Heating demand in Washington begins in September, when the statewide reference station’s average temperature first dips below the 65°F mark used to calculate heating degree days. The season builds through fall, peaks in December, and stretches across roughly ten months before the mean climbs back above 65°F in July.
When the heating season starts in Washington
The number that matters here isn’t a date on a calendar. It’s a threshold: 65°F. That’s the base temperature the National Oceanic and Atmospheric Administration uses to calculate heating degree days, and at Seattle-Tacoma International Airport, the 30-year normal mean temperature drops below that line starting in September. That’s the month heating demand technically begins, according to NOAA’s climate normals.
December is when the need is heaviest, with 711 degree days, the highest monthly figure on the chart below. From there, the season doesn’t really let go until spring gets underway, and the mean doesn’t climb back above the 65°F line until July. That’s close to ten months of the year where a Washington home, on average, needs some form of heat.
None of this means a homeowner should circle a date on the fridge calendar. A monthly normal is an average pulled from three decades of data. It tells you what a typical September or December looks like across Washington’s lowlands, not which specific evening the first cold snap will hit. Cold fronts don’t read spreadsheets, and they routinely show up before the monthly average would suggest.
Monthly heating degree days at Seattle-Tacoma International Airport
| Month | Heating Degree Days |
|---|---|
| January | 687 |
| February | 588 |
| March | 557 |
| April | 412 |
| May | 242 |
| June | 118 |
| July | 31 |
| August | 24 |
| September | 102 |
| October | 349 |
| November | 555 |
| December | 711 |
Every month on the chart carries at least some heating demand, even the height of summer. That’s the shape of a maritime climate: it rarely gets brutally cold, but it also rarely lets a furnace stay fully off.
How hard the winter works here
Add up all twelve months and Seattle-Tacoma lands at roughly 4,376 heating degree days a year, against about 265 cooling degree days. That lopsided ratio tells the real story: this is a heating climate first, with air conditioning as an afterthought for most of the year.
Degree days work as a rough stand-in for fuel use. A house that logs twice the degree days of another, similar house, will burn roughly twice the fuel to stay comfortable, all else being equal. So the annual total gives you a sense of the workload, but it hides an important detail: two places can share the same yearly number while feeling completely different to live in. A short, harsh winter and a long, mild one can average out to the same total.
Washington’s version, based on the monthly table above, leans toward the long-and-mild side. There’s no single brutal month that dominates the year. Instead, degree days are spread across ten months, with a broad, gentle peak in December rather than a sharp spike. That pattern matters for how a home’s heating system gets used: shorter, more frequent cycles across most of the year, rather than a few weeks of nonstop, high-intensity operation.
What most homes in Washington actually heat with
Federal survey data on how Washington households actually heat their homes points to a mixed setup, not a single dominant technology.
| System or Fuel | Share of Homes |
|---|---|
| Furnace | 47% |
| Central heat pump | 9% |
| Steam or hot-water boiler | not reported (estimate suppressed) |
| Natural gas (fuel) | 37% |
| Electricity (fuel) | 55% |
| Fuel oil or kerosene | not reported (estimate suppressed) |
| Propane | not reported (estimate suppressed) |
Furnaces are the most common equipment type, and electricity edges out natural gas as the more common fuel, according to the U.S. Energy Information Administration’s Residential Energy Consumption Survey. Where boilers, fuel oil, and propane show “not reported,” that doesn’t mean nobody in Washington uses them. It means the survey sample was too thin, or the estimate too unreliable, to publish a number. Treat those as unknowns, not zeros.
A furnace being the most common system says something about what fuel was cheap when Washington’s housing stock went up. It says nothing about what’s best for any particular house today. A furnace pushes heated air through ducts quickly and evenly, which suits a home with an existing duct system well. What it doesn’t handle gracefully is neglect: a clogged filter chokes airflow, forces the blower motor to work harder, and can shorten the unit’s life. That’s the one maintenance item furnace owners pay for when they skip it.
Run it once before you need it
Systems don’t usually fail in July. They fail on the first genuinely cold night of the year, which in Washington tends to land sometime after that September threshold passes and demand starts climbing through October and November. That’s also the exact week every heating technician in the state gets booked solid. Testing the system weeks early, while the calendar still says September, means any problem gets fixed on a slow week instead of a frantic one.
- Set the thermostat a few degrees above room temperature and run the system for a short cycle, just long enough to hear it kick on and settle into a steady hum. Rattling, screeching, or a burning-plastic smell (not the same as burnt dust) are reasons to stop and investigate.
- Expect a faint burnt-dust smell on the very first firing of the season. That’s normal: it’s dust that settled on the heat exchanger or coils over the summer burning off, and it should clear within a few minutes.
- Replace or clean the air filter. A clogged filter is the single most common cause of restricted airflow, and it’s the cheapest fix on this list.
- Walk the house and clear the space around vents, radiators, or an outdoor heat pump unit. Furniture blocking a vent, or leaves piled against an outdoor unit, forces the system to work harder for less output.
- Test every carbon monoxide alarm in the house by pressing the test button, and replace batteries if needed. Any fuel-burning furnace can produce carbon monoxide if it malfunctions, and a working alarm is the cheapest insurance against it.
- If the system won’t hold a steady cycle, trips a breaker, or the smell doesn’t clear, stop testing and call a licensed technician rather than troubleshooting further.
Even a system that passes every one of these checks can still be losing heat before it ever reaches a room. According to the U.S. Department of Energy, “in a typical house, about 20%-30% of the air that moves through the duct system is lost due to leaks, holes, and poorly connected ducts.” That’s not a flaw in the furnace or heat pump itself. It’s the ductwork quietly working against it, and it’s worth a look separate from the seasonal test above.
What turning it down is actually worth
The Department of Energy puts a real number on the habit of dialing the thermostat back: “You can save as much as 10% a year on heating and cooling by simply turning your thermostat back 7-10°F for 8 hours a day from its normal setting.” That’s an annual figure covering both heating and cooling, tied to a specific setback range held for a specific stretch of hours, not a per-degree rule that scales up or down.
In a state where heating demand touches ten months of the year, that setback has a lot of hours to work with. A household running a nightly or workday setback here isn’t applying it to a short, three-month season. It’s applying it across nearly the entire calendar, which is exactly the condition under which that annual savings figure was built.