A humidity controller for a forced air heating system, said system having a furnace and providing a furnace operation signal, a discharge duct for carrying a flow of heated air from the furnace to a temperature-controlled enclosure, said enclosure having an interior window surface cooler than the normal air temperature, a humidifier providing water vapor to the air in the discharge duct responsive to a humidity demand signal, and a return air duct carrying a flow of air from the enclosure to the furnace, said humidity controller including:
- a) a load factor calculator receiving the furnace operation signal, and responsive thereto calculating the furnace load factor, and providing a load factor signal encoding the load factor;
- b) a temperature calculator receiving the load factor signal, and providing a window temperature signal encoding a window temperature factor indicative of the estimated window temperature and based on the load factor signal;
- and
- c) a temperature analyzer receiving the window temperature signal and responsive to certain values of the window temperature factor encoded therein, providing the humidity demand signal to the humidifier.