Condensing gas furnace with PVC vent pipes and metal ductwork in a utility room
Generic photo: Wtshymanski · CC BY-SA 4.0

Bypass humidifier on a furnace: layout, bypass duct, water, drain and 24 V control (install outline)

A bypass humidifier hangs on the supply or return plenum. A short round duct (often 6 in) connects it to the opposite plenum, so the pressure difference across the blower pushes warm air through a wetted water panel and back into the return. A solenoid valve feeds water to the panel only when the humidistat calls and the furnace is heating; unused water runs to a drain or condensate pump. The jobs are: cut and seal two duct openings, run a water line and a drain, wire the 24 V valve through a humidistat and the furnace so it only runs with the blower. Any cut into the furnace cabinet, gas venting or board changes beyond the accessory terminal follow the maker's instructions; many makers state a qualified HVAC contractor should install.

Cross-checked ~180 min advancedBrands: Aprilaire, All

Before you start

  • Turn off power to the furnace before cutting into the plenum or wiring; check for wiring or the coil behind the spot you cut
  • Never cut into the furnace cabinet, heat exchanger area or vent pipe; plenum and duct only
  • Do not mount on a supply plenum directly above an AC coil drain pan without following the maker's placement rules; water must not drip on the coil or furnace
  • Gas furnace: venting and gas work stay with a licensed tech

Diagnostic steps

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Bypass humidifier on a furnace: layout, bypass duct, water, drain and 24 V control (install outline) · OnCall Tech