A whole-house humidifier adds moisture to the air distributed through the HVAC system, maintaining healthy indoor humidity levels during dry winter months. Optimal indoor relative humidity is 35-50%; levels below 30% cause dry skin, static electricity, respiratory irritation, wood cracking, and increased susceptibility to respiratory illness.
The main types are: bypass humidifiers (use airflow differential between supply and return ducts — lowest cost, require bypass duct), fan-powered humidifiers (have their own fan, more output, work with any system), and steam humidifiers (boil water to create steam — most output, highest precision, best for very cold climates, most expensive).
Whole-house humidifiers require annual maintenance: cleaning the water panel/pad every season, checking the solenoid valve, and descaling in hard water areas. A humidistat controls operation, usually mounted on the return duct or thermostat.