How long should I wait after changing the filter before my AC starts cooling again?

Give the system 20–30 minutes after changing a severely clogged filter. If the evaporator coil froze due to restricted airflow, you may need to let it thaw for 2–4 hours with the system in fan-only mode before full cooling returns.

My AC runs constantly but can't reach the thermostat setpoint on hot days. Is that normal in Fairfax?

On extreme heat days, it's normal for a correctly sized AC to run nearly continuously. If it's running non-stop on a moderate summer day and still can't catch up, you likely have an airflow problem, a refrigerant issue, or a system sizing problem.

Should I turn my AC off if it's not cooling, or keep running it?

If you suspect a frozen coil, switch to fan-only mode or turn the system off. For other issues like a dirty filter or dirty condenser, you can continue running while you address the fix. When uncertain, fan-only is the safest setting.

How do I verify an HVAC contractor is licensed in Virginia?

Virginia requires all HVAC contractors to hold an active license through the Virginia Department of Professional and Occupational Regulation (DPOR). You can verify any contractor's license status on their website.

Is a heat pump a good replacement for a failing AC in Northern Virginia?

Yes — modern variable-speed heat pumps are well-suited to NoVA's Climate Zone 4A. They qualify for the largest federal tax credits available in 2026 and Dominion Energy offers rebates for qualifying installations.

Back to BlogHeating · Fairfax, VA · Northern Virginia

When Should You Turn Your Heat On in Northern Virginia? A 2026 Homeowner's Guide

Chilly mornings, warm afternoons — so when do you actually switch from cooling to heating in Fairfax, Vienna, Reston, Alexandria, and Arlington? The 68°F rule, why the overnight low matters more than the afternoon high, the 60°F AC limit, and the carbon monoxide check to do first.

September 23, 2026 By the Fairfax Mechanical Team 9 min read

Short answer: In Northern Virginia, turn your heat on when indoor temperatures start slipping below the 68°F comfort range and can't recover on their own — usually mid-to-late October in Fairfax, Vienna, Reston, Alexandria, and Arlington. Watch the overnight lows rather than the warm afternoon highs, don't run the AC below about 60°F outside, and give the furnace a safety check before its first run.

The simple rule of thumb for turning your heat on in Northern Virginia

Every fall, homeowners across Fairfax, Vienna, Reston, Alexandria, and Arlington face the same small standoff: it's chilly in the morning but warm by afternoon, and nobody wants to be the first to switch the thermostat from cooling to heating. So when should you actually turn the heat on in Northern Virginia? The answer starts with a number.

Most Northern Virginia homeowners turn their heat on when indoor temperatures start dropping below the 68°F comfort range and can't recover on their own — which in our climate typically lands in mid-to-late October, though an early cold snap can move that up. The U.S. Department of Energy recommends around 68°F while you're home and awake as an efficient winter thermostat setting. If your home is consistently drifting below that and staying there, your heating system has earned its start.

68°FDOE-recommended setting while you're home and awake

Mid-to-late OctTypical Northern Virginia heat-on window

~60°FNever run your AC below this outdoor temperature

Watch the overnight lows, not the afternoon highs

Northern Virginia's fall "shoulder season" is deceptive — warm, sunny days followed by cold nights. The real trigger to watch is the overnight and early-morning low temperature. When mornings are consistently in the 40s and 50s and the house feels cold when you wake up, it's time to run the furnace or heat pump.

Toughing it out usually costs more

Trying to wait it out often just means running space heaters, which cost far more per unit of heat than your central heating system. If the mornings are cold enough that you're reaching for a portable heater, you've already crossed the line — run the real system instead.

Don't run your AC and heat carelessly in the same day

During the fall changeover it's tempting to bounce between cooling and heating. A key rule: avoid running your central air conditioner when it's below about 60°F outside — an AC isn't designed to operate in cold weather, and doing so can damage the compressor and cause oil-return problems.

If you own a heat pump, it handles the heating-and-cooling changeover automatically through its reversing valve, so there's nothing to flip. A traditional gas-furnace-and-AC system, though, requires you to switch the thermostat mode from cool to heat deliberately.

