Best Whole-House Humidifier by Home Size: Bypass, Fan-Powered or Steam

Match the gallons per day and rated square feet to your home. AprilAire rates the 500 at 12 gallons a day for up to 3,600 sq ft, the 600 at 17 gallons for 5,000 sq ft, the 700 at 18 gallons for 5,300 sq ft and the 800 steam unit for up to 10,300 sq ft. Those ratings assume a tightly built home, so go one size up for an older or drafty house.

A bypass humidifier uses furnace blower pressure and a bypass duct to move air through a wet pad, with no motor of its own. A fan-powered model has its own fan, so it needs no bypass duct. A steam humidifier boils water with electricity, so it delivers the most output and does not depend on how warm the furnace air is.

A bypass humidifier uses almost no electricity because it has no fan and evaporates water with the furnace's own heat. A steam humidifier uses about 2.7 kWh to turn each gallon of tap water into steam, roughly 47 cents per gallon at the 17.3 cent per kWh US average. At 10 gallons a day that is about $4.70 a day.

Keep it between 30 and 50 percent, the range the EPA recommends for indoor air. On very cold days aim for the low end, because humid indoor air condenses on cold window glass. Automatic controls like the AprilAire 500, 700 and 800 lower the target for you as the outdoor temperature drops.

AprilAire recommends replacing the water panel at least once a year, at the start of the heating season. Hard water can clog it sooner, so check it mid-season if your water is hard. The AprilAire 500 control has a light that tells you when to change it.

Many homeowners do, if they are comfortable cutting sheet-metal ductwork and connecting a water line. A bypass or fan-powered unit needs a duct opening, a cold-water supply, a drain line for the water that runs off the pad and low-voltage wiring to the furnace. A steam humidifier also needs its own electrical supply, so it is often a job for an HVAC contractor.

It is worth it if you run humidifiers in several rooms every winter. A furnace humidifier like the AprilAire 500 adds up to 12 gallons a day to the whole house from a plumbed water line, so there are no tanks to fill or clean. For one bedroom, a portable unit costs far less.

It can if the humidity is set too high. The EPA advises keeping indoor humidity below 60 percent and ideally 30 to 50 percent, and condensation on windows is the warning sign to turn it down. An automatic control that adjusts for outdoor temperature reduces that risk.

They work best on surfaces, not on moving air. A UV-C lamp at 254 nm shining on the cooling coil and drain pan continuously keeps mold and bacteria from growing there. The EPA notes that typical residential UV air cleaners may not expose passing air long enough to kill bacteria and mold spores.

Most HVAC UV-C lamps are replaced once a year. The Fresh-Aire UV Blue Tube ships with a lamp rated for a 1-year effective life, because UV-C output fades even while the bulb still glows. Replace it on the same schedule as the humidifier pad so both are done each fall.