The document provides installation instructions for an Aprilaire 550 whole-house humidifier. Key steps include cutting openings in the return and supply ducts to attach the humidifier housing and run bypass ducting. A licensed technician should perform final wiring connections between the humidifier and furnace. Proper installation also requires attaching a water supply line and drain hose. Homeowners should consult an Aprilaire dealer for current installation requirements.

1. Aprilaire 550 Humidifier Install
Aprilaire 550 Humidifier Install
There are many models of the Aprilaire Humidifier and there are variations in their install. The
purpose of this article is the give a rough idea what is involved in the installation of an Aprilaire
Humidifier. You should have a licensed Aprilaire dealer/contractor install your whole-house
humidifier.
Since the Aprilaire 550 humidifier was replaced by models 500A (automatic) and 500M (manual) in
2007, there may not be any model 550s around to install. In case there is, here are general
guidelines for installation. It is important to note that the wiring of this installation varies from one
heating unit to the next. (The newer models' install differ from the install of the 550 model. Consult a
licensed Aprilaire Humidifier Contractor. Also check local heating and air conditioning installation/
building codes for regulations).
Steps:
1) Turn off all power to the furnace/ heating unit
2) Mark the spot on the vertical return duct where you will cut an opening to attach the humidifier
to. The marking must be a level, square-like shape, same size as the circumference of the opening on
the back of the humidifier housing.
3) Cut along the lines of the markings making your opening.
4) Place the back of the humidifier housing into opening (should fit snuggly). Screw the housing into
the duct.
5) Mark a spot to cut on the supply duct where you will run duct from to the humidifier's damper
opening. Marking should be a circle and the size should be the same diameter as the bypass damper
opening. Cut out hole. Run duct from hole to the built-in bypass damper on the humidifier.
6) Use grey duct tape or mastic to seal duct at the supply air and bypass damper joints to prevent air
leakage.

2. 7) Mark a spot on the return duct NO LESS THAN 6 inches upstream from the Humidifier. The hole
should be a size large enough to insert the sensor that is located on the back of the Humidistat. Cut
out the hole and attach the Humidistat.
THE NEXT STEP SHOULD BE PERFORMED BY AN INDIVIDUAL HAVING KNOWLEDGE AND
EXPERIENCE WIRING AND INSTALLING HEATING AND AIR CONDITIONING SO THAT THIS
STEP IS PERFORMED SAFELY
8) Find where the power source enters into the heating unit. Remove heating unit's front cover.
Attach the transformer to the inside of the heating unit's housing in close proximity to the power
source. Find the junction box and remove its cover. Take the black and white wires from the
transformer and connect them to the black and white wires (black to black and white to white)
stemming from the power source inside the junction box. All wires should be connected tightly with
wire nuts and no wires exposed. Reaffix the junction box cover.
9) Run the red and white wires from the 24 volt transformer terminals located in the heating unit to
the Humidistat on the return air dust located no less than 6 inches upstream from the humidifier.
10) Connect the white wire on the A50 relay to the white wire on the blower motor in the heating
unit. Connect one of the yellow relay wires to the humidistat and the other to the solenoid valve at
the humidifier.
11) Sand the area of the hot water pipe where the water shut off valve will be attached.
12) Attach the water shut off valve. Turn the valve handle clockwise all the way in and then counter-
clockwise all the way out. Check for leaks.
13) Attach the other end of the water line to the humidifier solenoid. Check for leaks.
14) If not already done, attach drain hose to the bottom of the humidifier.
15) Run a pipe from the bottom of the humidifier's drain hose to a floor drain or a utility sink with a
drain.
16) Set the humidistat to manual mode since model 550 is to be manually operated.
17) Place the Water Panel Assembly back into the humidifier housing.
18) Attach the water feed tube and nozzle to the top of the Water Panel Assembly/ Distribution Tray.
19) Place end of the humidifier drain hose into the pipe.
20) Replace humidifier front cover.
21) Turn on power source. Turn on the