I have been using an add-on wood furnace (WF) that ducts into the plenum of my gas furnace (GF) for a couple seasons now. Heats the house really nicely all winter. Using little to no propane all season. A couple years before purchasing the WF I installed an Aprilaire 600 humidifier onto the GF. This also worked well. Since adding the WF the humidifier does not work properly as it is installed on the GF and the humidifiers duct runs back to the GF return duct where the air isnt being pulled back into the GF since its blower isnt running. To remedy this I would like to install the humidifier onto the wood furnace plenum. One of the concerns I have is running the duct from the humidifier to the WF return. Will this add too much moisture and possibly cause premature rusting to the WF? What do you think? Thanks for any suggestions.