-splits, or portables, and those homes get a shorter version of this list: clean the filter, check that the unit drains where it’s supposed to, and run it once before the first hot stretch. Sixty-eight percent of Utah homes also report using ceiling fans, which do nothing for humidity but let a household push the thermostat a degree or two higher without feeling it.
ENERGY STAR’s own warning is why the filter step never gets skipped in either version of this list: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A unit that’s been sitting all winter with a clogged filter can look fine on the outside and still be working harder than it should.
The days that ask for both
Two months on Utah’s calendar carry enough heating demand and enough cooling demand at the same time to count as asking for both: May and September. That’s a convention stated here for this page, at least 50 heating degree days and 50 cooling degree days in the same month, not a category NOAA publishes. In May, the state logs about 172 heating degree days and 63 cooling degree days; in September, about 59 heating and 163 cooling. Those numbers describe a familiar situation with no dedicated name until now: a house that wants heat before sunrise and cooling by mid-afternoon, in the same 24 hours.
The practical question is what to do with the thermostat during those weeks.
| Setting | What it does | What it costs |
|---|---|---|
| Manual, one mode only | Runs heat OR cool, whichever you pick, all day | Comfortable half the day, uncomfortable the other half |
| AUTO changeover | System switches between heat and cool on its own as the temperature drifts | Convenient, but a narrow gap between the two setpoints means the system reheats air it just finished cooling, or the reverse |
| Wide deadband (5-8 degrees between setpoints) | Lets the house drift within a comfortable range before either system fires | Fewer short cycles, lower run time, the standard fix for the narrow-gap mistake above |
| Windows open, systems off | Uses the actual outdoor air instead of either system | Free, and often the better answer on the mild days May and September specialize in |
The mistake worth naming directly: setting the heating and cooling targets close together, say 70 and 72, turns May and September into an expensive loop, where the furnace warms the house at six in the morning and the air conditioner cools that same air by three in the afternoon. Widening the gap, or just opening a window on the mild days, breaks that loop without touching the equipment at all.
And the changeover in the other direction
Utah’s heating demand starts building in October, the first month whose average temperature drops below the 65-degree mark NOAA uses to count heating degree days, and it climbs steadily to a peak in January before the mean finally lifts back above 65 degrees in June. That means the reverse changeover, from cooling down to heating up, is really an autumn task rather than a single date on the calendar.
What matters locally is what’s getting switched on. Utah homes lean overwhelmingly on furnaces, 90 percent of them, and mostly natural gas, 86 percent, with electric heat covering another 13 percent. Figures for central heat pumps, steam or hot-water boilers, and propane weren’t published in enough detail to report here, which means the state simply isn’t a boiler state in any way the data can confirm. That’s the detail that should shape an autumn checklist: filter changes, a furnace inspection, and vents cleared of summer clutter matter far more here than anything a radiator or a boiler system would need. The full pre-heating checklist, including the fuel-specific steps this page only touches on, belongs on the heating-season guide for this state, and September is the right month to go read it.