Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: If installed on the supply side, after the filter, and after the furnace heats the air - wouldn't the built in fan introduce dust and cold air in the supply?
Document: Useful Information that may not be obvious at first:The cutout is 14 3/4" H x 14 1/4" W -- cut very exactlyCan be mounted vertically in the "cold" (supply) or "hot" (heated) sideIf installed on the "cold" air intake side hot water is requiredIf installed on the "hot" side make sure an "A" coil with a solid side isn't blocking the humidifierThe humidifier is deep; allow 2' min.1/2" id tubing for the drain is not supplied -- and a drain is requiredWater connection is a compression fitting for 1/4" copperHookup between the humidifier and furnace controller needs 18/5 wireYou will need to install a supplied 24VAC transformer for just the solenoidBe prepared to install a two wire outside thermistorCool things:The humidistat/controller is neat.
Home Depot proved to be invaluable in helping me with the plumbing connections. I used shark-bite fittings instead of the supplied saddle valve (every plumber I talked to advised me against using the saddle valve). I also used plastic pipe instead of copper. Overall I spent about $70 on supplies. I installed the unit on the supply side, as was recommended, but I could not justify hooking it into the hot water line and watch energy just pour down the drain. The instructions indicated a cold water supply was fine as long as the unit was mounted on the supply side.
The fan is weak and I found the air was actually being blown out through the vents on the cover. Even if it were drawing air in through the vents, you would be pulling unfiltered, unconditioned air into the system. Out Furnace sits in a cold unfinished portion of our Michigan basement. When the furnace is running, regardless of the on/off state of the Aprilaire unit, there is warm air being blown out through the vents.
the unit will whistle loudly when the furnace fan is running. Buy some silver duct sealing tape ( not regular duct tape.. That will dry out and come off). Get the super sticky, thin shiny tape they sell for sealing seams and joining duct board. Seal all around the edges of the unit to seal up any air leaks.Lastly, you will need to supply the supply line of 1/4&#34; tubing. Curiously, they give you the saddle valve, but not the tubing to connect to it.
They just mention about inspecting. Replacing Solenoid valve alone will cost you about $70 + labor extra!3. Another major flaw if you have this installed on the hot water. The manufacturer should have recommended not using Saddle valve on hot water supply. Saddle valve will slowly accumulate sediments and close itself and you will spend hours trying to troubleshoot what is wrong.... that is if you notice that your humidifier is not putting out any humidity in the air. It will continue to consume electricity, run the fan and give you an illusion that everything is working just fine unless you are a good handyman who knows what to look for and if you have some kind of instrument to measure and detect that the humidity levels in your house have dropped.
There were 3 terminals on the lower left side of it with three light blue wires coming out of it - one of them was connected to the chassis/frame of the furnace with a loop connector and a sheet metal screw - just loop in your "C" connection at the same point - avoid splicing just use a loop connector on the same screw.Pay attention to the wiring diagram. Some of the wires go to the R-G-W terminals on your furnace board and at least one of them goes to the little green thermostat wire that USED to go to the furnace terminal.Make sure to install your Humidistat (the Humidifier control panel) to the RETURN side of the furnace ductwork where you can see it (the return side is where the air comes back to the furnace; think "air to furnace" - the air filter you change every month or so is on the return side).Buy different colors of 16-18 gauge wire.
Or a short length of 1/2" id tubing to get to the drain? One spends an amazing amount of time digging out just the right screw or tubing.The noise. Installed on the supply side this sucker makes a whistling racket. Now I've not balanced the intake side of the ducting, so the vacuum may be a tad high; but this thing sounds like the seventh fleet has just spotted something remarkably comely.What's unfair:"Warranty is void unless humidifier is installed by a qualified heating and air-conditioning contractor due to possible misapplication of product."
I have gone through 3 humidifiers in 10 years. Was looking for something quality built and with the possibility of lasting more then 3 years. Checked for it on Amazon and selected this one. Then called few local HVAC places and the price including install was $600. Ordered it on Amazon and had it installed by local HVAC tech for $60.00. It took him 2 - 3 hours to do it but it has everything, including outside temp censer. Works like a charm and the tech suggested to change filter every 2 months due to the water conditions.
I put it around the edges to ensure an air tight seal since it's on the supply plenum.You'll need copper tubing. The same exact size you use for an ice maker on a fridge. (They include the saddle valve)  Length depends on how far your supply plenum is from your water pipe that you're tapping into. They suggest you tap into a hot water pipe. Don't tap into PVC. Use the saddle valve on a copper pipe. I tapped into the copper pipe on the hot water side of my hot water heater about a foot above my water heater.
things:Aprilaire clearly is a bit behind the times. You'd better not have an energy efficient on-demand hot water heater if you install this humidifier on the "cool" side (supply side) of the furnace -- the water trickle to the humidifier may not trigger heating. "Position the template on supply plenum..."  It takes a moment to realize that the single sheet instructions are also the template -- but with no screw hole markings!What would it have cost for a bag of little things like mounting screws (they use #8 & #10)?
Answer: The fan draws air from within the duct through the moist evaporator panel and back into the duct. There is no room air pulled in through this action.