In line duct booster fan

I am wanting to put in an inline duct booster fan to make our bedroom more comfortable (too cold in winter, too hot in summer). I would like to wire it to the furnace blower so that when its on, the fan comes on its own at the same time so its fully automated with no manual control. In the manual it says to not wire the fan to the blower if the motor control is a variable speed tap type or a solid state speed control. I'm trying to determine what type of speed control my blower is. The furnace manual says that it is equipped with a 3 speed direct drive motor; low, medium and high. Does this mean its a variable speed or is it a multi speed blower? Furnace is about 15-20 years old and its high efficiency. Here is a picture of the control board if that helps.

 

The picture of the board helps a little but the heat speed and the A/C speed are two different taps.

The most effective way is to use a fan sail switch or a current sense relay module with a relay.

 

Your furnace fan motor will be 240 volts and most "duct fans" are 120 volts which requires you to install a relay and related wiring in your furnace .
A much more simple and likely cheaper solution is to buy a duct fan thermostat and install it on the duct.
It will sense heat in the duct whenever warm air is available.

 

Will that work for a/c as well?

 

The answer is in the description for the device.

 

Sounds good thanks. On another forum that I'm asking about this the consensus there seems to be that these booster fans are useless and nothing more than a novelty that doesn't make a big enough difference in terms of controlling the temperature in the room and that I should look to other methods such as adding a register or bigger duct to the room. Be nice to know if anybody has used these things and noticed any difference.

 

The other forum is correct. If the duct is to small to deliver the amount of air to the room then a fan is not going to help. Fix the duct issues would be the best thing.