Bill
HD probably removes humidity from the air when the temp. is fairly low but the humidity is high like when it's been cool and rainy. It probably only runs the compressor at a lower setting so it doesn't cost as much to run but will keep the house/room comfortable. Neo Plasma is a fancy name like Ion pure. It removes airborne contaminates and is supposed to make the air feel fresher. They say it makes the air feel fresh like right after a thunderstorm.