Bad Smells in Millwood, WA

AC smells aren't random. Each type points to a specific part of the system. Here's what's actually happening inside the equipment when your vents start producing odors.

Musty or Moldy Smell

This is the most common complaint we hear, especially in Millwood homes that were built during the building booms of the late 1990s and early 2000s. A lot of those homes came with builder-grade AC units that are now 15 to 20 years old - right at the age when components start to degrade and maintenance history gets spotty.

The root cause is almost always moisture that isn't draining properly.

Your evaporator coil - the indoor coil that absorbs heat from your air - also pulls humidity out of the air as it cools. That moisture is supposed to drip into a drain pan and exit through a condensate drain line. When the drain line clogs (dust, algae, debris), water backs up in the pan. Standing water in a dark, cool space is exactly where mold and mildew grow.

The smell travels with the air. Every time your blower runs, it pulls air across that contaminated coil and pushes it into your living space.

Burning or Electrical Smell

If it smells like something is burning - plastic, rubber, or a hot electrical component - your system is working harder than it should be somewhere.

Common sources: - A blower motor running hot due to worn bearings or a failing capacitor - Electrical connections that have loosened or corroded over time - A clogged filter forcing the motor to overwork - Dust burning off the heat strips (common on first use of the season - usually harmless if it clears within a few minutes)

If the burning smell is persistent or gets stronger, shut the system off and call. Running a system with an overheating motor or failing electrical component can cause more damage and creates a fire risk.

Dirty Sock Syndrome

This one has an actual name in the HVAC industry. It smells like a locker room - sour, stale, and unmistakable.

It's caused by bacteria and mold growing on the evaporator coil itself, not just in the drain pan. When the coil surface gets a thin layer of biological buildup, it produces that odor every time the system runs. It's more common in systems that sit idle for long periods or in homes with higher indoor humidity.

Chemical or Sweet Smell

A faint sweet or chemical odor - sometimes described as ether or nail polish - can indicate a refrigerant leak. Refrigerant is the fluid that makes cooling possible. It circulates through a closed loop, and if that loop develops a leak, you lose cooling capacity and introduce a chemical smell into your airspace.

Refrigerant leaks also mean your system is running inefficiently and working harder to compensate. Left alone, a leak leads to a frozen coil, compressor strain, and eventually compressor failure - the most expensive repair in an AC system.

Before you call, there are a few things you can check safely on your own. These won't fix the problem, but they'll help you understand what you're dealing with and give us useful information when we arrive.

Check your air filter. A clogged filter restricts airflow and can cause the system to run hot or allow moisture to build up in the wrong places. If it's gray and packed with debris, replace it. A 1-inch filter should be replaced every 30–60 days during heavy use.

Look at the area around your indoor unit. Is there standing water near the base? Is the drain pan visibly full or overflowing? That points to a clogged condensate drain.

Check your vents and registers. Pull a register cover off and look inside with a flashlight. Visible mold growth or heavy dust buildup in the duct near the register is worth noting.

Note when the smell is strongest. Does it happen at startup? Only when the system has been running a while? Only on humid days? That timing helps narrow down the cause.

Do not open the unit, touch refrigerant lines, or attempt to clean the evaporator coil yourself. Those steps require proper tools and training to do safely and correctly.