September is the closest thing Idaho gets to a break from the thermostat: it’s the state’s quietest month, with roughly 199 combined heating and cooling degree days. But “quietest” isn’t the same as “off.” Idaho’s winters run long and its summers push past 90 degrees often enough that the furnace and the air conditioner both stay in rotation most of the year. Here’s what that number actually means, and how to handle both changeovers.
The quietest month in Idaho
A degree day isn’t a temperature reading. It’s a measure of how hard a home’s heating or cooling system had to work on a given day, based on how far the daily average strayed from the 65°F baseline NOAA uses for this kind of tracking. Stack up a month’s worth of those and you get a number that describes demand on the equipment, not how warm the air felt outside.
Nationally, the calmest month anywhere in the country runs about 70 combined degree days. The highest calmest-month figure among the fifty states comes in around 246. Idaho’s September lands at roughly 199, which puts it much closer to the busy end of that range than the quiet one.
That position matters more than the label “quietest month” suggests. A state whose calmest month sits near 70 gets real weeks where neither the furnace nor the air conditioner has much to do. A state sitting up near 199 doesn’t get that. September is the lightest month Idaho sees all year, but the demand doesn’t disappear so much as it eases off for a few weeks before one system or the other picks back up.
Why the year doesn’t leave much room
The rest of the calendar explains it. Winters here run cold enough that heating stays the dominant demand for the better part of eight months, and summers bring real heat of their own, Boise averages about 47.5 days a year reaching 90°F or hotter. Between those two seasons, the handoff happens quickly rather than gradually, so any month caught in the middle tends to pick up a bit of both demands instead of neither. Treat September as the easiest window to schedule maintenance, not as a stretch when the house asks for nothing at all.
Shutting the heat down for the year
Once the calendar turns toward the mild stretch and the mean temperature is climbing back above 65°F (that happens in June here), it’s a reasonable time to put the heating system to bed for the season. Do it in this order:
- Switch the thermostat off the heat call, or over to cool, rather than cutting power at the breaker.
- Let the system finish whatever cycle it’s mid-way through before touching anything else.
- On a gas furnace or boiler, schedule the annual check of burners, venting, and safety shutoffs called for in the manufacturer’s guidance, that’s a job for a licensed technician, not a homeowner task.
- Shut down and drain any humidifier mounted on the ductwork. It’s the step almost everyone forgets, and a forgotten humidifier left full of water over the summer is a mold problem waiting to happen.
- Change or clean the filter now. ENERGY STAR’s guidance is direct: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.”
- Clear the area around the equipment of stored boxes, vacuum the supply and return registers, and reopen any vents that got closed off for winter.
Because Idaho’s mean temperature doesn’t climb back above that 65°F baseline until June, this shutdown makes the most sense once nights are consistently mild, not the moment a warm afternoon shows up in April. Shutting the heat off entirely any earlier, while the state is still deep in its roughly eight-month heating season, risks frozen plumbing if a cold snap follows. Once June arrives, that risk is gone.
The first run of the cooling season
Run the first cooling cycle of the year on a mild day, not the first 90-degree afternoon. A mild day gives room to fix whatever’s wrong before the House Actually Needs the system.
- Clear at least two feet of leaves, clippings, and debris from around the outdoor condenser so it can breathe.
- Check the condensate drain line for algae or clogs before the first hot week arrives, a blocked line can shut the system down or drip into the house.
- Change or wash the filter. ENERGY STAR puts a number on why this matters: “Airflow problems can reduce your system’s efficiency by up to 15 percent.”
- Run the system in COOL for a full cycle on a mild day, well before you actually need relief from the heat.
- Feel the air at a register farthest from the unit, not the closest one, weak airflow shows up there first.
Most of this applies directly in Idaho, where about 64% of homes run central air conditioning. But roughly one in five Idaho households cools with an individual unit instead, a Mini-Split, a window or wall unit, or a portable, and for those homes the outdoor-condenser and far-register steps translate more simply: clear the exterior grille, wash or swap the filter, and run one full cool cycle before the heat sets in. Overall, 81% of Idaho homes report having some form of air conditioning, and 70% also use ceiling fans, which take real pressure off the compressor on days that don’t quite justify running it hard.
The days that ask for both
Some months on the Idaho calendar don’t pick a side. A house can call for heat at six in the morning and for cooling by mid-afternoon, in the same week. Using a stated convention 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. By that measure, Idaho has two such months: June (61 heating, 143 cooling) and September (80 heating, 119 cooling). Neither is a contradiction; each is simply a month where mornings and afternoons make different demands on the same house.
| Situation | What to do | Why |
|---|---|---|
| Cool mornings, warm afternoons | Set the thermostat to AUTO changeover | Switches between heat and cool as each half of the day needs it, without manual toggling |
| Wide day-night swing | Widen the deadband between heat and cool setpoints to 4-6 degrees | Stops the system from cooling air it just paid to heat a few hours earlier |
| Mild afternoon, low humidity | Open the windows instead of running either system | Free ventilation covers the gap when outdoor air sits between the two setpoints |
The expensive mistake is a narrow gap between the two setpoints, say one or two degrees. That configuration heats the house in the morning, then turns around and cools that same conditioned air in the afternoon, running both systems against each other over a single day. A wider deadband, or a manual switch to fan-only on the mildest afternoons, avoids paying for that.
And the changeover in the other direction
Idaho’s heating season begins in October, the first month whose normal mean temperature (53.2°F) drops below the 65°F baseline, and it peaks in December at 1,020 degree days. It runs about eight months before the mean climbs back above 65°F in June. So the reverse changeover, cooling down, heating up, tends to land in that September-to-October window, right after the year’s quietest stretch ends.
Most Idaho homes are set up to make that switch through a furnace: 74% report a furnace as their main heating equipment, and natural gas fuels 61% of homes, with electricity covering another 26% and propane 6%. The state’s survey doesn’t publish a reliable figure for central heat pumps or steam and hot-water boilers here, which isn’t the same as saying nobody runs one, just that the sample was too thin to report. For most Idaho households, the pre-season checklist that matters is the furnace version, igniter, burners, filter, not the boiler routine of bleeding radiators. The full walkthrough for that changeover belongs in the heating-season guide on this site, worth reading once the mean temperature starts dropping back through the 60s.