I'm going to install a duct booster vent fan to help better pull/push
heated/cooled air to my slightly remote family room. The manufactorer of
the duct booster fan also sells a sensor that suppossedly will sense air
movement (i.e. will know when the furnace blower is on) and will turn
the booster fan on and off accordingly. However, I don't really trust
that the company's sensor will work well plus it has to be adjusted
differently (i.e. temperature settings) for heating versus air
conditioning (besides, the sensor isn't cheap). I would much prefer
wiring the booster fan directly to the furnace blower or
controller/limit switch so that whenever the main blower comes on the
booster fan also comes on. I'm very comfortable working with electricity
and have considered using my volt meter to locate a 120V hot line going
to the blower and tapping it for the low draw booster blower. I'd like
some advice on what I'm attempting to do. Is there a better way? Any
advice/thoughts are welcome.

This procedure will work on the older type
furnace that uses only a two wire system.
However, on the newer furnaces with circuit
boards you will end up with problems. These
motors can use up to eight wires and tapping
into the wrong wire will wipe out your motor
and possibly the circuit board as well. Many
homowners find this out the hard way when
they try to hook up a humidifier to the motor
to accomplish exactly what you are proposing.