The quietest stretch of the year here is June, when heating and cooling demand together add up to about 70 degree days, one of the lowest quietest-month totals recorded anywhere in the fifty states. Shut the heat down as spring turns to early summer, then get the cooling system ready before the first truly hot stretch arrives, and expect to flip the switch back the other way once November’s cooler air settles in for good.
The quietest month in California
Degree days measure demand, not temperature. A day with 20 heating degree days means the house needed roughly 20 degrees’ worth of heating effort below a comfort baseline, added up over that day; it isn’t a reading off a thermometer. That distinction matters here because June’s total of about 70 combined heating and cooling degree days sounds abstract until you compare it against the national spread: the calmest month anywhere in the country runs about 70, and the warmest quietest-month figure recorded runs about 246. California’s June sits right at the floor of that range.
That placement answers the question this page exists to settle. A total near the national low end means June really is a pause, not just a handoff between seasons. The house isn’t quietly switching from asking for heat in the morning to asking for cooling in the afternoon; it’s genuinely asking for very little of either, most days. Compare that with a state whose quietest month still lands near 246, where the year never fully releases its grip, heating fades out just as cooling ramps up, and there’s no real gap between the two. Reference-station data for this state (Los Angeles Intl AP) shows no month here carrying enough of both heating and cooling demand at once to count as a genuine overlap month, which is exactly what a low quietest-month figure predicts.
What this means in practical terms: June, and the weeks bracketing it, is the window when a household can reasonably expect neither system to be doing much work. It’s the natural moment to close out the heating season properly rather than just letting it fade unattended, and it’s also, not coincidentally, the moment before summer’s heat load builds. That second part matters too, since this reference station still averages a handful of days a year reaching 90 degrees or hotter, so “quiet” here means light demand on both ends, not the absence of any weather worth planning for.
Source: NOAA NCEI, U.S. Climate Normals 1991-2020.
Shutting the heat down for the year
Do this in order, not all at once, and start at the thermostat rather than at the breaker or the fuel line.
- Turn the thermostat down so the system stops calling for heat, rather than cutting power outright.
- Let the system finish its current cycle and let the blower complete its normal shutdown run.
- Leave gas valves, pilots, and ignition components alone unless the manufacturer’s guidance for your specific furnace or boiler calls for a seasonal shutdown step. That’s a manufacturer decision, not a routine chore.
- Inspect or change the filter now. ENERGY STAR is specific about this: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” That’s every month, all season, not just at the changeover.
- If ductwork carries a humidifier, shut it down and drain it. It’s tied to the heating system and gets forgotten more often than any other component in the house.
- Clear the area around indoor equipment, vacuum registers and returns, and put away any portable heaters for storage.
One caution worth stating plainly: never plan to shut heat off entirely somewhere freezing pipes are a real risk. At the Los Angeles reference station, heating degree days drop to a handful by June and stay low through summer, and the coldest months (December and January) still sit well short of the totals that push plumbing toward frost. That’s the mild-coastal picture. Inland valleys, high desert, and mountain communities in this state run a very different winter, and homeowners there should treat a full shutdown with more caution than this reference station suggests.
The first run of the cooling season
Do this on a mild day, not the first hot one. A fault that shows up when it’s 75 degrees out and you’re just testing the system is an easy fix; the same fault discovered at 95 degrees, with the household already trying to live through the heat, is a much worse afternoon.
- Clear leaves, debris, and anything growing against the outdoor condenser unit.
- Check the condensate drain line for clogs; a blocked line is one of the most common causes of a system that runs but doesn’t cool.
- Change or wash the filter, whether that’s a central system’s return filter or the filter in an individual unit.
- Run the system in COOL for a full cycle on that mild day, long enough to hear it settle into steady operation.
- Feel the air at the register farthest from the equipment, not the nearest one; weak airflow at the far end shows up before anything else does.
ENERGY STAR puts a number on why the filter step matters more than it looks: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system that turns on and blows cold air can still be running well below its real capacity, and a dirty filter is the cheapest cause of that gap.
How this checklist applies depends on what’s actually installed. Statewide, 72% of homes here run some form of air conditioning, but only 54% run a central system; the rest, close to a quarter of households, cool with individual equipment: ductless mini-splits, window units, wall units, or portables. A household running a window unit skips the outdoor-unit and condensate-line steps for a central system and instead checks the unit’s own filter and drain, usually right at the chassis. And with a reference-station average of just 2.4 days a year reaching 90 degrees or hotter, most of the cooling season here is mild by national standards, which is exactly why testing the system before the first genuine heat wave is realistic rather than a luxury.
Source: U.S. Energy Information Administration, RECS 2020; U.S. ENERGY STAR.
The days that ask for both
A house that wants heat at six in the morning and cooling by three in the afternoon isn’t malfunctioning; it’s just living through a month where mornings and afternoons sit on opposite sides of comfortable. For this page, a month counts as asking for both when it carries at least 50 heating degree days and at least 50 cooling degree days at once, a line stated here for clarity, not a category NOAA itself publishes.
By that measure, no month at the reference station actually crosses both thresholds together; June, the closest candidate, carries far more cooling demand than heating demand by the time it arrives, and April and May tilt the other way. But that monthly average hides what happens day to day. Spring and fall around here are known for wide swings between morning lows and afternoon highs, and it’s entirely normal on an individual day in April or October to want a jacket at sunrise and shorts by early afternoon, even in a month where the aggregate numbers don’t show a true overlap.
| Situation | What it does | What it costs or requires |
|---|---|---|
| Thermostat set to AUTO changeover | System switches between heat and cool automatically based on the setpoints | Convenient, but a narrow gap between the two setpoints causes short-cycling |
| Wide deadband (several degrees between heat and cool setpoints) | Gives the house room to drift before either system fires | Fewer cycles, lower run time, still automatic |
| Narrow deadband | Heats in the morning, then cools that same air in the afternoon | The costly mistake: paying to condition the same air twice |
| Windows open instead | Uses the natural swing between a cool morning and a warm afternoon | Free, and often the better answer on a genuinely mild day |
The expensive habit to avoid is setting the heat and cool points too close together, six degrees apart or less. That configuration heats the house at dawn, then air-conditions the same warmed air a few hours later, running both systems in the same day for no real gain in comfort. A wider gap, or simply opening windows on the mildest days, does the same job for less.
And the changeover in the other direction
Heating demand at the reference station starts climbing again in November, the first month whose average temperature drops below the 65-degree baseline NOAA uses to count heating degree days, and it keeps building until it peaks in December. That gives most households here a fairly short runway, one autumn month, to get the heating side ready before real demand shows up.
What that pre-season list looks like depends on what’s actually installed. Statewide, 62% of homes heat with a furnace and 64% run on natural gas, so the checklist most households need centers on furnace-specific items: the filter again, the burner and igniter, and clear airflow at supply and return vents. Central heat pumps make up a small share, 3%, and steam or hot-water boilers an even smaller one, 1%, not zero, just uncommon; a household on a boiler system has a different list entirely, bleeding radiators and checking pressure rather than swapping a furnace filter, and that version of the checklist belongs in this site’s dedicated heating-season guide rather than here.
This page’s job ends at the handoff: know that November is when the reference climate here starts asking for heat again, know what your own system actually is before you assume the generic furnace checklist applies, and treat the fall changeover with the same unhurried, mild-day approach that works so well in spring.