Connecting Carrier Infinity furnace to Aprilaire humidfier
The simplest way to hook up is to the W terminal and the C terminal. Like this. 

The HUM terminal may be 120v or 24v if it's next to the low voltage connections (R C W G Y not necessarily exactly like this) then it will be 24v ALWAYS check with a volt meter. If HUM is off by itself or next to E. A. C. terminal then it's 120v. Either way it will only be live when the burner and the fan are on