Your smoke detector should not go off when the furnace turns on at any time during the year. Unless you are seeing visible smoke I first suggest that you really take a look at your smoke detector and determine whether or not it is a combination smoke detector carbon monoxide detector. If it is a smoke detector then the first thing that you should do is replace it. It is probably defective. We do replace smoke detectors as we have a full scale electrical department.
If it is a combination smoke detector/carbon monoxide detector you should turn your furnace off, open the winds and air out your home. Give us a call because you may have a serious condition that could be life threatening.
If you can see smoke coming out your vents then chances are the blower motor has burned up. It will smell terrible. The motor will have to replaced and the system cleaned out. It isn’t a really big deal and it can usually just be performed on a normal service call.
Many of the manufacturers have labels made for their motors that are misleading. If you purchase a ½ horsepower motor to replace your existing ½ horsepower motor it may burn up (no warranty from the seller on motors at all) because of a labeling issue. The reason is that the manufacturer does not state how they came up with the horsepower measurement.
Horsepower can be running load horsepower, low speed horsepower, high speed horsepower, maximum horsepower or break horsepower. All of these readings are different and the wrong size motor means a second trip to the supply house and another expenditure. Match the rpm’s, the horsepower and the amperage ensuring that you get the right motor mounting system. There are many types of motor mounts and they are not interchangeable.
Bear in if you bought the wrong motor the first time and didn’t install it, you now have a spare. The supply houses will not take back motors. They are to easily destroyed during installation and transportation.