The closest Arizona gets to a real pause is March, and even that month asks for both heat and cooling rather than neither. What follows is when to shut the heating system down for the year, when to bring the cooling online, and what to do with the days in between that want both.
The quietest month in Arizona
March is the calmest month of the year statewide, and it still carries about 170 combined degree days of demand, heating and cooling added together. Degree days measure how hard a system has to work to hold a comfortable baseline, not a temperature reading. Set that 170 against the rest of the country: the lowest quietest-month total in any state runs around 70, the highest around 246. Arizona’s March sits well toward the busy end of that range.
A state whose quietest month lands near 70 gets weeks where neither the furnace nor the air conditioner has anything to do. Arizona’s 170 doesn’t describe that. It describes a changeover, a stretch where cool mornings still call for a little heat and warm afternoons already call for a little cooling, inside the same 30 days.
The shape of the Arizona year is a long, heavy run of cooling demand through spring, summer and well into fall, a short, lighter run of heating demand in winter, and two brief hinge points where the two overlap instead of one cleanly replacing the other. Those hinges are the nearest thing to a lull this climate offers, and even they carry real demand running both directions at once.
That changes how a homeowner should approach the thermostat calendar here. In a four-season climate with a genuine dead zone, the routine is simple: shut the heat off, wait, then bring the cooling on. Arizona doesn’t offer that gap. The useful version of the advice, covered below, is less about finding a window to do nothing and more about managing a handoff: which system to stand down first, when to bring the other one up, and what to do on the days that call for both within a few hours of each other.
Shutting the heat down for the year
Heating demand here fades fast once the calendar turns. December carries the heaviest load of the year, January and February taper, and by March the system has little left to do. That’s the window for shutting the heating side down properly, one step at a time rather than flipping a breaker.
- Stop calling for heat at the thermostat first, not by cutting power at the panel. Switch it to cool or off and let the system stand down on its own rather than interrupting a cycle mid-run.
- Let the current heating cycle finish before doing anything else.
- For a gas or propane furnace, leave the fuel-burning components alone. Pilot lights, ignition systems and venting are governed by the manufacturer’s own guidance, not a seasonal habit.
- Inspect, clean, or change the filter now, at the changeover, rather than waiting. ENERGY STAR’s own guidance is direct: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” The changeover is a natural monthly marker to build that habit around.
- Shut down any humidifier tied into the ductwork. It’s the step almost everyone forgets, and a humidifier left running after the heating season just adds unwanted moisture to air about to be cooled.
- Clear the area around the furnace or indoor air handler, vacuum nearby vents and returns, and note anything that looked or sounded off during the last heating cycles of the season.
None of this means leaving the system fully off through every cold snap that might still arrive. At the reference weather station used for this state’s normals, winter lows rarely get near freezing, so shutting the heating side down for the season doesn’t put plumbing at risk the way it would in a colder climate. That’s not universally true across every part of Arizona. In the higher, colder counties, a hard freeze is still possible into early spring, and heat should stay available on a low setting through those cold snaps rather than being switched off entirely.
The first run of the cooling season
Do this on a mild day, not on the year’s first hot one. A mild day gives room to fix whatever turns up, a weak capacitor, a clogged line, a filter nobody changed since fall, without living through a house that won’t cool while it’s 105 outside. Waiting for the heat to force the issue is the single most common way this turns into an emergency instead of a checklist.
- Clear the outdoor condensing unit and the space around it. Grass clippings, leaves and shrubs growing into the cabinet restrict airflow and make the system work harder for the same result.
- Check the condensate drain path for blockages. A clogged line is one of the most common reasons a cooling system trips out or backs up water where it shouldn’t.
- Change or wash the filter, whichever the system uses, before the first real run of the year.
- Run the system in COOL for a full cycle on that mild day, and listen for anything unusual: grinding, rattling, a compressor that won’t kick on.
- Feel the air at a register far from the air handler, not the closest one. A weak far register with a strong near one usually points to a duct or airflow problem rather than the compressor itself.
ENERGY STAR’s warning applies directly here: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system can be mechanically sound and still underperform because something as simple as a dirty filter is choking the airflow.
Most of that list is written for the equipment most Arizona homes actually run. Statewide, 94% of homes use some form of air conditioning, and 86% run a central system, the kind this checklist is built around. The remaining share cools with individual equipment: ductless mini-splits, window units, wall units or portables. For that group, the outdoor-unit and duct steps mostly don’t apply; the useful version runs to a clean or replaced filter, a clear exterior grille or condenser, and a test run on a mild day before the heat sets in. Ceiling fans, used in 86% of homes here, don’t replace any of this, but running them alongside the first cooling cycle takes some load off the compressor.
The stakes for getting this right before summer are higher here than in most states. Arizona averages about 168.9 days a year at 90°F or above, a measure of the total heat load a cooling system has to carry. A breakdown discovered in July doesn’t mean a rough afternoon. It can mean a rough week or more before parts and a technician are available, which is the real argument for testing the system on a calm day in spring rather than finding out the hard way.
The days that ask for both
Anyone who’s lived through an Arizona spring knows the feeling: the house wants a little heat at six in the morning and a little cooling by three in the afternoon, sometimes in the same day. That’s not a malfunction. It’s what the degree-day numbers for March actually show.
The convention used on this page, stated here and not a category NOAA itself publishes, is that a month counts as asking for both when it carries at least 50 heating degree days and at least 50 cooling degree days in the same 30 days. By that measure, two months in Arizona qualify: March, with roughly 65 heating degree days against 105 cooling degree days, and November, with roughly 85 against 88. Both months sit almost evenly split between the two kinds of demand.
| Situation | What it does | Watch for |
|---|---|---|
| AUTO changeover mode | Lets the thermostat pick heat or cool on its own as the temperature crosses the setpoints | Convenient, but only as good as the setpoints behind it |
| Wide deadband between setpoints | Leaves a gap between where heat kicks in and where cooling kicks in | Reduces short-cycling between the two systems in the same day |
| Narrow deadband between setpoints | Heat and cooling setpoints sit close together | The house heats in the morning, then cools the same air it just paid to warm by afternoon, an expensive pattern most people never notice |
| Opening windows instead | Uses outside air directly during the mild hours of a shoulder-season day | Often the simplest and cheapest option on days that don’t swing to extremes in either direction |
The narrow-deadband mistake is worth calling out, because it’s invisible on a utility bill until the pattern’s been running for weeks. A thermostat set to heat at 68°F and cool at 70°F will happily run both systems against each other on a day like this. Widening that gap, or switching to windows open during the mild parts of the day, is usually the better answer during March and November.
And the changeover in the other direction
The reverse handoff, cooling winding down and heating coming back on, tracks the same climate data from the other side. Heating demand here starts building in December, the first month where the average temperature at the reference station (Phoenix Sky Harbor) drops below the 65°F baseline NOAA uses to count heating degree days. December also carries the heaviest heating load of the year, with the season running about three months before the average climbs back above 65°F in March.
What that means in practice is a much shorter fall pre-season checklist than the spring one. Statewide, 55% of homes heat with a furnace and 25% with a central heat pump; steam or hot-water boiler use wasn’t reported in enough homes here to publish a figure. Natural gas serves 36% of homes and electricity 51%, with propane a small share at 2%. For most Arizona households, the fall changeover is a furnace or heat pump story: filter, airflow, a listen for anything unusual, the same categories covered on the shutdown side of this page, just run in reverse as December approaches. The full version of that checklist, including equipment-specific steps for a heat pump versus a gas furnace, belongs in the heating-season guide for this state rather than repeated here.