I have a General humidifer on a newly installed furnace. (It probably goes without saying that it contains a vertically oriented metal screen) I've had problems with it getting musty-smelling and consequently causing me to have allergy symptoms.
I've noticed that the water valve turns off only when the furnace fan turns off. Shouldn't the water turn off when the gas valve turns off, so that the when the fan runs for a couple more minutes the humidifier screen gets a chance to dry out? Most furnaces do this on the A/C setting.