Since forced hot air systems dry the air while they heat it, humidifiers are used to add moisture back to the air. A humidistat checks the humidity level and turns the humidifier on when necessary. The humidistat may be attached to the ducting near the heater to measure the return air, or it may be located next to the thermostat in the house. Generally 40% humidity is a good setting. If it gets very cold, 40% humidity may cause the warm humid air to condense on the cold windows. If this happens, simply lower the setting 5% at a time for a few hours to see if the situation 'clears' up.