Maryland doesn’t get a true dead zone in its degree-day calendar. NOAA’s 1991-2020 climate normals put September, the state’s quietest month, at about 199 combined heating and cooling degree days, well above the national low of roughly 70. The honest answer runs on a calendar, not a single date: shut the furnace down sometime in late spring, start the air conditioner before the first real heat wave, and expect May to ask for both.
The quietest month in Maryland
Degree days measure demand, not temperature. A day with 20 heating degree days means the average temperature sat 20 degrees below the 65°F comfort baseline that NOAA uses as its reference point; it’s a workload number, not a weather report. That distinction matters here because a low-sounding month can still mean the furnace or the air conditioner ran plenty, just not as hard as in the extreme months.
September is that low point for Maryland, with roughly 199 combined heating and cooling degree days for the month. Set that against the national spread: the calmest month anywhere in the fifty states runs about 70 combined degree days, and the highest quietest-month figure recorded is around 246. Maryland’s 199 sits well up in that range, closer to the crowded end than to the empty one.
That placement answers the real question of this page directly. A state whose quietest month sits near 70 gets stretches of actual quiet, weeks where neither the furnace nor the air conditioner has much reason to run. Maryland’s number doesn’t support that story. The year here moves from a long, demanding cold season into a shorter but genuinely hot one, and the point where they meet is brief rather than restful. There’s no month in this state when the house asks for nothing at all; September is the lightest workload of the twelve, not an off switch.
Practically, that means the two changeovers covered below, shutting the heat down and firing the air conditioner up, sit close together on the calendar rather than bracketing a long stretch of shutdown. Plan around that reality instead of a pause that the numbers don’t back up.
Shutting the heat down for the year
Maryland’s heating season runs long, close to eight months by NOAA’s measure, with January carrying the heaviest load at 951 heating degree days. That length is the reason to treat this changeover as a process rather than a single flip of a switch, and to hold off until overnight lows are reliably staying above freezing. Cutting heat off too early, while a cold snap can still return, risks frozen plumbing in an unheated house, a real concern in a state where winter holds on into spring.
- Stop calling for heat at the thermostat first, either by raising the setpoint or switching the mode, rather than cutting power at the equipment.
- Let the system finish whatever cycle it’s mid-way through instead of interrupting it.
- Have fuel-burning equipment checked according to the manufacturer’s guidance rather than shutting off gas supply or a pilot light as a routine, do-it-yourself step.
- Change the filter at this point rather than skipping it until fall. ENERGY STAR’s standing advice: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.”
- Shut down and drain any whole-house humidifier tied into the ductwork. It’s the one component almost everyone forgets, and a pad left wet all summer is asking for mold.
- Clear registers and vents of anything piled against them over the winter, and tidy the area around the indoor unit before it sits idle.
With 64% of Maryland homes heating with a furnace and 19% with a central heat pump, most of these steps apply as written. The smaller share, about 7%, on a steam or hot-water boiler needs an added step at this changeover: bleeding radiators and checking the expansion tank, since that system doesn’t share a filter or blower with the summer cooling side.
The first run of the cooling season
Do this on a mild day, not the first hot one. A mild day gives room to find and fix a problem, a compressor that won’t kick on, a clogged drain, without living through a 90-degree afternoon while it gets sorted. Maryland sees an average of about 24.6 days a year at 90°F or above, so waiting for the heat to arrive before testing the system is waiting for the worst possible moment to discover trouble.
- Clear leaves, mulch, and anything else piled against the outdoor unit, leaving open space on all sides.
- Check the condensate drain line for clogs before the system runs hard enough to need it.
- Change or wash the filter, following the same monthly standard used at heat shutdown.
- Run the system in COOL for a full cycle on a mild day, not just a few minutes.
- Feel the air at a register far from the unit, not just the closest one, to catch weak airflow reaching the far end of the house.
ENERGY STAR frames the reason for that filter step plainly: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system can be mechanically sound and still lose that much output to a dirty filter or blocked vents, which is why the check belongs at both changeovers, not just one.
These steps apply cleanly to the 80% of Maryland homes running a central air-conditioning unit. The remaining 20% relying on individual equipment, ductless mini-splits, window units, wall units, or portables, skip the outdoor-unit clearing in favor of checking the unit’s own filter and the seal around a window or wall sleeve. About 66% of Maryland households also run ceiling fans, which stretch the comfortable range on either side of the AC startup date and can push that first full cooling run a little later into the season.
The days that ask for both
Every Maryland household with a thermostat that logs cycles recognizes this pattern: heat calls at six in the morning, cooling calls by mid-afternoon, same house, same week. Here’s the month where NOAA’s numbers actually back that up: May, with about 109 heating degree days and 90 cooling degree days. A month counts as asking for both, in this guide’s own terms, when it carries at least 50 heating degree days and at least 50 cooling degree days in the same thirty days. That’s a line drawn here for clarity, not a category NOAA itself publishes, and May is the one month in Maryland that clears both bars.
| Strategy | What it does | When it fits |
|---|---|---|
| AUTO changeover | Thermostat switches automatically between heat and cool as the temperature drifts | Convenient, but a narrow gap between the two setpoints can heat the house in the morning and cool that same air by afternoon, running both systems for no gain |
| Wide deadband | A several-degree gap held between the heating and cooling setpoints | Cuts down on same-day back-to-back cycling; the standard fix for a month like May |
| Windows open | No equipment runs at all | Best on the mild, low-humidity stretches within May, when neither system is truly needed |
The expensive version of this month is a thermostat set with almost no gap between its heating and cooling points, say 68°F to heat and 70°F to cool. That gap gets crossed twice a day without any real change in comfort, and the house ends up paying to heat the morning air and then paying again to cool the same air that afternoon.
And the changeover in the other direction
Maryland’s heating demand starts building again in October, the first month whose average temperature drops back below the 65°F baseline NOAA uses to count heating degree days, and the load climbs fast toward January’s peak of 951 degree days. That’s the season this guide’s cooling advice hands off to the reverse process: pilot lights and fuel checks scheduled ahead of the first cold snap, humidifier pads reinstalled, filters changed one more time before the furnace or heat pump carries the load through winter.
Which checks matter most depends on what’s actually installed. With furnaces running 64% of Maryland homes and central heat pumps another 19%, filter and airflow checks cover most households directly. Homes on a steam or hot-water boiler, about 7% of the state, follow a different list built around the water side of the system rather than a filter. Fuel sources split close to evenly too, with natural gas at 49% of homes and electricity at 41%, so the pre-season check for a gas furnace and the pre-season check for an electric heat pump aren’t the same conversation. That fuller changeover, and the specifics for each equipment type, belongs in this site’s heating-season guide rather than here.