Heating demand in the Phoenix area doesn’t show up until December, and even then it’s mild by national standards, peaking that same month and fading out again by March. For most of the year, the concern here is keeping the house cool, not warm. Here’s what the thirty-year climate record says about when a furnace or heat pump actually has work to do in Arizona.
When the heating season starts in Arizona
At Phoenix Sky Harbor International Airport, the National Oceanic and Atmospheric Administration’s 1991-2020 Climate Normals show December as the first month whose average temperature (55.8°F) drops below the 65°F base line used to count heating degree days. That’s the month to watch, not a date. A monthly normal is an average built from thirty years of data, and the first genuinely cold evening of any given year almost always shows up before the calendar catches up to the average.
December is also the coldest month on the normals, generating 288 heating degree days, more than any other month in the record. From there, demand tapers through January and February, and the monthly mean climbs back above 65°F by March, closing out a heating season that runs roughly three months.
The monthly table below tells a more detailed story than a single start-and-end date ever could. It’s worth noting that no month from May through September carries any heating degree days at Sky Harbor. Air conditioning owns that stretch of the calendar; a furnace or heat pump has nothing to do.
| Month | Heating degree days |
|---|---|
| January | 256 |
| February | 159 |
| March | 65 |
| April | 14 |
| October | 6 |
| November | 85 |
| December | 288 |
Notice that October and November already carry a handful of degree days even though the monthly average temperature hasn’t dropped below the 65°F base yet. That’s a normal quirk of how degree days are counted: they’re built from daily readings, and a handful of cool desert nights can add up even in a month whose overall average stays mild. It’s a small reminder that “the season starts in December” describes the typical pattern, not a hard switch that flips on a single date.
How hard the winter works here
Phoenix Sky Harbor logs about 874 heating degree days a year on the same 65°F base. Heating degree days work as a rough stand-in for fuel use: a house that needs roughly twice the degree days of another, similar house, burns roughly twice the fuel to stay comfortable over a season. At 874, Arizona’s total sits at the mild end of the national spectrum, and the monthly table above shows why: three months of real but modest demand, a firm December peak, and nothing that qualifies as a brutal cold spell by the standards of northern states.
The same station logs about 4,765 cooling degree days a year, more than five times the heating total. That single comparison says more about a Phoenix-area utility bill than the heating numbers ever will. A long, mild heating season and a short, sharp one can post the same annual degree-day total on paper; here, the shape is unmistakably mild and short, and the annual total confirms what the monthly breakdown already showed.
For a household budgeting fuel or electricity, the takeaway isn’t the total by itself. It’s that heating in this part of Arizona is a three-month, low-intensity job wedged into a calendar dominated by cooling season on the other end.
What most homes in Arizona actually heat with
The U.S. Energy Information Administration’s Residential Energy Consumption Survey (RECS 2020) breaks down what Arizona households actually have installed:
| Heating equipment | Share of homes |
|---|---|
| Furnace | 55% |
| Central heat pump | 25% |
| Steam or hot-water boiler | estimate suppressed, not reported |
On fuel, natural gas heats 36% of homes and electricity powers 51%, with propane covering about 2%; the fuel oil and kerosene share, like the boiler share, was suppressed as too small a sample to publish reliably. That’s not the same as zero. It means the survey couldn’t produce a trustworthy number, not that nobody in the state uses those systems.
A 55% furnace share reflects what got installed when these houses were built and what fuel was cheap at the time, not a recommendation for what any particular home needs today. Furnaces push heated air fast through ducts, which suits the short, punchy demand pattern shown in the table above: turn on, warm the house quickly, shut off again once the desert sun does the rest of the work. Where furnaces struggle is efficiency lost to the duct system itself, and where they punish neglect most is the air filter. A clogged filter restricts airflow, forces the blower to work harder, and can shorten the life of the unit. Heat pumps, running a quarter of homes here, tend to do well in a climate this mild since they rarely face the deep-cold conditions that cut their efficiency in colder states.
Run it once before you need it
A heating system that hasn’t run since March tends to fail on the first properly cold night, which in this climate usually lands sometime in December. That’s also the worst possible night to discover it, because every technician in the Phoenix area is already booked solid. The fix is to test everything in November, well before the December peak identified above. Work through these steps in order:
- Turn the thermostat to heat and run the system for ten to fifteen minutes, listening for banging, squealing, or a burner that won’t catch.
- Expect a faint burnt-dust smell on the first firing of the season. That’s normal, it’s just dust burning off the heat exchanger, and it should clear within a few minutes.
- Check the air filter and replace it if it’s visibly gray or clogged. A restricted filter is the single most common cause of an underperforming furnace.
- Walk the house and clear anything blocking a vent, a floor register, or the outdoor unit if a heat pump is in play. Furniture, rugs, and stored boxes are the usual culprits.
- Test the carbon monoxide alarm’s function button and confirm it has working batteries. Combustion heating systems can produce carbon monoxide if something’s wrong with venting, and a working alarm is the cheapest safeguard against it.
- If the system won’t stay lit, trips a breaker, or still smells like something burning after a few minutes, stop and call a licensed technician rather than troubleshooting further.
Even a furnace or heat pump in perfect working order can still shortchange a room. The U.S. 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.” That’s air that never makes it to the register, which explains a lot about a room that stays stubbornly cool no matter how long the system runs.
What turning it down is actually worth
The U.S. 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 figure covers heating and cooling together over a full year, not a per-degree formula and not a heating-only number.
In a climate where the heating season runs about three months and stays relatively mild, most of that 10% figure is doing its work on the cooling side of the year, given how dominant air conditioning is here. Still, an 8-hour daily setback through December, January, and February, overnight hours when nobody’s awake to feel the difference, costs nothing and stacks directly onto whatever the air conditioning season is already saving.