Heat Pump Smells Like Burning or Musty: What Each Means

Burning dust on the first cold day

What it is. Dust settles on the heating parts over the months they sit unused, and it burns off the first time they get hot. Trane says a smell like burning toast the first time you turn on the heating should go in 10 to 15 minutes. Carrier says a burning dust smell can last from about 30 minutes to a few hours, and Lennox calls a light burning smell at first heat normal.

Why a heat pump can do it twice. The backup heat strips live in the indoor unit, and Trane says on a correctly sized heat pump they may seldom run. So the first really cold night that calls them in can bring the same smell, weeks after the heat pump itself started heating.

A brand new system. Lennox and Trane say a new furnace can smell of chemicals the first time it heats, from its protective coatings, and Trane says to let it run a few hours with the windows open. If yours is a new heat pump with a gas furnace backup, open a window and give it those hours.

When it is not dust. Trane says if the smell persists or worsens, turn the unit off and call. Carrier adds a smell that keeps coming back. Burning hair usually means pet hair in the filter and ducts, so change the filter.

Musty, damp or dirty socks

What it is. Trane calls it dirty sock syndrome: mould and mildew on the indoor coil, in the ductwork or around the drain. Carrier says mould and mildew on the coil or in the drain line is the most common cause of an air conditioner smell, and a heat pump cools the same way.

Start with the filter. Trane says to clean or replace it first. Lennox says a clogged filter can let mould grow in the ducts, and to change filters every three months: how often to change the filter.

Then the drain. The coil makes water every time the heat pump cools, and it drains through a pan and a small pipe. Standing water there is what grows the smell: clearing the drain line.

Only at the start of the season? Lennox says a musty smell can simply mean the system has not run for a while. If it stays, Trane advises having the coil cleaned by a professional.

Burning plastic, hot metal or electrical

Switch it off first. At the thermostat and at both breakers. Then find which part the smell is strongest at, because that tells the technician where to look.

Indoors. Lennox says a metallic smell can be a serious electrical problem, and names an overheated blower motor or wiring harness as major causes of faulty wiring: what a failing blower motor does.

With a gas furnace backup. Trane says electrical damage in a furnace can lead to a cracked heat exchanger or an overheating blower motor. Keep it off until a technician has looked.

Outdoors. Trane’s advice is the same at the outdoor unit: a burning electrical or plastic smell there means shut it off and call.

One room only. American Standard says burning plastic can be debris in the ductwork rather than the system. With the system off, lift the register in the room where the smell is strongest and look for a toy or a piece of plastic on the duct.

A dead animal or rubbish smell

Trane says rats, mice and squirrels can get into the duct system and not get out, and a dead animal there smells awful even when the system is off. It also says small animals shelter in outdoor units in the colder months.

Follow the smell to the strongest register. If the source is out of reach inside the ducts, that is a job for a duct cleaner or a technician.

Rotten eggs or sulphur

A heat pump with electric backup burns no gas, but a gas furnace, water heater, stove or fireplace in the house can still leak, and the return ducts carry the smell to the vents. A rotten egg smell anywhere in the home is gas until proved otherwise. Trane says leave the house and call the gas company from outside.

With a gas furnace backup, an exhaust or car fume smell indoors also means get out and call from outside. Carbon monoxide travels with it and has no smell, Lennox says, so keep an alarm. More here: what a furnace smell means.