What is a whole-house humidifier?

Check the plenum right above or beside the furnace for a plastic box about the size of a toaster oven, with a small copper or plastic water line tapped off a nearby pipe, a drain tube heading to a floor drain, and a wire pair running back to the furnace board. Bypass models add a round duct arcing from the supply side to the return, and that duct carries a damper handle marked for summer and winter. Control is either a dial on the box or a humidistat mounted on a hallway wall beside the thermostat.

Winter air is dry because heating makes it dry: outdoor air at 20 degrees and 70% humidity falls under 10% relative humidity once you warm it to 70 indoors. The unit puts that moisture back, targeting 30 to 40%, the band where static shocks stop, wood trim quits shrinking, and sinuses settle. Two failure patterns dominate. A water panel left in for two seasons turns into a brick of scale and quietly evaporates nothing, so humidity never rises. A stuck solenoid valve does the opposite, running water to the drain around the clock and adding $10 to $30 a month to the water bill until somebody notices the trickle.

Replace the water panel, sometimes called the evaporator pad, once a year before heating season, and twice a year on well water or municipal water above about 10 grains of hardness. The pad runs $15 to $30. While it is out, scrub the housing with a 1:3 vinegar and water mix, flush the drain, and confirm the feed line actually drips. Interval and hard-water exceptions live in how often to clean a whole-house humidifier; the step-by-step is in how to clean a whole-house humidifier.