Why is there condensation on my AC vents?

Condensation on AC vents is very common in Ottawa summers and usually comes down to one of two causes: warm, humid air meeting the cold surface of your vents, or an issue with your system that's allowing too much moisture to build up.

The most common cause is simply humidity. When your home's indoor humidity is high — especially during Ottawa's hot, humid July and August — the cold metal of your supply vents acts like a cold glass of water on a summer day. Moisture in the air condenses on the surface. This is especially noticeable on vents in rooms with poor airflow, basements, or areas near exterior walls. It's not necessarily a sign that anything is wrong with your AC system.

However, there are a few system-related causes worth investigating. A dirty or clogged air filter restricts airflow across your evaporator coil, causing it to get excessively cold and freeze up — this can lead to excess moisture throughout the system. Similarly, if your AC is oversized for your home, it short-cycles (turns on and off too quickly), which means it cools the air without properly dehumidifying it. Low refrigerant can also cause the coil to freeze and create moisture problems.

In Ottawa's climate, humidity swings significantly between spring and late summer. Running a standalone dehumidifier or ensuring your AC is properly sized and maintained goes a long way. If you're seeing water actually dripping from vents (not just surface condensation), that's a stronger signal of a system issue like a frozen coil or blocked condensate drain.

Start with these simple checks: Replace your air filter if it's been more than 3 months, make sure all supply and return vents are open and unobstructed, and check that your condensate drain line isn't clogged. If the problem persists or you're seeing actual dripping, it's worth having a technician inspect your system before the issue causes water damage or mold.

If you'd like a professional eye on your AC system, Ottawa HVAC Pro offers inspections and tune-ups — a good tune-up runs $150–$200 and can catch these issues before they become bigger problems.