May is Oklahoma’s quietest month for heating and cooling combined, but the number attached to it, about 201 degree days, still sits closer to the busy end of the national scale than the calm one. That single figure decides almost everything below: whether you get a real break between systems, and when each changeover actually makes sense.
The quietest month in Oklahoma
May carries roughly 201 combined heating and cooling degree days at the Oklahoma City reference station, according to NOAA’s 1991-2020 Climate Normals. To put that in context, the calmest month anywhere in the fifty states runs about 70 degree days, and the highest calmest-month figure recorded is about 246. Oklahoma’s 201 lands well up that scale, closer to the crowded end than the empty one.
Degree days aren’t a temperature. They’re a running tally of how far the outdoor air strays from a comfort baseline, added up hour by hour across the month. A day that swings from 45 degrees at dawn to 80 by mid-afternoon can rack up both heating demand and cooling demand before dinner, and that single day contributes to both totals. So a degree-day number tells you how much work the house is doing, not how warm it felt outside.
A lull, not a shutdown
Here’s the honest read: 201 degree days in a single month is not a stretch of weeks where the house asks for nothing. It’s the sound of a year turning over rather than resting. Mornings still cool enough to want the furnace running, afternoons already warm enough to call for the air conditioner, sometimes inside the same 24 hours. That’s a fundamentally different situation from a state where the calmest month barely registers a heartbeat of demand.
What this means in practice: don’t plan on flipping the whole system off in May and forgetting about it until fall. The smarter framing is that May (and, as the next sections show, October) are the two months when Oklahoma homes are managing both directions of comfort at once, rather than either month offering a genuine pause. There is no calendar month in this state when the house asks for absolutely nothing from either system. The closest thing to a break is a short window inside May when demand from both sides is low enough that the thermostat can coast rather than fight, and that’s the window worth using for the maintenance steps in the next two sections.
Shutting the heat down for the year
Because Oklahoma’s heating season runs a full seven months, October through May, with January alone piling up 832 degree days, this changeover should happen after the season’s last real cold spell, not on the first warm afternoon. Do it in this order:
- Set the thermostat so it stops calling for heat, rather than cutting power to the furnace or heat pump at the breaker.
- Let the system finish whatever cycle it’s mid-way through before touching anything else.
- Leave gas supply valves and pilot assemblies alone; on any modern furnace or boiler, shutdown and startup procedures for the fuel train are governed by the manufacturer’s manual, not by a general house checklist.
- If the ductwork feeds a whole-house humidifier, shut that down too. It’s the single step almost everyone forgets, and a humidifier left running into a cooling season can push moisture into ducts that no longer need it.
- Pull the filter and either clean or replace it. ENERGY STAR’s maintenance guidance is direct on timing: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” That’s a once-a-month habit for all three types of equipment, not a seasonal afterthought, and a changeover is a natural checkpoint to reset the schedule.
- Clear the area around indoor equipment, vacuum visible dust from registers and returns, and note anything that looked off during the winter, an odd smell, a noise, a room that never got warm, so it’s on record before you forget it over the summer.
With natural gas heating 52 percent of Oklahoma homes and furnaces the dominant equipment type at 69 percent, most of the fuel-burning caution above applies directly here. And because winter in this state is real, seven months of measurable heating demand with January alone accounting for more degree days than any other month, this changeover is not the moment to disable backup heat sources or drain systems that protect against a hard freeze. Save that for well after the last cold snap has passed.
The first run of the cooling season
Do this on a mild day, not on the first 95-degree afternoon. That’s the whole point of running the cooling test early: a mild day gives a fault time to surface and get fixed before it turns into a household emergency. Oklahoma City averages about 70.7 days a year at 90 degrees or hotter, so there’s a real cost to discovering a dead capacitor or a clogged condensate line on day one of that stretch instead of weeks ahead of it.
- Clear leaves, grass clippings, and anything stacked against the outdoor condenser unit, giving it at least a couple feet of open air on all sides.
