How to Install a Bypass Humidifier

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How to install a bypass type whole house humidifier in your furnace or air handler ductwork.

A bypass humidifier, also called a whole-house humidifier, is an excellent way to improve the comfort and indoor air quality of your home as well as lessen the chance for asthma, allergies, and other sinus conditions. While smaller humidifiers are often used in a room, a bypass humidifier effectively supplies moisture to all rooms of the home and does not require any additional floor space. Humidifiers are mainly used during winter months when the outside humidity levels are below 25%, but in some areas of the country they may be used during other times of the year.

Bypass humidifiers also require less maintenance and use less power than portable humidifiers as the unit only operates while your central air handling unit is running. There are no buckets to fill or frequent cleaning of filters. A bypass flow humidifier typically requires air filters to be changed every 6 months to 2 years depending on the usage.

Wall-mounted Digital Humidistat (Aprilaire)

Recommended humidity levels in the home should be between 25 and 50 percent. As the humidity level approaches the high end of this range, you may notice condensation collecting on the windows and there is an increased chance for mold and mildew growth. If your home has been weatherized, meaning that most of the air leaks have been sealed, the home may require less humidification since normal household activities like cooking, bathing, and dishwashing will add moisture to the air.

Some bypass humidifier manufacturers are Honeywell, Rudd, Rheem, Trane, Aprilaire, Carrier, and GeneralAire.

Tools and Materials

6 inch duct

6 inch tin collar

Wire nuts

Plastic tubing for draining


Bypass gate

Bypass humidifier kit

Foil Duct tape


Torpedo Level

Work gloves

Sheet Metal Shears (Tin snips)

Electric drill and bits

Flat and Phillips screwdrivers

Flash light

Utility knife

Measuring tape

Adjustable wrench


Before you begin installing the humidifier, you need to assess your system components to determine if a whole house bypass humidifier is appropriate for your home.

1. Verify that the location of the air handler will not experience temperatures below freezing. Not only would this freeze the piping and cause a leak, extremely low temperatures can cause the humidifier system to malfunction. Attic spaces are not good locations for bypass humidifiers.

2. Calculate the volume of your home in cubic feet to determine the proper size bypass humidifier. Measure the length, width, and height of each occupied room on every floor and add them together to determine the total cubic feet.

3. Determine whether or not you need a permit to perform the work on your HVAC system. Typically the work involves a simple water connection and a 120V electrical connection. Most municipalities will not require a permit or inspection for this project, but they may offer some guidance to ensure that the work is performed safely.

4. Most homes have sheet metal ductwork with fiberglass insulation wrapped on the outside. Some homes have fiberboard ductwork. These material may not be appropriate for use with a whole house bypass humidifier system. Carefully read the specifications for the type of humidifier you are planning to install to determine if it can be used with fiberboard ductwork. You may have to install a different type such as a spray mist humidifier. Humidifiers typically cannot be mounted on fiberboard ductwork.

Installing the Humidifier

1. Shut off the power to your air hander or furnace.

2. Determine the best location for the humidifier. Most units can be installed in the side of the housing of the furnace or in the supply ductwork. The position of the blower and ductwork will dictate the best location for your application.

Typical Installation Locations

The humidifier installation kit will come with a template. Tape the template to the desired location on the furnace or air duct. Use a torpedo level to position the template in a horizontal position. Use a pencil or awl to mark the locations for the mounting holes and humidifier flow hole.

Humidifier Components

3. Remove the template and drill the mounting holes according to the instructions. Typically there are 8 holes, 4 for mounting the unit to the ductwork and 4 that form the corners of a rectangle. Wear leather gloves when working with sheet metal.

4. Four of the holes will be the four corners for the humidifier bypass flow. Cut the rectangular hole where the humidifier will be attached using the sheet metal shears.

5. Screw the humidifier to the plenum with the screws provided.

6. Install the bypass gate to the ductwork downstream from the humidifier. There will typically be a 6-inch duct collar that is installed in the duct and connected to the humidifier by way of a flexible duct. Trace the template for the hole and cut it out with the sheet metal shears after drilling a pilot hole.

7. The water supply connection is similar to the ones used on ice makers and water dispensers in refrigerators using copper or plastic tubing and a saddle valve. Shut off the water to the closest cold water supply line. Open a faucet to relieve the pressure.. Install the saddle valve and tubing and connect the other end of the tubing to the humidifier with the compression fitting supplied in the kit.

Run the tubing in a neat fashion from the saddle valve to the humidifier and use clips to keep the tubing close to the ductwork or piping.

8. Connect the drain line, if applicable, to the bottom of the humidifier and run the tubing to the closest drain or sump pit.

Electrical Connections

Most humidifiers will use a low-voltage power system to energize a solenoid valve to control the flow of water through the humidifier. A transformer is installed on a junction box located on the side of the unit.

1. Mount the transformer in a convenient location.

Note: Install transformer on the outside of a grounded metal junction box using only a 7/8 in. knockout hole. Place the transformer mounting tabs into the knockout hole and firmly tighten the locking screw.

Wiring connections and grounding means for the transformer and enclosure shall be in accordance with the National Electrical Code (NEC) and the Canadian Electrical Code (CEC).

2. Connect wires to the 120V side of the transformer.

Humidistat Wiring

The humidistat works the same way as a thermostat, but it senses room humidity and sends a signal to the bypass humidifier opening and closing the solenoid valve on the water supply line.

The humidistat uses 2-wire 18 to 22-gauge thermostat wire to send the signal to the unit. Typically the humidistat is mounted close to the main thermostat so the two can be adjusted at the same time. A new wire will need to be run from the humidistat location to the humidifier on the furnace.

Typical Humidistat Wiring

Wire the humidifier solenoid valve, current sensing relay or sail switch, humidity control and transformer. Refer to the humidity control installation instructions for mounting and wiring information.


The furnace blower must be on for the humidifier to operate.

1. Open the saddle valve on the water supply line.

2. Set the thermostat setpoint 10°F above the room temperature.

3. Set the humidistat to a high humidity setting, or place it in the TEST position.

4. Observe the water running out of the drain line to be sure the humidifier is working properly.

5. Check all piping connections for leaks.

6. Reset the thermostat and humidistat to a comfortable setting.

1 comment

Roberta Baxter
Posted on Oct 12, 2011