Things You'll Need
Turn off the power source that is connected to the furnace blower.
Carefully disconnect the wires from the coolant terminal, which is located on the back of the unit. The wires are several different colors.
Connect the black wire to the manometer. The manometer tells you the CFM number. CFM stands for "cubic feet per minute." The black wire is usually the highest blower, so that’s why you should start with that one. The reading will vary, but you will likely see a decimal number, such as 0.88 or 0.89. Your furnace should have a sticker that tells you the normal range for each of the wire colors.
Check the other wires. Your furnace's manual tells you whether or not the furnace is blowing low, medium or high. Each wire has a different speed that it blows when connected. While the black is high, the blue is medium, and the yellow is often low. This isn’t always the case, though, so it’s important to check the speed with the manometer.
Attach the wire that is emitting the speed you want — low, medium or high — to the coolant terminal. The wire you choose should be connected to the bottom right of the coolant terminal. For instance, if the black wire was originally at the bottom right and you now want to use the blue to determine blower speed, you should put the blue wire where the black wire originally was.