September is the closest New Hampshire gets to quiet, but even then the house rarely asks for nothing. Heating demand starts as early as September and doesn’t fully let go until June, so there’s no calendar month here without a call for heat, for cooling, or for both. The honest answer: you’re not shutting a system off for good, you’re managing two changeovers a year, spring and fall, plus a stretch of days in between when the thermostat swings from heat to cool inside the same 24 hours.
The quietest month in New Hampshire
September is New Hampshire’s quietest month, with combined heating and cooling degree days coming to about 177. For comparison, the lowest quietest-month figure anywhere in the fifty states runs around 70, and the highest around 246. New Hampshire’s number sits well above that low end, more than double it, closer to the busier side of the national range than the calm side.
Degree days measure demand, not temperature. A day’s count reflects how far the average temperature strayed from the comfort baseline used to calculate it, not what the thermometer read at noon. That distinction matters here, because a reader glancing at “177” might assume it describes a mild month. It doesn’t. It describes a month where the house is still working, just less than it does in January or July.
With a combined total that high, September isn’t a real pause. It’s demand that has shrunk, not vanished. There’s no month in New Hampshire when the house asks for nothing at all. The shape of the year here runs long and cold: heating dominates from early fall straight through spring, with only a brief summer stretch where cooling outweighs it. Early autumn is where total demand bottoms out, but it doesn’t reach zero, it just splits between the two systems within the same week, sometimes the same day.
Summer heat is real here but compressed. The reference station averages about 10.9 days a year at 90°F or above, meaning genuinely hot weather is the exception, not the rule. That short, sharp cooling window is part of why the quietest month still carries real weight on the degree-day ledger, it’s just weight that gets divided between two different needs instead of resting on one.
Shutting the heat down for the year
Because heating demand here runs across roughly nine months of the year, the actual shutdown window is brief and it isn’t fixed to a date. It arrives whenever the household stops needing overnight heat consistently, usually somewhere in late spring. When it does, the order matters.
- Stop calling for heat at the thermostat rather than killing power at the breaker. Let the controls end the season, not a hard cutoff.
- Let the system finish whatever cycle it’s mid-run on before touching anything else.
- Have fuel-burning equipment, furnaces and boilers, inspected according to the manufacturer’s guidance. That guidance governs pilot lights and gas supply, and it’s not a job to freelance.
- Change or clean the filter. As U.S. ENERGY STAR puts it: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” Season’s end is a natural moment to do it, not the only one.
- Shut down any humidifier fitted to the ductwork, drain it, pull the pad, close its damper. It’s the step almost everyone skips.
- If the house runs a steam or hot-water boiler, about a quarter of New Hampshire homes do, bleed the radiators and note the system pressure now, so a slow leak or a stuck valve shows up before the next cold snap, not during it.
- Clear the area around the equipment and vacuum out vents and returns while everything is idle and accessible.
Given how quickly demand returns here, never leave the system fully disconnected or the plumbing without a heat source through the shoulder season. “Shutting down” in New Hampshire really means servicing, not unplugging.
The first run of the cooling season
Run the first cooling cycle on a mild day, not the first hot one. A fault caught in May is a simple repair. The same fault discovered in July, mid-heat-wave, is a wait.
- Clear the outdoor unit and everything growing or piled around it.
- Check the condensate path for blockages before the humid months arrive.
- Change or wash the filter, the single most-touched maintenance item.
- 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 the nearest one, to catch weak airflow early.
U.S. ENERGY STAR notes: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” A system can be mechanically sound and still underperform if that airflow check gets skipped.
That checklist applies fully to the roughly one in five New Hampshire homes running a central air conditioner. Statewide, 78% of homes have some form of cooling, but only 21% run central systems; 59% rely on individual equipment, window units, wall units, ductless mini-splits, or portables, and 58% use ceiling fans alongside whatever else they’ve got. For that larger group without central air, the pre-season check looks different: reseating and resealing a window unit, cleaning its coil, checking the mounting; for a Mini-Split, clearing the outdoor pad and washing the coil. The filter step applies across all of it.
The days that ask for both
September is the one month here that clears a threshold worth naming: at least 50 heating degree days and at least 50 cooling degree days in the same month. It comes in at 98 heating degree days and 78 cooling degree days, both above that line. That threshold is a convention used on this page, not a category NOAA publishes, but the pattern it describes is real: a house calling for heat at six in the morning and for cooling by mid-afternoon, in the same week, sometimes the same day.
| Setting or action | What it does | Caution |
|---|---|---|
| Fixed HEAT or COOL mode | Forces the system to answer only one kind of call | The other need goes unmet until someone switches it back |
| AUTO changeover | Lets the system decide which call to answer first | Convenient, but can mean more short cycles as the system flips back and forth across a single day |
| Wide deadband between setpoints | Gives the house room to drift before either system fires | Fewer starts, but a wider swing in comfort between them |
| Open windows | Lets outside air do the work during mild hours | Best option when the outdoor temperature already sits between the two setpoints |
The expensive mistake is a narrow gap between the heating and cooling setpoints, say 68°F for heat and 70°F for cool. That narrow band pushes the house to heat in the morning and then cool in the afternoon the very air it just paid to warm, running both systems against each other on the same day.
And the changeover in the other direction
Heating demand here starts as early as September, the first month whose normal mean temperature drops below the 65°F baseline used to count heating degree days. It climbs fast through October (402 heating degree days) and November (706), peaks in January at 1,221, and doesn’t ease until the mean climbs back above 65°F in June. That means the reverse changeover, cooling down for good, heating up in earnest, lands for most New Hampshire households right around September and October, the same stretch where the “asks for both” month tips over into straight heating.
What matters most in that fall changeover depends on what the house actually burns. Furnaces are the largest single category at 53% of homes, and for them it’s the standard filter-and-burner routine. Steam or hot-water boilers run in about a quarter of homes, and for those it’s pressure and bleeding, not a coil. Fuel oil or kerosene supplies 40% of homes here, a high share, so the tank and the flue deserve attention before the season starts, not just the filter. Natural gas (28%) and propane (15%) systems follow their own manufacturer guidance; a central heat pump’s share isn’t published for this state, which means the survey found the estimate too unreliable to report, not that none exist.
The full fall version of this checklist, in order and by equipment type, belongs in this site’s heating-season guide rather than repeated here.