Turning on your home's AC has an expectation of fresh, cold air blowing throughout your house. Instead, you may notice that the air has a strange odor to it, all while still being cold. Air conditioners that are producing an odd smell are not normal. You'll need to identify the smell to learn what the problem could be.
The odor of ammonia will almost smell like a skunk is in your air vents and will require immediate attention to fix the problem. This smell is typically due to gas leaking in the system somewhere. What you are smelling is methyl mercaptan, an odor that is designed to smell bad to draw your attention to it.
The gas leak will cause the odor to circulate through your HVAC system, and if you can smell the odor in your home, it means that gas is in your home as well. You should call an HVAC technician to solve the problem for you and get you, your family, and your pets out of the home until the problem is resolved.
Is there an odor coming from your vents that reminds you of rotting eggs? There is a high chance that a rodent tried to find shelter in your air vents, got stuck, and died inside the vents somewhere. This usually happens in the winter, because rodents like to live in places that are warm.
The good news is that rodents can be very small, and the carcass will eventually deteriorate on its own. In a worse case scenario, you leave the carcass where it is and the smell eventually goes away after a couple weeks. Unfortunately, this may not be an option in the peak of summer when you want to use your air conditioning the most. You can have your vents professionally cleaned, which will get rid of the smell immediately.
The smell of mildew will be from trapped moisture, which can grow fungus somewhere within the system. This is caused by an air conditioner removing moisture in the air to make the air feel more comfortable and that moisture getting stuck and growing fungus.
A likely cause is a clogged drain in the pan that collects the moisture that the system is removing from the air. Cleaning the drain and the fungus in the pan will resolve the issue quickly.
For more help with smells coming from your HVAC system, speak with an HVAC contractor in your area such as Advanced Air Quality Services.