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The Reason Why Your Furnace Smells When You Turn it On and How to Get Rid of It

Deposition of dust, mold, and mildew can be the top reasons for the bad odor when you turn on the furnace. It is a common phenomenon, especially during winter. The odor should reduce within a few minutes or hours. If the problem persists, you need to clean up the furnace parts. Some of the parts in which you need to clean are the filters, duct, blower motor, gas burner, heat exchanger, valves, and igniter.

Furnace Filters

The internal wire-frame of the filter can accumulate dust and mold during it working. The volume may seem to be too little when you observe it externally. Air passes through panels of the filters and deposit dust. High capacity mechanical filters can also absorb the moisture and humidity. It could lead to the growth of mold and mildew.

The deposits can become dry due to the heat. But the internal layers of the mold still carry humidity. When you turn on the furnace, heat can cause fumes of mold to escape into the exteriors. So, the smell spreads. Once the humidity becomes dry, the small may stop temporarily. But the phenomenon can continue whenever you switch on the furnace.

Larger capacity furnaces have multiple layers of filters. Hence, the thickness of duct and mold layers can also become higher with time. So, it is very important for you to clean the filters regularly. You also need to check the efficiency of filters every six months. If there are any damages, you have to replace the filters. It is one way of keeping the furnace free from odor when you switch it on. It can also increase the lifespan of filters and furnace.

Furnace Ducts

Deposits within the furnace ducts can have direct impact on the burner. Blocked ducts can push the escaping air back into the furnace. Air pressure in the duct reduces considerably and the reverse flow of air carries the dust and mold into the burner. The burning of these particles can cause a foul smell. The smell may last for days and weeks since the flow of air is continuous. Sometimes the small may make you feel some internal part of the furnace is burning.

Check the supply air ducts and return ducts also. So, the first step is to get the duct layout plan. The sizing of inlet and out may be different for each room. You have to start cleaning from the outlet from the burner. Similarly, you have to start cleaning from the inlet of the kitchen, bathroom, bedroom, living room, etc. Once the process is complete, you can wait for some time and switch on the furnace. The system should not produce any foul smell if the problem was with the ducts.

Blower Motor

Blower motor fan can have dense deposits of dust. The possibility of mold could be considerable, though not high. However, moisture can cause rusting of the fan. There is also a possibility of corrosion due to heat and humidity. The combinational effect can be a foul smell.

Check the fan and clean it. Remove the rust. You may have to remove the blades and replace them if the corrosion is too high.

Heat Exchanger

The S-shaped heat exchanger can separate the toxic gases from the air which passes through it. Soot and dust deposits can block the space between S-layers. The result is a reverse flow of gasses, which may cause a foul smell.

Initially, you may not be able to recognize the heat exchanger as the source. However, the odor of carbon compounds will be different from that of dust and mold. With experience, you can detect the difference and diagnose the problem to be with the heat exchanger.

Furnace Valves

Furnace valves may carry water (in Hydronic furnaces) and hot air (in blower furnaces). Pressure and temperature variations within the valves can cause fumes. It can result in an upsurge of deposited dust and mold within the various parts of the furnace. They can cause foul smell within the furnace and outside when you switch it on.

Sometimes there could be cracks and damages or rusting of the valves. In such cases, they can leak. The leaked hot water or hot air can trigger the dust and gasses to escape. It could be one of the reasons for foul smell.

The possible methods of fixing are repair and replacement. Apart from valves, you also need to check the T-joints, U-bends, screw and bolts for corrosion and rusting. Replacing them can also reduce the foul smell from your furnace.

Furnace Burner

Furnace burner typically consists of burner block, throat, air inlet and outlet, fuel connection, pilot gas, and pilot holder, which can be roots of heat. The heat refractory system can keep the excess of heat away and cool the burner when it is off. The probability of soot within the interior and exterior surfaces can also result in emission of soot and foul smell.

One remedy is to clean the entire burner. However, cleaning the burner is a complex process. You need to know the structural and functional aspects of the burner parts. Some of the parts are detachable, while the others are non-removable. So, you need to open each part carefully and clean. You may clean the burner only after disconnecting the fuel supply pipes.

Note: It is a good practice to clean the burner using a handheld vacuum. Use a soft steel brush to clean the exteriors. Make sure you clean the burner after it cools down. Or the vacuum can create fire accidents.


These are some of the reasons for which the furnace can smell after you witch it on. It is a good practice to hire a professional furnace repair professional  rather than trying the DIY method unless you are an expert with experience.

You may feel the professional solution is expensive. The price you pay for the services could be negligible compared to the safety and security of your property and family. Search for a reliable furnace repairer in your region and call today for preventive maintenance.