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 BlogHome Comfort · Fairfax, VA · Northern Virginia

Will Your HVAC System Work During a Power Outage? What Homeowners Need to Know

Your HVAC stops when the grid goes down, and that includes a gas furnace, which needs power for the blower, controls, and ignition. Why that happens, how portable and standby generators compare, and what safe backup power really requires.

October 14, 2026 By the Fairfax Mechanical Team 10 min read

Short answer: Virtually all central HVAC systems stop working during a power outage because they depend on electricity, and this includes gas furnaces, which need power for the blower motor, controls, and ignition, not just all-electric air conditioners and heat pumps. To keep your HVAC running during an outage, you need a properly sized backup generator connected through the correct electrical transfer equipment, such as an automatic transfer switch, professionally installed. Because HVAC systems draw significant startup and running power, generator sizing and compatibility matter enormously, and the installation must be done safely and to code. Fairfax Mechanical is an experienced, licensed Virginia HVAC contractor that helps Northern Virginia homeowners plan backup power for their HVAC the safe, correct way, never with dangerous shortcuts.

Will your HVAC system work during a power outage?

When a Northern Virginia storm knocks out the power, your HVAC system goes silent right along with the lights, leaving your home without heating or cooling at exactly the moment you may need it most.

A summer outage during a heat wave or a winter outage during a deep freeze is not just uncomfortable. It can be a genuine safety concern. So why does your HVAC stop when the power goes out, and what can you actually do about it?

Why your HVAC stops during an outage, even a gas furnace

The most common surprise for homeowners is learning that a gas furnace will not run during a power outage either. It burns gas, sure, but it relies on electricity for the blower motor that distributes the heated air, the thermostat and control board, and the electronic ignition that lights the burners. No power means no blower, so even if the gas is flowing, no warm air reaches your rooms.

Air conditioners and heat pumps are entirely electric. The compressor, outdoor fan, and indoor blower all need power, so they stop completely as well.

The bottom line

No electricity, no central heating or cooling, regardless of your fuel type.

Your backup power options, compared

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1. Standard grid power, no backupThis is the default. When the utility grid is down, your HVAC is down. Fine for brief outages, but it offers no protection during extended or severe-weather outages.
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2. Portable generators and their real limitationsA portable generator can power some household essentials, such as a refrigerator, lights, and a few outlets, but central HVAC systems, especially air conditioners and heat pumps, draw substantial startup and running power that many portable generators simply cannot supply. Two critical safety points: a portable generator must always be operated outdoors, well away from the home, because its exhaust produces deadly carbon monoxide, and it should never be connected directly to your home's electrical wiring without proper, professionally installed transfer equipment. Doing so creates serious shock, fire, and backfeed hazards.
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3. Standby (whole-home) generatorsA permanently installed standby generator, correctly sized and connected through an automatic transfer switch, can power your HVAC system and the rest of your home automatically when an outage hits. This is the reliable, hands-off solution for homeowners who cannot afford to lose heating or cooling, but it requires correct sizing for the HVAC load, professional installation, permits, and a fuel source, typically natural gas or propane.

What proper generator and HVAC planning involves

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Correct generator sizingFor your HVAC's startup and running load, plus your other essential circuits. Undersizing means the generator cannot actually start the compressor.
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The right transfer equipmentAn automatic transfer switch or a proper manual transfer setup, so the generator powers your home safely and legally, never through an unsafe direct connection.
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Professional installation and permittingCoordinating the electrical and HVAC sides of the job.
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Fuel planningA natural gas line or propane supply sized for extended outages.
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Matching the generator to your specific systemVariable-speed and larger HVAC systems have particular power characteristics.

Why this matters in Northern Virginia

Our region experiences both violent summer storms and winter weather capable of causing multi-hour or multi-day outages. Losing HVAC during a Zone 4A heat wave or a hard freeze is more than an inconvenience. It can be hazardous for infants, older adults, and anyone with health conditions, and a prolonged winter outage can even risk frozen pipes.

Planning backup power before storm season means you are never left without heating or cooling when it counts.

Why choose Fairfax Mechanical

We are an experienced, licensed Virginia HVAC contractor, and we help you plan backup power for your HVAC safely and correctly, with proper sizing for your system's real load, the right transfer equipment, and professional, code-compliant installation, never unsafe shortcuts. We serve Fairfax and all of Northern Virginia.

“The worst time to size a generator is the night the power goes out. Do it in the spring, on a calm afternoon, with real numbers.”Fairfax Mechanical Technical Team, Fairfax, VA

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Frequently Asked Questions

Does a gas furnace work during a power outage?

No. A gas furnace needs electricity for the blower motor that distributes heated air, the thermostat and control board, and the electronic ignition that lights the burners. Even with gas flowing, no power means no blower and no warm air reaching your rooms.

Can a portable generator run my central air conditioner?

Usually not. Central AC and heat pumps draw substantial startup and running power that many portable generators cannot supply. A portable generator is generally best suited to running a few essentials like a refrigerator, lights, and a few outlets.

Can I connect a portable generator directly to my home's wiring?

Never. It must be connected through properly installed transfer equipment. A direct connection creates serious shock, fire, and backfeed hazards, and it is illegal in most jurisdictions. Portable generators must also always run outdoors, well away from the home, because of carbon monoxide.

What is an automatic transfer switch?

It is the equipment that safely switches your home's power between the utility grid and a backup generator, and it is what makes a standby generator come on automatically and legally during an outage.

Why choose Fairfax Mechanical?

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