I want to install an Aprilaire 400 by-pass humidifier in a system that has a fiberboard supply line and a metal return. The house was built in 1987 and we bought it in 2008. I changed the return line to metal when we replaced the oil furnace. The new furnace is a Thermo Pride two stage OH6. I've read putting humidifiers in older fiberboard lines can cause problems with mold but was wondering if having an all metal return where the Aprilaire would be mounted would solve the problem. Thanks for the help!