By-Pass Versus Power Humidifier
The term By-Pass or Power refers to the driving force behind the air flow. The Power humidifier (aka Fan Power) uses a fan to draw the air thru the humidifier. The Bypass humidifier uses a 6" diameter duct that connects the supply to the return duct.
In the Bypass configuration the air leaving the furnace is routed back to the supply. The warm air leaving the furnace takes this short-cut of least resistance and bypasses the long trip down the duct work, in to the house, and back through the return. The Bypass is the 6" round duct work shown to the left. During the the hot humid A/C season the Bypass duct should be closed with a butterfly valve since the air flow through the humidifier would not be needed.
The Power humidifier uses a fan to draw air in from the duct and through the humidifier and exhausts back out in to the duct. The air is drawn in through duct at the center of the humidifier and back out in to the duct at the sides. The power humidifier should be mounted in vertical warm air flow.
The HE225 and HE265 are Bypass humidifiers. The HE365 is a fan powered humidifier.