Shut off the furnace and wait for it to cool.
Twist the knob clockwise on the water valve that controls flow through the pipe to turn off the water.
Spray the hex nut at the pipe connection with lubricant on all sides. Wait at least 15 minutes for the lubricant to work into the threading.
Twist the nut counterclockwise to unscrew it from the pipe using the adjustable pipe wrench.
Hold a bucket below the connection to catch excess water. Set aside the bucket when the pipe drains to a trickle.
Clean the threading on the pipe as well as the threads inside the nut with old rags to remove grime. Wipe the connections dry with a clean rag.
Wrap the threading on the pipe with Teflon plumber's tape,which is temperature resistant and effective in stopping minor drips. Wrap the threads in a counterclockwise direction and cut the tape close to the pipe with scissors.
Reattach the pipe to the connection on the furnace with the nut. Twist the nut clockwise by hand as far as it will turn, then finish the connection by turning the nut with the pipe wrench.