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Not November. According to thirty years of National Weather Service normals for Islip, buildings on Long Island begin drawing heat on September 23. By the time the first cold snap makes anyone think about it, the season is already a third of the way through its first thousand degree days.
Figures calculated from National Weather Service daily climate normals for Islip, Long Island MacArthur Airport, 1991 to 2020. A heating degree day is one degree of daily mean temperature below the 65°F base.
A heating degree day counts how far the day's average temperature fell below 65°F. A day averaging 55°F produces ten of them. A day averaging 65°F produces none. The number tracks how hard a building has to work to hold its setpoint, which makes it a far better measure of demand on your equipment than the temperature on a thermometer.
New York uses the same measure. NYSERDA publishes monthly heating and cooling degree day data by region and defines the heating season as October through March. That definition works for energy accounting. It does not describe what your equipment experiences, because the equipment starts working before the calendar says it should.
Run the daily normals for Islip and the picture is specific. The normal mean temperature sits at 71°F on September 1 and holds at or above 65°F through September 22. On September 23 it reads 64°F, and heating demand begins. From there it does not stop until spring.
September produces sixteen heating degree days. October produces 285. Nobody notices the transition because it happens one degree at a time, spread across five weeks.
That gradual slope is the reason heating problems get discovered late. A furnace with a cracked igniter or a boiler with a failed circulator will run through October without complaint, because October only asks it for a fraction of what January will. The fault is there the whole time. The load just has not arrived to expose it.
Every bar is a single day of normal heating demand at Islip. Drag the slider to move through the fall and see what the building is being asked to do on any given date.
The building is not calling for heat on a normal day this early. This is the window where service gets booked at a normal rate and on your schedule rather than someone else's.
Source and method. Daily normal mean temperatures come from the National Weather Service New York office almanac for Islip, Long Island MacArthur Airport, covering the 1991 to 2020 normals period. The September almanac and the October almanac are published as PDFs and updated by the forecast office.
Heating degree days were calculated as 65 minus the normal daily mean, with negative values counted as zero. Normals describe a thirty-year average and no individual year matches them exactly.
The chart above has three inflection points, and each one changes what a building owner should be doing.
The first heating degree day of the season. A single degree, on a day nobody would call cold. Nothing in the building responds yet, and that is the point: this is the last stretch of the year when a heating contractor has open schedule.
NOAA's 1991 to 2020 normals put the average first fall frost at the Islip station on October 28, at thirty percent probability. Frost is when a marginal system gets its first real test, and it is also when the calls start.
November delivers twice October's demand and thirty-six times September's. Anything that was going to fail under load is going to do it now, and the technician who could have come in three weeks ago is booked.
A house that loses heat is a bad night. A restaurant that loses heat closes. A warehouse with freeze-sensitive stock loses product, and a medical or office building with no heat sends people home.
Commercial systems also take longer to fix. Rooftop units need roof access and sometimes a crane. Boiler parts on older equipment are not on a truck. A commercial repair that would take an afternoon in September turns into a multi-day job in December when everyone in the trade is running emergency calls.
The gap between those two outcomes is roughly six weeks of lead time, and the chart shows exactly where that window sits.
Four stops between now and the first hard freeze. Each one is easier and cheaper than the one after it.
Zero heating degree days on a normal day, and open calendars across the trade. A system that has sat idle since March gets started, tested and adjusted with no pressure and no one waiting on it. If a part is needed, there is time to order it.
The first 112 degree days of the season land in this stretch. Fire the heat on a cool morning and pay attention to the start sequence, the noise and how long the building takes to come up. Faults announce themselves at low load before they fail at high load.
Demand is climbing and first frost arrives at the end of the month. This is the last stretch where a repair can be scheduled rather than dispatched. Parts availability is still normal and so are lead times.
580 degree days in November alone. From here, every heating fault is an emergency by circumstance rather than by severity, and the cost of that is measured in downtime as much as in the invoice.
