In Colorado, the quietest point in the heating-and-cooling cycle falls in June, but “quiet” needs a caveat: the degree-day count that month is far from zero. There’s no month here when the house asks for nothing at all, though June comes closer than any other. The real work is in the changeover, not some mythical off-switch, and that’s what the rest of this page walks through.
The quietest month in Colorado
Degree days measure demand, not temperature. They tally how far each day’s average temperature strays from 65°F, in either direction, and add that up across the month. A cold day adds heating degree days, a hot one adds cooling degree days. By that measure, June is Colorado’s quietest month, combining to about 199 degree days total between the two categories.
Set that number against the country as a whole. The mildest month anywhere in the fifty states runs around 70 combined degree days. The highest such figure, in the state where even the calmest month still demands real conditioning, reaches about 246. Colorado’s 199 sits well into the upper half of that range, closer to the states where the year never fully lets go than to the ones with a genuine seasonal lull.
That placement matters for what this page can honestly promise. A total near 70 would mean weeks where neither the furnace nor the air conditioner has any reason to run at all, real downtime. A total near 199 doesn’t describe that kind of month. It describes a June where mornings can still call for a few degrees of heat and afternoons can already ask for cooling, with the overall volume of demand simply lower than in the depths of winter or the peak of summer.
So the honest answer to when the heat can go fully off and stay off, with the cooling system doing all the work from there: there isn’t one clean date, and this page won’t invent one. What exists instead is a changeover, a period when the two systems trade places rather than both going silent together. That’s the subject of the rest of this guide: not a pause, but a handoff, and the maintenance that makes each side of it safe.
Shutting the heat down for the year
The end of the heating season in Colorado isn’t a single date. Statewide, heating demand doesn’t taper off until the monthly average climbs back above the 65°F baseline, which happens around June. Shutting the system down properly, once that point arrives, is less about flipping a breaker and more about sequencing.
- Turn the thermostat down so the system stops calling for heat, rather than cutting power at the breaker or switch first.
- Let the equipment complete its final cycle undisturbed. A furnace or boiler powered off mid-cycle can be left in a state it wasn’t designed to sit in for months.
- For any fuel-burning furnace or boiler, follow the manufacturer’s guidance on seasonal shutdown. The pilot light and gas supply on modern equipment are governed by that guidance, not by a routine seasonal habit.
- Inspect and, if needed, change the air filter. ENERGY STAR’s standing advice applies here as much as anywhere: “Inspect, clean, or change air filters once a month in your central air conditioner, furnace, and/or heat pump.” The seasonal changeover is a natural checkpoint for this even if the monthly habit has slipped over winter.
- If the ductwork carries a whole-house humidifier, shut off its water supply and let the pad or drum dry out. It’s tied directly to the furnace and gets forgotten almost every year once the heat stops running.
- Clear the area around the furnace or boiler of stored items, and vacuum visible dust from vents and the unit’s exterior panels.
None of this means killing the heat entirely for the summer in a way that leaves pipes exposed to a freeze. Colorado’s heating season stretches across roughly nine months, from September through May by the monthly averages, with December and January carrying the heaviest load of the whole year. By the time June arrives and the system is genuinely being shut down for the year, overnight lows have moved well past any freeze risk. That timing is the point: shutting down early, before the mean temperature has actually turned, is the mistake worth avoiding, not the shutdown itself.
The first run of the cooling season
The best day to start the air conditioner for the season is a mild one, not the first 90-degree afternoon. A mild day leaves room to find a problem, a weak capacitor, a clogged drain line, a filter nobody replaced since fall, without living through a house that won’t cool down while the temperature outside keeps climbing.
Colorado averages about 39.6 days a year at 90°F or hotter. That’s real heat load, enough that a system caught off guard has consequences, but not so much that there’s no window to test things early on a cooler day first.
- Clear leaves, grass clippings, and anything stacked around the outdoor condenser unit. Restricted airflow around the unit is one of the most common causes of poor cooling.
- Check the condensate drain line for blockages. A clogged line is a leading cause of water damage once cooling season is underway.
