There are several conditions that can result in a strange smell when the heat is on in your living space. If you smell something that isn’t right when your furnace is on, here are some possible reasons.
Furnace Smells Like Burning – Common Reasons
Some of these reasons are “no big deal,” and will go away on their own or with a simple “fix” from you like a fresh air filter.
Others represent more serious issues that should be immediately referred to a pro furnace technician.
Note: We’re primarily talking about furnaces in this article – either gas/oil/propane furnaces or an electric furnace. Space heaters often smell like burning due to the first issue – dust.
Normal Dust at the Beginning of the Heating Season
This is something that will often happen the first time the heat is turned on after not being used for a few months. Dust will collect in the heat ducts and even inside the furnace itself when the system is sitting idle.
Sitting idle? If you have central air conditioning that uses the blower in the furnace to circulate air, then your furnace has been collecting a little bit of dust and debris all through the AC season. Fire up that furnace, whatever kind it is, and you’ll get the “something is burning” odor from normal dust, pet hair and other typical debris.
As soon as it gets hot, this accumulated dust will burn off and the smell will travel throughout your home.
This is a fairly normal condition that will happen year after year the first time the heater is used after several months of non-use. This will not usually last for more than an hour or so, and should not be a matter of concern.
Very Dirty Air Filter
The air filter in your furnace is there to trap dust, dirt, pet hair and other airborne particles as they are drawn into the cold air return ducts. The filter is supposed to be replaced at least every 3 months to ensure proper circulation of warm air through the system. In a dusty environment it should be changed even more frequently.
If the filter hasn’t been changed in a long time, and especially if it has collected a lot of food crumbs or pet hair, it might give off a moldy or musty smell. This will not go away after a short time, but will continue, and will keep getting worse as the filter continues to get dirtier. Remove the filter, and take a whiff of it. Smell nasty? This might very well be your problem.
Get rid of the smell with these steps (If this is the problem):
1 – Once you’ve replaced the nasty old filter, turn the temperature down to well below the temperature inside your house (to prevent wasting as much energy)
2 – Set the fan function from Auto to Fan – and the fan will run all the time.
3 – Open a door and one or two windows, and let the house air out while the blower fan circulates fresh air through the house.
Change the filter regularly according to the User’s Manual for your heater.
Foreign Object Inside Hot Air Duct
If a child’s plastic toy or other object has fallen into the hot air duct, it will not usually be a problem. But if it happens to be very close to the heater, or if it has somehow gotten all the way back into the plenum, it could begin to melt and give off a burning plastic smell in the house.
If your heat registers are in the floor, it is fairly easy to see down into them with the aid of a flashlight. They can often be simply lifted off to retrieve a foreign object that may have fallen in. But if you cannot see an object, and if there is still a smell coming through the ducts, it may be time to have the heat ducts thoroughly cleaned by a professional.
Defective Component Inside the Furnace
A strong electrical, rubber or metallic smell that comes when the heater is on could be from bad wiring, an overheated fan motor, a worn-out bearing or other component in the furnace itself. These are not things that most homeowners have the know-how or tools to fix, and a competent, licensed HVAC technician should be called in to diagnose and repair the defective part.
A Leak in the Gas Line
Both natural gas and liquid propane gas have a chemical added to them that smells like rotten eggs, which is a strong sulfur-like odor. This is so that if there is a leak anywhere in the gas line to your heater, you will be able to smell it. If this is what you smell, you should leave the house immediately and call your local gas company to come and check it out.
Pro Tip: On your way out the door, turn the thermostat to the Off position to ensure the furnace won’t come on when there is a gas leak.
A Leak in the Oil Line(Oil Furnaces)
Not as common as they once were, some home heaters use oil to provide the fuel for the furnace. Normally, you should not be able to smell it. If you do, it might smell like hot or even burning oil if it is contacting a hot part of your furnace.
It is also possible that some oil is leaking from a joint or coupling in the line and is making a puddle on the floor near your furnace. In this case, it could also contaminate anything on the floor nearby and possibly cause a fire hazard. An oil leak is a problem for a licensed professional to fix.
Cracked Heat Exchanger
This can be a very dangerous situation, but will probably only be diagnosed by a competent HVAC technician. The burning gases that heat your home are supposed to remain inside the heat exchanger and exit the house via the vent without mixing with air inside your home. If the heat exchanger has cracked from years of use, the dangerous fumes could be coming into your house through the hot air ducts. This can cause headaches, nausea and even asphyxiation, which is death by suffocation. If you suspect that this is the problem, contact your local HVAC technician right away, and leave/stay out of the home until the situation is fixed.
Defective Vent Pipe
Every home heater or furnace will have a vent or exhaust pipe for the burned gasses to escape to the outdoor environment. If there is a crack, a loose seam or joint in this pipe, some of the burned gasses will be allowed to get into the living space of the home. This is another dangerous condition that needs to be quickly fixed by a licensed HVAC technician.
If You Have a Portable Heater (Space Heater)
In some areas, a home may not have central heating, but will use a portable heater that sits on the floor. If there is a burning smell coming from one of these units, there may also be visible smoke. In some cases, the cause of the smell might be obvious, such as a toy or other object being on top of the heater or inside the radiating fins on the side.
If you are having a problem with a portable heater that is giving off a strange smell, it should be taken out of the living space until the cause can be determined. Once the issue is resolved, the portable heater can again be brought back into the home.
