Whole-house humidifiers, by capacity

Nearly 35 gallons a day, which is twice the biggest evaporative here and three times any portable. A steam humidifier boils water with electrodes and injects the steam into the ductwork, so unlike every evaporative unit it does not care whether the supply air is warm - it makes its own heat. That is what lets it serve a 10,000 sq ft house, and it is also why it needs a dedicated 240V circuit and why installation runs $1,800 to $4,200. There is no water panel to buy, which is the one running cost it removes; the 11.5 kW it draws while steaming is the one it adds.

The fix for the bypass units' one real flaw. A fan-powered humidifier has its own blower, so it can run independently of the furnace - on a mild day when the heat barely cycles, this one still humidifies and a 500 or 600 does not. It also needs no bypass duct, which makes it the easier install where the furnace is tight against a wall. Eighteen gallons a day, 5,300 sq ft claimed for a tight home, and around 3,000 for an average one by Aprilaire's own guidance. The load calculation lands at about 4,400.

Eighteen gallons a day against a published 4,500 sq ft - almost exactly what the load calculation says 18 gal/day supports, which makes this the most precisely honest coverage claim of any whole-house unit here. Like the Aprilaire 700 it carries its own blower, so it humidifies on mild days when a bypass unit sits idle, and Honeywell specifically recommends it with variable-speed and multi-stage furnaces where the burner cycles less. It undercuts the Aprilaire 700 by $60 on hardware.

The best-selling whole-house humidifier in North America, and the default recommendation from most HVAC contractors for a reason: seventeen gallons a day for $220 of hardware and $25 a year of water panels. The published 5,000 sq ft is the tight-construction figure, and Aprilaire says plainly that an average home of about 2,000 to 2,500 sq ft is the realistic target - a maker volunteering that its headline number applies to a minority of houses. The load calculation puts 17 gal/day at around 4,200 sq ft, neatly between Aprilaire's own two figures.

Honeywell's direct answer to the Aprilaire 600, matching it at seventeen gallons a day, and the only unit in this catalog that states its coverage in cubic feet rather than square feet - 18,000 of them, which at an eight-foot ceiling is 2,250 sq ft. That is a notably conservative way to quote a machine whose output supports around 4,200, and it is the correct unit to use, since humidity fills a volume rather than a floor. The PerfectFLO tray meters water onto the pad instead of flooding it, which is where the water-saving claim comes from.

Bolted to the ductwork, plumbed to a water line, and never filled by hand again - that is the entire argument for going whole-house, and the Aprilaire 500 is the small end of it. A bypass humidifier borrows warm air from the supply duct, passes it through a wet evaporator panel, and returns it to the return duct; it only humidifies while the furnace is running, which is its main limitation. Twelve gallons a day matches the biggest portable console in this catalog, and the 3,600 sq ft claim sits about a fifth above what the load calculation supports. Aprilaire publishes separate figures for tight, average and loose construction, which is a more honest way to state coverage than anyone selling a plug-in unit manages.