If your furnace smells like burning dust the first few times it runs in the fall, that is almost always normal and should fade after a few hours of run time. A rotten egg smell is a different story: that can be natural gas, and it means you leave the house and call for help from outside. Hot plastic or electrical smells mean shut the furnace off and call us. Most other smells fall somewhere in between, and we will walk through each one below.
Your nose is one of the best early warning systems a house has. Plenty of furnace problems get smelled days before anyone hears a noise or feels a cold room. So it is worth knowing which smells you can shrug off and which ones deserve a phone call.
The dusty burning smell on the first cold day
Every year, the first real cold snap on the Front Range brings a wave of calls that start the same way: “I turned the heat on and the whole house smells like something is burning.”
Here is what is going on. All summer, dust settles on the burners, the heat exchanger and inside the ductwork. The first time the furnace fires up, that dust gets hot and burns off. It smells like a toaster that has not been cleaned in a while, and it can be strong enough to set off a sensitive smoke detector near a supply register.
That smell should fade within a few hours of the furnace running. Opening a window for a bit helps. If it is still going strong the next day, or it smells sharper than dust (more like plastic, rubber or wiring), turn the furnace off and give us a call.
A dirty air filter can make the first-run smell worse and keep it hanging around longer. If you cannot remember the last time you changed yours, the start of heating season is a good time to do it.
One more thing we see every winter: a toy, a sock or a crayon that slipped through a floor register over the summer. Once the warm air starts moving, it lets you know. If the burning smell is strongest at one register, pull the cover and take a look with a flashlight.
Rotten eggs or sulfur: get out first
Natural gas has no smell of its own. Utilities add a chemical that smells like rotten eggs or sulfur so that a leak gets noticed. If you smell it near the furnace, the water heater or anywhere in the house, do not try to track it down.
Get everyone out of the house, pets included. Do not flip light switches, unplug anything, use the thermostat or try to relight anything on the way out, because any of those can make a spark. Once you are outside and a safe distance away, call 911 or Xcel Energy’s gas emergency line. Call us after the gas has been dealt with and everyone is safe, and we will come find out what happened with the furnace.
We would much rather hear that it turned out to be nothing than have anyone take a chance on this one.
Electrical, hot plastic or burning rubber
A sharp smell of hot plastic, melting wire or burning rubber usually points to something electrical. On a furnace that might be a blower motor that is overheating, a wire that has worked loose and is getting hot, or a part on the control board that is failing.
This is a turn-it-off smell. Set the thermostat to off, then flip the furnace’s power switch to off. That switch looks like an ordinary light switch and is usually on the side of the furnace or on a nearby wall or ceiling joist. If you see smoke or the smell is getting stronger, get out of the house and call 911.
Please do not open the furnace up to look for the source. There is line voltage in there, and a hot motor can stay hot for a while after the power is off. Call us and tell us what you smelled and when. Our tech will find it.
Musty, damp or mildew smells
A musty smell when the heat kicks on is not dangerous the way gas or electrical smells are, but it is still telling you something. The usual suspects:
- A filter that has been in too long, especially in a damp basement.
- A whole-home humidifier pad that has not been changed and has started to grow something.
- Moisture in the ductwork or around the furnace, sometimes from a condensate drain that is backing up on a high-efficiency furnace.
- Ducts that run through a crawl space and have picked up that crawl space smell.
The filter is the easy first step, and the humidifier pad is the second. Colorado air is so dry that a lot of homes here run a humidifier all winter, and the pad in those units wears out and needs replacing on a regular schedule (the manual for yours will say how often). If both of those are fresh and the smell sticks around, it is worth having us look at the drain and the ducts. Our page on indoor air quality covers what we can do when the air in a house just does not smell or feel right.
Exhaust, fumes or a chemical smell
This is the one we want everyone to take seriously. A furnace burns gas, and the leftover gases from that burning are supposed to leave through the flue or vent pipe and go outside. If you smell something like car exhaust, fumes or a sharp chemical odor when the furnace is running, those gases may not be going where they belong.
Carbon monoxide itself has no smell at all, which is exactly why it is dangerous. But the smell of exhaust can be a sign that the venting has a problem, and venting problems are one of the ways carbon monoxide ends up inside a home.
If your carbon monoxide alarm is sounding, or anyone in the house feels unwell, get everyone outside and call 911 from there. If the alarm is quiet and everyone feels fine, turn the furnace off at the thermostat and call us so we can check the venting and the burners before you run it again.
Smells that are probably fine (and one that is not)
A few smells are mostly harmless. A faint burning smell after the house has been closed up for a vacation usually means dust had time to settle, same as in the fall. A new furnace can also give off a light “new appliance” smell for its first few days as the coatings on the parts heat up for the first time. Both should fade on their own.
What is not fine is any smell that keeps getting stronger, comes with smoke, or comes with the furnace shutting itself off. Those go straight to the off switch and a phone call.
What you can check before you call
For anything other than gas, electrical or exhaust smells, there are a couple of things worth doing on your own. Change the filter. Check the humidifier pad if you have one. Make sure nothing is stored up against the furnace, like boxes, paint cans or laundry, because furnaces get hot and stored things near them can scorch or give off fumes. Look in your registers for anything that fell in.
If none of that takes care of it, we are happy to come out. When the smell comes along with no heat at all, our no heat page has a short checklist of what is safe to try first and what to leave alone.
How a yearly inspection heads off most of these
Most of the smells on this list come from dust, dirt, worn parts or moisture building up over time. A yearly furnace inspection is when our techs clean what typically gets dirty, check the electrical and look over the drain and the moving parts. We cover what that visit includes on our furnace and AC maintenance page, and if a repair turns up, our furnace repair and installation page explains how we handle it.
If something smells off, call us
If something smells wrong and you are not sure what it is, call us at 303-464-7000 or request service online. You will get a real person who will tell you honestly whether it sounds like something to watch or something to fix today. Our $89 diagnostic fee is waived if you go ahead with the repair, and if it turns out to be simple, we will say so.
Furnace giving you trouble?
If the heat is out, start with our no heat checklist. For everything else, see furnace repair and installation. Furnace repairs start with an $89 diagnostic fee, waived with the repair.
