May is the answer. Missouri’s heating and cooling demand bottoms out that month, at roughly 207 degree days combined, the lowest of any month on the calendar here. But that number doesn’t mean the furnace and the air conditioner both go quiet for four weeks. It means the opposite: this is the one month when the house calls for both, in the same week, sometimes the same day.
The quietest month in Missouri
Degree days aren’t a temperature reading. They’re a measure of demand, tallying how far the day’s average temperature strays from 65 degrees Fahrenheit, the point NOAA uses as the line where a house stops needing either system. Add up every degree of strain across a month and you get the total. May’s combined figure of about 207 degree days sits well above the lowest quietest-month total recorded anywhere in the country, which is around 70, and it’s not far off the highest quietest-month figure nationally, near 246. Read against that range, Missouri’s calmest month is closer to the busy end than the sleepy one.
That’s the honest answer to the question this page exists to settle: there isn’t a real pause here, a stretch of weeks where the thermostat sits untouched and both systems rest. May is the closest Missouri gets, and even May carries enough heating demand and enough cooling demand to count as a month that asks for both, not a month that asks for nothing.
Picture the shape of the year instead of chasing more numbers. Winter here is long and heavy, stretching from late fall through early spring, with a deep trough in the coldest months. Summer runs the other direction with real intensity, not just warmth but sustained heat load that keeps the air conditioner working hard for weeks at a stretch. Between those two extremes, spring and early fall are brief hinge points rather than settled seasons. The house tips from calling for heat in the early morning to calling for cooling by mid-afternoon, and back again the next cool front. That’s the character of a mid-continent climate: short transitions, long extremes, no real rest for the equipment.
Practically, this changes how you plan the two changeovers below. In a state with a genuine lull, you’d shut heating down and simply leave both systems off for a few weeks. In Missouri, the shutdown and the startup happen close together, and the days in between deserve their own approach, covered later in this piece.
Shutting the heat down for the year
Do this on a mild day, not a cold snap that circles back a week later. Missouri’s heating demand doesn’t fully clear until the monthly average climbs back above 65 degrees in June, so shutting the system down too early just means restarting it in a hurry.
- Stop calling for heat at the thermostat first, rather than cutting power at the breaker or the switch. Let the system respond to the setpoint change on its own terms.
- Let the furnace or heat pump finish its current cycle completely before touching anything else. Interrupting mid-cycle can leave components stressed unnecessarily.
- Check any fuel-burning equipment for what the manufacturer’s guidance actually requires at shutdown. Beyond that, leave gas supply lines and pilot assemblies alone; that’s not a routine homeowner task on modern equipment.
- Inspect and, if needed, replace the air filter. 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.” That’s the standard year-round, not just at shutdown, and it applies whether the equipment is gas, electric, or a heat pump.
- Shut down any humidifier mounted on the ductwork. It’s tied to the heating cycle and almost everyone forgets it’s even running until a dry winter reminds them.
- Clear dust, vacuum registers, and tidy the area around the furnace or air handler while it’s idle and accessible.
One thing this page won’t tell you to do: turn the heat off entirely for weeks at a time. Missouri’s heating season runs roughly eight months, from October into May, with January carrying the heaviest load of any month recorded here. A house left with no heat source at all during that stretch risks frozen plumbing the first hard cold front that rolls through. Set it back, not off.
The first run of the cooling season
Do this on a mild day, well before the first genuinely hot one. That’s the whole point: a fault caught in April, when 75 degrees feels comfortable either way, gets fixed on your schedule. The same fault discovered in July, with the reference station here averaging close to 33 days a year at 90 degrees or hotter, gets fixed on the repair company’s schedule instead.
- Clear leaves, mulch, and debris away from the outdoor condenser and give it a couple of feet of open space on every side.
- Check the condensate drain line for clogs or standing water before the system runs under real load.
- Change or wash the filter. ENERGY STAR is specific about what skipping this costs: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system in perfect mechanical condition can still underperform on a dirty filter alone.
- Switch the thermostat to COOL and let the system run a complete cycle on a mild day, not a scorcher.
- Feel the air at a register far from the unit, not the closest one. Weak airflow at distance often shows up before it shows up nearby.
Most of this list applies fully here. Missouri runs air conditioning in 95% of homes, and 83% of homes run it through a central system, which is exactly the setup these checks are built around. The remaining share, about 19% of homes, relies on individual equipment instead, meaning ductless mini-splits, window units, or portables. For that group, the outdoor-condenser and duct-airflow steps matter less; the filter step still matters just as much, and checking each unit’s own drain or coil takes its place. Ceiling fans show up in 81% of homes here too, and running them alongside the air conditioner lets you keep the thermostat a degree or two higher without losing comfort.
The days that ask for both
This is the situation nobody quite explains: heat clicks on at six in the morning, cooling clicks on by three in the afternoon, same house, same week. In Missouri, that’s not an edge case. Using a simple convention stated 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 at the same time. By that measure, both May and September qualify in this state. The house genuinely needs heat and cooling within the same short stretch, and the thermostat strategy has to account for that instead of picking one mode and forgetting it.
| Setting | What it does | Trade-off | Best use |
|---|---|---|---|
| Manual HEAT or COOL only | Locks the system to one mode | Misses the opposite need entirely on swing days | Deep winter or deep summer, not May/September |
| AUTO changeover | System switches modes on its own as temperature crosses each setpoint | Can cycle back and forth if setpoints sit close together | Genuinely mixed days, with a wide enough gap between setpoints |
| Wide deadband (setpoints several degrees apart) | Gives the house room to drift before either system fires | Slightly less tight temperature control | Nearly all shoulder-season days here |
| Open windows | No equipment runs at all | Only works when outdoor humidity and temperature actually cooperate | Mild mornings and evenings in May or September |
The expensive mistake is a narrow gap between the heating and cooling setpoints, say a single degree apart. On a swing day, that setup heats the house in the morning, then spends the afternoon cooling the same air it just paid to warm. Widen the gap and let the house coast through the middle of the range before either system has to work.
And the changeover in the other direction
Missouri’s heating demand starts building in October, the first month whose normal average temperature drops below the 65-degree line NOAA uses to count heating degree days. It builds through fall, peaks hard in January, and doesn’t fully release its grip until June. That’s the cue for the reverse of everything above: cool the house down through September, then start easing into the pre-heating checklist as October arrives.
What that checklist should actually cover here depends on what’s running. Furnaces heat 76% of Missouri homes, making them by far the dominant equipment, with central heat pumps accounting for another 10%. Natural gas fuels 59% of homes and electricity 31%, with propane at 7%. Boiler figures for this state weren’t published in the survey, an estimate suppressed rather than a genuine zero, so a smaller number of homes here almost certainly run steam or hot-water heat even though the data can’t size that share precisely. For the furnace-and-heat-pump majority, filter checks and a pre-season test run cover most of it. For any household on a boiler, the maintenance list runs differently, and that detail belongs in the dedicated heating-season guide on this site rather than here.