A furnace water sensor is a device that is used to detect the presence of water in a furnace system. It is usually installed in the condensate drain pan or in the condensate trap of the furnace. The water sensor works by detecting the presence of water in the condensate drain pan or trap and then sending a signal to the furnace control board. This signal will then shut off the furnace in order to prevent further damage to the system. The water sensor is an important part of the furnace system as it helps to protect the system from water damage and can also help to improve the efficiency of the system. It is important to keep the water sensor in good working condition in order to ensure that the furnace system is working properly and efficiently.