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GeneralAire 81

24v Drum Bypass Model with Manual Control

(4 Reviews)


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    • Pro Installation

The GeneralAire Drum Humidifier is based on the evaporative pad method of adding moisture to the air.  A polyurethane pad forms the surface of a slowly rotating drum which is hydrated as it turns in an automatically controlled level of water and is installed on a vertical plenum surface of any forced air furnace.  It is recommended that the humidifier be mounted on the return air plenum.  This humidifier can be installed with the outlet to the right, or to the left by reversing the motor mounting plate and bypass collar. 

Standard Features:

  • Unique design allows for right or left-hand installation
  • Removable front cover and water reservoir slides out
  • Split drum permits easy separation from removable polyurethane foam sleeve
  • Fits vertical plenum of any forced air heating system
  • Includes 6" diameter bypass assembly, 1/4" tubing and saddle valve
  • M3 Select-Control Manual Humidistat included
  • High capacity, 18 gallons per day


The two main types of standard humidifiers are the bypass and power fan.

Bypass humidifiers can be installed on either the supply or return plenum and you run a bypass duct to the plenum the unit isn't installed on.

Power-Fan humidifiers can be installed on a vertical run of either the supply or return plenum and uses a fan to distribute humidity into the air stream.

Maximum Coverage Area

Maximum coverage area means this humidifier will perform well for home sizes up to the square footage indicated, provided that the home is “tight." A "tight" home is typically one that is sealed well around windows and doors and does not leak air to/from the outdoors.

Maximum Coverage Area
4500 Square Feet
Water Capacity (GPD)

The amount of water vapor that can be delivered into the home, measured in gallons per day (GPD), that the humidifier can deliver into the space.

Water Capacity (GPD)
18 Gallons/Day
Electrical Requirements

The electrical requirements either 24 volts (bypass humidifiers) or 120 volts (power fan humidifiers) that needs to be supplied to the unit.

Electrical Requirements
24 Volts
Remote Sensing Bulb

A remote sensing bulb is a small (about ¼” round) sensor that mounts outside the home and connects by light gauge wire (18 to 22 gauge, 2-conductor) to the humidifier controller. This sensor provides temperature readings to the computerized controller which allows the humidifier to automatically adjust humidity levels in the home based on outside air temperature.

Remote Sensing Bulb
Not Available For This Model
Bypass Duct Diameter
6 inches
Plenum Opening Size
8W x 7 1/2H inches
12 Inches
11 1/2 Inches
Limited Warranty
1 Year