There's Somethin' A'burnin' in the Basement: How You Can Trace, Track, and Repair What Makes That Smell
Furnaces leave behind a variety of unpleasant odors when they break down. Most of these odors are burning, smelly, and stinky. Here is how to track down these odors to find out what is making those smells, and exactly how to fix it so those smells do not continue.
Following Your Nose
It helps to have a really good sense of smell that is not hampered by tobacco smoking. Then you will smell these furnace odors sooner, and you will be able to find them faster. Otherwise, you should just follow your nose. Trace the smell to the point where it is strongest, and then get down to find the exact location. Both the smell and the location can tell you a lot about what is broken. Knowing what is broken can also help you determine how to fix it.
Describing the Smell
Is it a rotten egg smell? Then you have a leak in the gas line. Does it smell like burnt ozone? Then electrical wiring is fried from too much power or pests chewing through the wiring. Does it smell like a diesel engine backfired? Then your oil-fueled furnace has dripped or leaked and the burning or burnt fuel is creating that smell. Once you can describe the smell, and exactly where it is coming from, it is time to fix the problem.
Fixing the Problem
To fix the problem with the electrical furnace, shut it off completely. You do not want the electricity to continue flowing if there are damaged wires burning inside the furnace. You also do not want the furnace running when you try to repair it. Most people do not have the electrical skills to fix this, so do not attempt to do so on your own. Additionally, you should only open the furnace's trap door to reveal the area where the smell is coming from. Leave it exposed and ready for a furnace service to examine and repair it.
To fix the problem with an oil furnace, turn it off. The oil would continue to boil and burn if you leave it on. Look for the leak and wipe it off, including the burnt stuff. Detach this section of the line, clear it, and clean it. Then either replace it or repair it by tightening it back into place. Turn the furnace back on and release the oil valve. Check the part you just replaced for leaks and sniff for smells. If it is good, then let it go.
To fix a gas leak, open the basement windows. Then turn off the gas by closing the valve. Turn off the furnace, if it is on. Then get a furnace contractor to help immediately.