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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Are HVAC UV Lights Worth It? A Northern Virginia Homeowner's Guide

Coil-sterilizing vs. air-sterilizing UV lights, why Zone 4A humidity makes coil mold a real problem, honest expectations for air purification, and who benefits most — an HVAC UV light guide for Northern Virginia homeowners.

September 22, 2026 By the Fairfax Mechanical Team 8 min read

What HVAC UV Lights Actually Are

If you've been researching indoor air quality, you've probably seen HVAC UV lights pitched as a way to kill mold, bacteria, and viruses inside your system. For Northern Virginia homeowners — where humid summers make mold on the coil a real problem — they can be genuinely worthwhile, but only when you understand what they actually do.

UV-C germicidal lamps install inside your ductwork or air handler, and there are two types — the difference matters. Coil sterilization lights shine continuously on the evaporator coil and drain pan, the cold, damp surfaces where mold, mildew, and biofilm love to grow. Air sterilization lights sit in the ductwork to disinfect air as it passes.

The Strongest Benefit: A Cleaner Coil

In Northern Virginia's Zone 4A humidity, the evaporator coil is constantly wet during cooling season, making it a magnet for mold and biofilm. That buildup insulates the coil, restricts airflow, breeds musty odors, and clogs the drain — a common cause of water leaks. A coil UV light keeps that surface clean, which protects airflow, efficiency, and indoor air quality. This is the most proven, worthwhile use.

ContinuouslyHow coil-sterilizing UV treats the wettest surfaces

45–55%The indoor humidity target UV works alongside

~1 yearTypical UV bulb replacement interval

Honest Expectations on Air Purification

UV is part of a plan — not a magic replacement

Air-sterilization claims are more variable, because air moves past the lamp quickly. UV can help, but it works best as part of a plan alongside good MERV 11–13 filtration and humidity control (ideally 45–55%) — not as a replacement for either.

Who Benefits Most

✓
Allergy or asthma sufferersCleaner coil and reduced airborne irritants help sensitive households.
✓
Recurring musty smellsA mold-laden coil is often the source — UV keeps it from returning.
✓
Visible coil moldA sign the damp surfaces need continuous treatment.
✓
High indoor humidityZone 4A summers keep the coil wet — exactly what mold loves.

Maintenance Note

UV bulbs weaken over time and typically need replacement about once a year to stay effective — even when they're still glowing. A UV system without a fresh bulb is quietly doing nothing.

The most proven use of HVAC UV light is the least glamorous one: keeping the wettest, dampest surface in your system clean.Fairfax Mechanical Technical Team, Fairfax, VA

Why Choose Fairfax Mechanical

We're a licensed Virginia HVAC contractor with an experienced local team, and we install UV systems as part of a complete indoor-air-quality strategy — filtration, humidity, and UV working together, sized correctly for your system. Serving Fairfax, Vienna, Reston, Alexandria, Arlington, and all of Northern Virginia. Book at fairfaxmechanical.co/contact or learn more at fairfaxmechanical.co/faq.

Musty smells or recurring coil mold? Let's build your full air-quality plan.

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Learn more at fairfaxmechanical.co/faq.

Frequently Asked Questions

Do HVAC UV lights really work?

Yes — coil-sterilization UV lights are the proven use, keeping the wet evaporator coil and drain pan clean in a humid climate like Northern Virginia's. Air-sterilization benefits are more variable and work best alongside MERV 11–13 filtration and humidity control.

How often do UV bulbs need replacing?

UV bulbs weaken over time and typically need replacement about once a year to stay effective — even if they're still glowing.

Are HVAC UV lights safe?

When professionally installed inside the ductwork or air handler, UV-C lights are shielded from the living space and operate safely. DIY placement is not recommended.

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