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.

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HVAC System Troubleshooting: A Northern Virginia Homeowner's Guide

The same logical sequence our technicians use — no power, weak airflow, no cooling, no heat, and short cycling — adapted for Northern Virginia homeowners, plus when to stop and call a licensed pro.

September 17, 2026 By the Fairfax Mechanical Team 9 min read

Troubleshoot Before You Call

Before you schedule an HVAC repair, a short, systematic troubleshooting pass often reveals the problem — and sometimes solves it outright. Here's the same logical sequence our technicians use, adapted for homeowners across Fairfax, Alexandria, Arlington, Vienna, and the rest of Northern Virginia.

20–30%Conditioned air NoVA ducts can lose to leaks

2 feetClearance your outdoor condenser needs on all sides

90–95°FExpected heat pump supply air temperature

No Power or No Response at All

✓
Start at the thermostatReplace the batteries and confirm it's set correctly.
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Check the electrical panelLook for a tripped HVAC breaker and reset it once.
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Find the service switchMany systems have a switch near the indoor unit that looks like an ordinary light switch — make sure it's on.

Weak Airflow From the Vents

The number-one cause is a clogged air filter, which chokes the entire system. Replace it, then confirm supply and return registers are open and unobstructed by furniture, rugs, or drapes.

In Northern Virginia homes, ducts can also lose 20–30% of conditioned air to leaks, so persistent weak airflow may point to duct problems.

No Cooling in Summer

After the filter, check that the outdoor condenser is running and clear of debris, with about two feet of clearance on all sides.

Ice means stop

If you see ice on the indoor coil or refrigerant lines, shut the system off and let it fully thaw — running a frozen system can damage the compressor.

No Heat in Winter

Confirm the thermostat is on "Heat." A gas furnace that ignites then shuts off may be tripping a safety limit on restricted airflow. With a heat pump, remember that brief cool air during the defrost cycle is normal, and that supply air around 90–95°F is expected.

Short Cycling — Turning On and Off Rapidly

This often traces to a dirty filter, poor thermostat placement (near a vent, in direct sun, or by the kitchen), or an oversized or struggling system. It's worth a professional look, because short cycling wears out equipment.

Know When to Stop

The DIY boundary

Once troubleshooting points toward refrigerant, gas, the heat exchanger, or internal electrical components, it's time for a licensed technician — both for safety and to protect your warranty.

Half of the "broken" systems we're called to in Fairfax and Alexandria are actually a filter, a breaker, or a light switch. The other half deserve a real diagnosis — not a guess.Fairfax Mechanical Technical Team, Fairfax, VA

Why Call Fairfax Mechanical

When the basics don't solve it, our licensed, EPA-certified team diagnoses the true cause instead of swapping parts at random. We serve all of Northern Virginia with same-day service and honest, written recommendations.

Troubleshooting come up short? Same-day diagnosis across all of Northern Virginia.

Book a Diagnosis

Frequently Asked Questions

Where do I start when my HVAC won't turn on?

The thermostat: replace the batteries and confirm the settings. Then check the electrical panel for a tripped HVAC breaker and reset it once. Finally, check the service switch near the indoor unit — it looks like an ordinary light switch.

Why is airflow from my vents so weak?

Most often a clogged air filter, then blocked or closed registers. If both check out, leaky ductwork may be losing 20–30% of your conditioned air.

Is it normal for a heat pump to blow cool air in winter?

Briefly, yes — during the defrost cycle. Supply air around 90–95°F is also normal for heat pumps.

When should I stop troubleshooting and call a pro?

The moment the problem points toward refrigerant, gas, the heat exchanger, or internal electrical components.

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