Why does my AC or heat smell bad?

What you can safely check

1. 1Treat a rotten-egg smell as a gas leak

   Do this:Do not switch lights or appliances on or off and do not use your phone inside. Get everyone out of the home, then call your gas utility or 911 from outside.

   You should see:Everyone accounted for outside, and the call made from a safe distance.

   When to stop:This is the end of the do-it-yourself list — do not go back in until the utility or the fire department says it is safe.

2. 2Switch off for a burning or electrical smell

   Do this:Set the thermostat to Off, and if you can reach the electrical panel safely, switch off the AC and air handler breakers too.

   You should see:The smell should stop growing with the system off, and nothing should feel hot near the indoor unit or the panel.

   When to stop:Stop and leave the home if you see smoke, sparks or the smell gets stronger — call 911 from outside.

3. 3Let the first heat run of the season burn off

   Do this:A light, dusty burning smell the first time the heat runs is usually settled dust on the heat strips or the furnace, and it should fade quickly as it burns away.

   You should see:The smell fading steadily instead of holding or strengthening while the heat runs.

   When to stop:Stop and switch the heat off if the smell is strong, electrical or getting worse — that is not dust.

4. 4Replace the filter

   Do this:Slide the filter out and hold it up to a light. A damp, dusty or loaded filter can smell musty on its own, so replace it with the same size if you cannot see light through it.

   You should see:The musty note should fade over the next day or so of normal running.

   When to stop:Stop and switch cooling off if you see ice or moisture at the indoor unit — follow the freezing-up guide first.

5. 5Look at the drain pan and line

   Do this:Find the pan under the indoor unit and the plastic drain pipe leaving it. Standing water in there feeds the mold and mildew behind a musty smell.

   You should see:A dry pan, and an open, flowing drain outlet outside while the AC runs.

   When to stop:Stop and keep cooling off if the pan holds water — the drain needs clearing, and the smell will not leave until it is.

6. 6Note where it smells strongest

   Do this:Sniff at a supply vent, the return grille and the indoor unit itself, and note whether one room is worse. It saves the technician real time.

   You should see:A simple description: which smell, where it is strongest, and when it shows up.

   When to stop:Stop at sniffing — never remove panels or grilles beyond the filter slot to hunt for the source.

Likely causes

• Natural gas leak

  Gas utilities add a rotten-egg odor to natural gas so leaks can be noticed. A gas leak is never a do-it-yourself job.

  Handled by a technician

• Overheating motor or electrical fault

  A hot, electrical or plastic smell can mean a motor or wiring is overheating.

  Handled by a technician

• Dust burning off the heater

  Heat strips and furnaces collect dust while unused. A light burning smell the first time the heat runs should fade quickly.

  Handled by you

• Mold or mildew in the system

  Moisture on the coil, in the drain pan or in the ducts lets mold grow, which gives a musty or mildew smell.

  Handled by a technician

• Dead animal or pest in the ducts

  A strong rotting smell that does not go away can mean an animal has died in a duct or the air handler.

  Handled by a technician

• Dirty filter

  A filter loaded with dust and moisture can smell stale.

  Handled by you