- Check the condensate drain path for algae or blockage before the system runs long enough to need it.
- Change or wash the filter. ENERGY STAR is specific about why this matters more than it sounds: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system can be mechanically sound and still underperform badly if the filter is choked.
- Switch the thermostat to COOL and let it run a full cycle on a mild day, watching for it to actually shut off once it reaches the setpoint rather than running continuously.
- Feel the air at a register farthest from the equipment, not the closest one. Weak airflow at the far end of the house shows up long before it’s noticeable up close.
This checklist applies almost universally in Oklahoma, since 83 percent of homes run a central air-conditioning unit, per EIA’s Residential Energy Consumption Survey. But 96 percent of homes report having air conditioning of some kind, which leaves a meaningful slice, around 13 percent, running on individual equipment instead: ductless mini-splits, window units, or portables. If that’s the setup, swap the outdoor-unit check for inspecting the unit’s own vents and drainage, and the condensate-line check applies to the individual unit rather than a whole-house line. Ceiling fans, present in 87 percent of Oklahoma homes, are worth turning on during this same test run since they let the AC coast at a slightly higher setpoint once the cooling cycle proves itself.
The days that ask for both
Every Oklahoma homeowner recognizes this without needing a name for it: heat clicking on before sunrise, air conditioning clicking on by mid-afternoon, same house, same week, sometimes the same day. NOAA’s normals show two months here that cross a useful line, at least 50 heating degree days and at least 50 cooling degree days in the same month, and that’s a threshold stated for this discussion, not a category NOAA itself publishes. May and October both clear it. That’s not a contradiction in the house’s systems; it’s simply what a transition month looks like when mornings still run cool and afternoons already run warm.
The practical question is what to do with the thermostat during those weeks. The choices differ enough in cost and convenience to lay out directly:
| Approach | What it does | Cost or tradeoff |
|---|---|---|
| Manual mode switching | You flip between HEAT and COOL by hand as the day’s weather demands | Free, but requires attention twice a day |
| AUTO changeover | Thermostat switches modes on its own between a heating and cooling setpoint | Convenient, but a narrow gap between setpoints makes the system cool the same air it just paid to heat that morning |
| Wide deadband | A generous spread, often several degrees, between the heating and cooling setpoints | Lets natural swings in outdoor temperature do the work instead of triggering both systems in the same day |
| Windows open | Systems off entirely, natural ventilation handles comfort | Best option on the mildest of these transition days, free, and the only real pause available |
The expensive mistake is a narrow deadband on AUTO changeover, something like a 1 or 2 degree gap between the heat setpoint and the cool setpoint. That configuration heats the house at 6 a.m. and cools it again by 3 p.m., burning energy twice on the same air in the same day. Widening that gap, or simply switching to open windows on the mildest of these transition days, is the cheaper answer both months offer.
And the changeover in the other direction
Heating demand in Oklahoma starts building in October, the first month whose normal mean temperature, 61.1 degrees, drops below the 65-degree baseline NOAA uses to count heating degree days. It climbs fast from there, peaking in January’s 832 degree days, and doesn’t ease back below that 65-degree line again until May. Practically speaking, that means most Oklahoma households are flipping from cooling to heating sometime in October, right around the same weeks the “asks for both” table above already covers.
The autumn version of the checklist matters more here than the spring one, given how much winter work the system is about to do. With furnaces running in 69 percent of homes and central heat pumps in another 11 percent, and boiler-based heat too rare in state surveys to publish a figure, this is squarely a furnace-and-heat-pump state rather than a hot-water-radiator one. That shapes what the pre-season check should focus on: filter condition heading into the season’s heaviest month, ignition and burner performance on furnaces, and defrost-cycle checks on heat pumps rather than bleeding radiators or checking boiler pressure. The fuller version of that list, and the timing questions specific to a seven-month heating season, belongs in the heating season guide for Oklahoma on this site.