Before your first heat run of the season, do a safety check

After sitting idle all summer, a furnace often produces a brief burning or dusty smell on first startup as accumulated dust burns off the heat exchanger — this is usually normal and fades within an hour. But the first cold morning is precisely when neglected heating systems fail, because a small problem hidden all summer surfaces under load.

✓
Replace the air filterA clean filter protects airflow, efficiency, and equipment from day one of heating season.
✓
Clear all supply and return ventsBlocked vents by furniture, rugs, or drapes starve rooms of heat and stress the blower.
✓
Keep the area around the equipment clearNothing flammable stored near the furnace or air handler — ever.
✓
Run it once on a mild dayTesting early means you find a problem while it's still an easy fix, not on the coldest night of the year.

The carbon monoxide safety step that matters most

Before you rely on a gas furnace all winter, it should have a professional heat-exchanger and combustion inspection — the front-line defense against carbon monoxide, a colorless, odorless gas produced by incomplete combustion or a cracked heat exchanger. Test your CO detectors on every level and near bedrooms before heating season begins, and replace any unit past its rated service life.

Never skip this one

CO gives you no warning — no smell, no color, no sound. A working detector on every level plus one annual professional heat-exchanger inspection covers the vast majority of the risk in any fuel-burning Northern Virginia home.

Save money from the very first day

Program a thermostat setback schedule so the heat eases off when you're away or asleep. If you have a heat pump, use a gentle, heat-pump-friendly setback so a big temperature swing doesn't trigger expensive auxiliary heat. Keep the thermostat around 68°F when home, and use free solar heat — open south-facing blinds on cold, sunny Northern Virginia days and close them at night to trap the warmth in.

"The first cold morning is when we get the most no-heat calls — and almost all of them were preventable in September. Run the heat once on a mild afternoon, change the filter, and get the heat exchanger checked. That's the whole difference between a smooth October and an emergency in November."Fairfax Mechanical Technical Team, Fairfax, VA

Not sure your furnace is ready for its first cold morning?

Book a Fall Heating Check-Up

Frequently Asked Questions

When should I turn my heat on in Northern Virginia?

Most Northern Virginia homeowners switch from cooling to heating when indoor temperatures start dropping below the 68°F comfort range and can't recover on their own — typically mid-to-late October in Fairfax, Vienna, Reston, Alexandria, and Arlington, though an early cold snap can move that up. The U.S. Department of Energy recommends around 68°F while you're home and awake as an efficient winter setting. If your home drifts below that and stays there, your heating system has earned its start.

Is it OK to run my air conditioner when it's cold outside?

Avoid running your central air conditioner when it's below about 60°F outside. An AC isn't designed to operate in cold weather, and doing so can damage the compressor and cause oil-return problems. If you own a heat pump, it handles the heating-and-cooling changeover automatically through its reversing valve, so no manual switch is needed.

Should I expect a burning smell the first time I run my furnace?

Yes — a brief burning or dusty smell on first startup is usually normal in Northern Virginia homes. It's accumulated summer dust burning off the heat exchanger and typically fades within an hour. Smoke, a persistent odor, or a repeated smell after several runs is not normal and warrants a professional look.

What should I check before my first heat run of the season?

Replace the air filter, clear every supply and return vent, and make sure nothing flammable is stored near the furnace or air handler. Then have the furnace's heat exchanger and combustion inspected professionally before you rely on it all winter — that's the front-line defense against carbon monoxide.

Does Fairfax Mechanical service furnaces and heat pumps across Northern Virginia?

Yes. Fairfax Mechanical is an experienced, licensed Virginia HVAC contractor providing furnace and heat pump service across Fairfax, Vienna, Reston, Alexandria, Arlington, McLean, Herndon, and all of Northern Virginia. Book your fall heating check-up at fairfaxmechanical.co/contact or learn more at fairfaxmechanical.co/faq.

Need help with your AC?

Free estimate for any HVAC service in Northern Virginia.

Get My Free Estimate →