Heating season in Idaho generally opens in October, when the state’s climate reference point, Boise Air Terminal, sees its average temperature drop below the 65°F mark used to calculate heating demand. Demand builds through fall, peaks in December, and the season runs roughly eight months before the state warms back above that threshold in June.
When the heating season starts in Idaho
That October start date isn’t arbitrary. It comes from a specific number: 65°F, the base temperature the National Oceanic and Atmospheric Administration uses to calculate heating degree days. Any day the average temperature falls below that line adds to the tally, and the size of the gap determines how many degree days that day contributes. October is the first month where Boise’s 30-year average mean temperature, 53.2°F, drops far enough below 65°F for heating degree days to accumulate at meaningful volume.
December carries the heaviest load of the year, with 1,020 heating degree days at the reference station. That’s the coldest month, on average, over the three decades NOAA used to build this normal. But a 30-year average smooths out a lot of variation. The first hard freeze in any given year can land well before October ends, or a mild stretch can push real cold into November. Treat the month, not a date, as the honest marker.
Here’s the monthly breakdown, station by station data drawn from NOAA’s 1991-2020 Climate Normals for Boise Air Terminal, ID:
| Month | Heating Degree Days |
|---|---|
| January | 1,018 |
| February | 770 |
| March | 614 |
| April | 424 |
| May | 200 |
| June | 61 |
| July | 5 |
| August | 8 |
| September | 80 |
| October | 377 |
| November | 742 |
| December | 1,020 |
Notice that even in summer, the count never quite hits zero. That’s a reminder that these are averages built from thousands of individual days, and a cool July night in the Treasure Valley can still nudge the furnace on for a few minutes. One more thing worth remembering: this table describes one weather station in the Boise area. Higher-elevation counties in central and northern Idaho run colder and start accumulating heating degree days weeks earlier than the valley floor does.
How hard the winter works here
Boise’s annual total comes to about 5,320 heating degree days, against roughly 1,045 cooling degree days for the summer side of the ledger. That imbalance tells you something plain: heating dominates the Idaho energy year far more than air conditioning does. Degree days function as a rough stand-in for fuel use, so a house that racks up twice the degree days of another, similar house elsewhere, burns roughly twice the fuel to stay at the same indoor temperature.
What the annual figure doesn’t tell you is shape. A place can rack up 5,320 degree days with a long, mild season stretched over eight months, or with a short, brutal one crammed into four. Idaho’s monthly table above answers that question for you. The load builds gradually from October, spends three consecutive months, November through January, above 700 degree days, and eases off slowly through spring rather than dropping off a cliff. That’s a long-haul heating season rather than a short sharp one, and it matters for anything from filter change schedules to how much a system’s efficiency rating actually shows up on a winter bill.
None of this swaps in for a real energy audit of a specific house. Insulation, window age, and how tightly a home is sealed all move the needle more than the regional climate number ever will. But the degree-day total is a fair way to size up how much winter a given part of the country asks a heating system to do, year after year.
What most homes in Idaho actually heat with
The Energy Information Administration’s Residential Energy Consumption Survey puts furnaces well ahead of any other heating equipment in Idaho homes, and natural gas ahead of any other fuel.
| System / Fuel | Share of homes |
|---|---|
| Furnace (equipment) | 74% |
| Natural gas (fuel) | 61% |
| Electricity (fuel) | 26% |
| Propane (fuel) | 6% |
Three other categories, central heat pumps, steam or hot-water boilers, and fuel oil or kerosene, exist in the survey data but their estimates were suppressed as statistically unreliable. That’s not the same as zero. It means too few Idaho households in the survey sample used those systems for the agency to publish a confident number, not that nobody in the state runs one.
A three-quarters share for furnaces says something about when Idaho’s housing stock went up and what fuel was cheap and available at the time, not what belongs in a house being built or renovated today. A furnace paired with natural gas heats a room fast, and it’s a familiar, well-serviced system across the state, which keeps parts and technicians easy to find. Its weak point is the ductwork carrying that heat around the house, not the furnace itself. A duct system with a few disconnected joints or crushed runs in a crawlspace can quietly rob a house of comfort no matter how well the furnace itself is running. Filters matter more here than people expect, too: a forced-air system pushes far more air through a clogged filter than most homeowners realize, and that struggle shows up as higher bills and shorter equipment life. Homeowners weighing wood as a supplemental or backup heat source in Idaho’s colder counties can find that covered separately in this site’s wood heating guide.
Run it once before you need it
Every heating call center in the Treasure Valley fills up on the same nights, the first hard cold snaps of the season, which is exactly why the test run belongs in October, not November. Work through it in this order, and stop wherever the problem outruns what a homeowner should tackle alone.
- Check the filter first. It’s the cheapest, fastest thing to inspect, and a dirty one is the single most common cause of a system that runs constantly without heating a house properly.
- Clear the space around registers, radiators, or an outdoor heat pump unit. Furniture pushed against a vent or leaves packed around an outdoor unit block airflow before the system ever gets a fair test.
- Run the system for a short cycle and listen. Banging, grinding, or a burning smell that lingers past a few minutes are signs worth noting, not ignoring.
- Expect a faint burnt-dust smell the first time the system fires up after months idle. That’s dust burning off the heat exchanger, and it’s normal as long as it fades within a short run.
- Test the carbon monoxide alarm. A furnace burning natural gas or propane is a combustion appliance, and a working alarm is the cheapest insurance in the house.
- Walk the ductwork where it’s visible, in the attic, basement, or crawlspace, looking for disconnected joints or crushed sections. The Department of Energy notes that “in a typical house, about 20%-30% of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.” A furnace can be in perfect working order and a house still runs cold if the ducts feeding it are that leaky.
- Call a technician if anything above raised a flag: an odd smell that doesn’t fade, a noise that doesn’t stop, or an alarm that doesn’t test clean. That’s a professional inspection, not a weekend project.
What turning it down is actually worth
The Department of Energy states it plainly: “You can save as much as 10% a year on heating and cooling by simply turning your thermostat back 7-10°F for 8 hours a day from its normal setting.” That’s a yearly figure tied to a specific setback and a specific stretch of hours, not a rate that scales up or down by the degree.
In a state where the heating season runs close to eight months, from October’s first chill through the tail end of spring, that 8-hour setback window has a lot more days to work with than it would somewhere the season lasts half as long. A setback overnight or during the workday isn’t a dramatic change to how a house feels, but stacked across an eight-month season, it’s hours upon hours where the furnace simply isn’t working as hard.