Seven checks a building owner or maintenance lead can run without tools. Your progress saves in this browser, so you can work through it over a few days.
Nothing checked yet. Start anywhere.
Everything on this page describes a thirty-year average. Individual years run warmer or colder, and a season that starts late can still deliver a brutal January. The value of normals is not prediction, it is planning: they tell you when demand typically arrives so you can be ahead of it rather than reacting to whatever this particular year does.
The other limit worth naming is the station. Islip MacArthur sits in the middle of Suffolk County, a few miles from the water. The South Shore runs slightly milder in early fall, the North Fork and inland Brookhaven slightly cooler. The shape of the curve holds across Long Island even where the exact numbers shift by a degree or two.
What does not change is the sequencing. Demand begins in late September everywhere on the island, climbs through October, and roughly doubles in November. Any building that waits for the first cold snap to think about heat has already spent five weeks running equipment nobody checked.
September, before the first heating degree days accumulate on September 23. That timing gives a technician an idle system to test properly, leaves room to order a part if something is worn, and avoids the scheduling crush that starts with the first cold week.
August works too and is easier to book. What does not work is November, when every heating contractor on the island is running emergency calls and a routine service visit competes with buildings that have no heat at all.
It measures how far a day's average temperature fell below 65°F. A day averaging 50°F produces fifteen heating degree days. A day averaging 68°F produces none. Adding them up over a month gives you the heating load that month placed on a building.
Utilities and energy analysts use the measure to compare seasons and normalize energy bills against weather. For a building owner it answers a more practical question: how hard has my equipment actually been working, regardless of what the thermostat says.
Because load exposes faults that low demand hides. A furnace with a weak igniter, a boiler with a sticking zone valve or a heat exchanger with a developing crack will all run acceptably through October, when the building only needs a few hours of heat a day.
November roughly doubles October's demand. The same component now cycles more often, runs longer and reaches higher temperatures. Nothing new broke. The load finally arrived to reveal what was already there.
Closely enough to plan on. The figures here come from Islip MacArthur Airport, which sits near the middle of Suffolk County. South Shore locations near the water tend to hold warmth a little longer in early fall, and inland or North Fork locations cool marginally faster.
The differences run to a degree or two, which shifts the first heating degree day by a matter of days, not weeks. The shape of the curve, and the timing of the November jump, holds across Suffolk and Nassau.
Yes, and the reason is that a system which finished last season is not the same as a system which will finish this one. Six months of sitting idle lets seals dry, dust settle on burners and controls drift. Combustion, safeties and heat exchanger condition are checked because the consequence of a fault in those areas is not inconvenience.
An annual visit also generates a record, which matters for warranty terms on newer equipment and for a commercial building with inspection obligations.
Everyone in the region fires their heat within the same forty-eight hours, and every system that was going to fail on startup fails at once. That is why response times stretch in the first genuinely cold week of the year, and why the same repair is harder to schedule then than it was a month earlier.
The number of failures does not spike. The number of failures discovered on the same day does.
Commercial or residential, the September window is the one worth using. Send the details and we will come back with dates inside weekday hours. If a building already has no heat, call 631-710-6257 instead, day or night.
The equipment type, its rough age, the address, and when it was last serviced. If it did anything odd at the end of last season, mention that too, because it is usually the same thing that fails first this year.
Restaurants, retail floors and offices rarely close for maintenance. Early mornings, evenings and weekends are normal for us, so say what fits your operation.
Inside weekday hours, Monday to Friday, 8am to 5pm. Emergency service runs 24 hours a day, every day of the year, and that one is a phone call.
Commercial buildings, restaurants, warehousing, medical and office properties, and homeowners across Suffolk and Nassau County, since 1999.
Temperature Control Systems has run Long Island heating seasons since 1999, across more than 6,000 completed projects, with emergency cover every day of the year.
