A collegemate and close friend live in a apartment that requires air conditioner for it to be comfortable, but this means that she has to have the A/C and furnace running when it is summer time and winter, respectively, then she depends on the Heating and A/C system honestly to survive. This one time, she felt some different plastic stink from her furnace. It had been a long time before it was used since my buddy and I had just come from a long summer time season. It is possible for one to feel the burning stink after a while of furnace disuse. Typically, this results from accumulated dust and dirt, which must burn first before the furnace can heat up. It should take a few minutes before the burning stink dissipates! However, when the scent tends towards plastic, then you have to be cautious. This could mean several things, with the first being that some plastic material may have been accidentally left inside the furnace and is now burning when you light it. It is also possible that wires could be burning or worn out and in need of repairs. The latter case is the most typical occurrence, then components that need to be replaced in this case include capacitors that are often plastic coated and will stink burnt plastic when they malfunction, but others that could lead to this stink also have the fan belt and plastic-coated wires; When these components wear out, they are easily affected by heat and, as a result, will burn. In her case, the pieces were worn out and needed urgent replacement. However, always call an Heating and A/C company to assess the disfigurement and advise otherwise. If the stink intensifies with time despite switching off the furnace, you need to contact the fire maintenance to be on standby just in case something drastic happens.