I vote both. We normally put humidifiers, by-pass or power, on cold water and they do fine. Once in a while they can't keep up and we put them on hot water for a little extra capacity. Usually this is in a very large home where the space is larger then the humidifier can handle but the owner didn't want to spend the money for steam or 2 humidifiers. This gets them by but the house isn't at the optimal humidity level.