What are Those Strange Smells Coming from My Air Vents?

Your HVAC system runs most of the time, sometimes cooling, sometimes heating, and always circulating air. Occasionally, you may notice there are odors that come in with the air flow. They can be unpleasant, annoying, or barely noticeable. But whatever the smell, it’s usually worth finding out why it’s there.

Problem: Musty/Moldy Odor

A smell like mold or mildew is a common one, but if you’ve been noticing a musty or moldy odor from your vents, it can be difficult to pin down the source. It’s important to do this, however, because while mold and mildew aren’t serious problems for your equipment, they can lead to respiratory health issues due to poor air quality. There are a few reasons you might be experiencing this.

• Running the air conditioner a lot can cause condensation within the unit. If that moisture doesn’t drain properly it can get into your ductwork and become a breeding ground for mold. Then your blower moves the odor (and the spores) throughout the house.

Solution: You need to have an HVAC technician find where the mold build-up is, and have it cleaned up properly.

• The problem might be an air conditioner that is too large for your home size. Bigger is not always better because your air conditioner doesn’t just cool the air, it dehumidifies it as well. If your unit is too big, it cycles too quickly and doesn’t run long enough to dehumidify the air.

Solution: Talk to a HVAC provider to see if your unit is too large and go over your options for a high efficiency replacement.

• Sometimes a musty odor is the result of bacteria growing on the coils of your air conditioner or heat pump.

Solution: The easy remedy is a routine maintenance call and/or tune-up.

Problem: Burning Odor

Sometimes when you turn on your furnace for the first time of the season you’ll get a slight odor like a hot iron or something scorched. As long as this doesn’t linger too long, you don’t have to worry. Dust gathers on the heat exchanger of the furnace when it’s not being used and then burns off when the furnace gets turned on for the first time in a while.

• If your vents smell like something might be overheating, there may be a more serious problem. The air filter may be dirty and clogged which restricts air flow and causes the electric resistance heaters to overheat, or it may be a problem with the wiring within the motor.

Solution: Turn off your furnace right away before doing anything. With the power off you can check your air filters, and if they’re fine and you suspect a problem with the wiring, contact an HVAC professional right away. You should not attempt to repair an electrical problem yourself.

Problem: Acetone Odor

• If you smell something that reminds you of fingernail polish remover, you could have a refrigerant leak. This is sometimes a problem in older and/or improperly maintained systems. This not only drastically reduces your system’s efficiency, it can pose a hazard as it is flammable

Solution: Contact an HVAC technician to inspect your system, find the leak, repair it, and replenish the refrigerant.

Problem: Urine Odor

• If you smell urine coming through your vents, you likely have a pest infestation of some kind. Mice in particular nest in and around equipment and ductwork, and where they nest, they urinate and defecate.

Solution: Find where the mice are living. In cooler temperatures it may be around the air handler (it’s warm there) and then you need to clear them out. After that you’ll probably need to clean your ducts and install some kind of air filtration system.

Problem: Rotten Egg Odor

• If you heat with natural gas, you should always be concerned with any rotten egg smell which likely comes from a leak in the system. Natural gas is actually odorless, but has a harmless chemical added to it (mercaptan) which does smell like rotten eggs, so you’ll know there’s a leak.

Solution: Turn off your gas immediately, open windows, and call an HVAC professional to come out and inspect your equipment. In the meantime, don’t use any electronics or light any candles near the area – sparks and open flames could cause an explosion. In fact, it’s a good idea to leave the area until you have the problem addressed.

Problem: Stale Odor

• Not as dangerous as a gas leak but much more common is that stale odor that greets you at home. This is a ventilation issue, and energy efficient windows, good insulation, and snugly installed doors only add to the problem.

Solution: Open the windows every now and again and let the fresh air outside circulate through your house.

Keeping the air in your vents smelling clean and fresh is usually just a matter of keeping your air filters changed regularly. When an odor results from something else, you should have your HVAC inspected by a trusted local company like Air Specialist. An annual maintenance contract often catches small problems before they become big ones.

Air Specialist Heating & Air Conditioning is a Houston-area HVAC contractor that has been serving the Houston Metro area with quality service since 1970.