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My home has 2 forced hot air furnances, one in the basement which services the 1st floor and one unit in the attic which services the 2nd floor. I had a contractor install an April Aire bypass humidifier (model 600) for the 2nd floor unit. They informed me that they couldnt put water lines in the attic due to freezing, so they put the humdifier in the basement with a duct feeding up to the Attic furnace's return. The intake for the humdifier was settup on the basement unit's supply duct. So basically the intake comes from the 1st floor supply and puts it up to the 2nd floor return duct. I measured the air coming from the 1st and 2nd floor vents, and only noticed a small jump from 20% RH on the first floor (no humidifier) and 24-26%RH on the 2nd floor vents. The problem I see is that if the basment furnace is not on, the humidified air will not run up to the attic. Any advice on this situation? Should I just switch the humidifier to be used on the basement unit? Thanks