Run humidifier while furnace is not heating but fan is running?
I have had to replace ductwork on systems that allowed the humidification while the fan was not running. In one case the duct work buckled and collected enough water to come through the Sheetrock, several others the ductwork rusted out. There needs to be water sensors and if moisture is detected below the humidifier it needs to be shut down as the water is collecting there, if the air is not circulating a in duct humidifier will only cause problems if on with no flow in my experience.