Found fresh air pipe frozen in 90. Removed fresh air pipe off top of furnace to allow customer to use heat until pipe defrosts on its own. Showed customer how to reattach fresh air pipe in a few days. Informed him furnace will be pulling air from basement in mean time and he may want to leave basement door slightly open to make sure enough air gets to unit. Checked piping outside and no visible signs of blockage from outside. When I removed fresh air pipe near unit he pipe began dripping water which indicates the pipe is frozen somewhere along run indoors. System is currently now running.