- Change or wash the air filter, on the same monthly schedule used for the furnace.
- Run the system in COOL for a full cycle on a mild day, letting it reach the thermostat’s setpoint rather than shutting it off after a few minutes.
- Feel the air at a register far from the unit, not just the nearest one. Weak airflow at the far end of the house often shows up before anything else does.
How much of this applies depends on what’s actually installed, and Colorado leans toward central systems more than many states: 82 percent of homes run some form of air conditioning, and 57 percent of that is a central unit tied to the same ductwork the furnace uses. The remaining stretch, roughly a quarter of homes, cools with something individual, a ductless mini-split, a window unit, or a portable, which means the duct filter and register checks above apply mainly to the central-system majority. Seventy-two percent of homes also run ceiling fans, which do real work here: they don’t lower the air temperature, but they cut how hard the AC has to work to make a room feel comfortable on the state’s 39.6 heat-load days a year.
ENERGY STAR puts a number on why the filter and airflow checks matter more than they look: “Airflow problems can reduce your system’s efficiency by up to 15 percent.” That’s a working system running at a real cost, not a broken one, the kind of loss nobody notices on the thermostat, only on the bill.
The days that ask for both
There’s a specific kind of week no single seasonal guide accounts for: heat needed at six in the morning, cooling needed by mid-afternoon, in the same house, in the same 24 hours. Colorado has two months a year that fit this pattern by a stated threshold used on this page, not a published NOAA category: a month counts as asking for both when it carries at least 50 heating degree days and at least 50 cooling degree days in the same month. By that measure, June and September are Colorado’s two swing months, the ones where demand doesn’t lean cleanly toward one system or the other.
This isn’t a contradiction in the equipment. It’s a description of what the house is actually doing, losing heat overnight in a state with wide daily temperature swings, then gaining it back fast once the sun is up.
| Thermostat approach | What it does | Where it costs you |
|---|---|---|
| Manual mode switching (HEAT to COOL by hand) | Gives full control over which system runs and when | Easy to forget, leaving the wrong system idle on the day it’s actually needed |
| AUTO changeover | Lets the thermostat pick heat or cool automatically based on the setpoints entered | Can short-cycle both systems in a single day if the setpoints sit close together |
| Wide deadband between setpoints | Sets a gap, often several degrees, between the heating and cooling targets | None, this is the fix for the short-cycling problem above |
| Opening windows instead | Free ventilation on mild mornings and evenings when outdoor air matches indoor comfort | Only works when outdoor humidity and temperature actually cooperate |
The expensive mistake shows up in that second row. A narrow gap between the heating setpoint and the cooling setpoint, something like heat at 70 and cool at 71, means the furnace runs at dawn and the air conditioner spends the afternoon cooling the same air the furnace just paid to heat. Widening that gap to four or five degrees, and letting the house drift naturally between them, is usually enough to stop paying for both directions in the same day. On the mildest mornings in June or September, opening a window costs nothing at all.
And the changeover in the other direction
The reverse changeover in Colorado starts earlier than most people plan for. By the monthly averages, September is the first month whose mean temperature drops back below the 65°F baseline used to count heating degree days, meaning the heating season effectively opens then and doesn’t fully release its grip until June, nine months out of twelve, with December and January carrying the heaviest load of the year.
That’s a long season, and prepping for it depends on what’s actually installed. Furnaces run in 77 percent of Colorado homes, which makes the standard pre-season furnace checklist, filter, ignition system, exhaust venting, the right one for most of the state. But 9 percent of homes heat with a steam or hot-water boiler, a system with its own pre-season routine built around bleeding radiators and checking system pressure rather than filters. Natural gas fuels 76 percent of homes here and electricity another 16 percent; the state’s central-heat-pump share wasn’t published in reliable enough numbers to report, which doesn’t mean it’s absent, just that the survey sample was too thin to publish a figure.
The detail of that autumn changeover, the fuel checks, the ignition tests, the venting inspection, belongs in this site’s dedicated heating-season guide rather than repeated here. What matters for now is the timing: with a season this long, waiting for the first cold snap to think about it means starting behind, not ahead.