I think I got it?! (obviously you have seen the Aprilaire 700 schematic)? I am going to wire some type of transformer (I'm not sure where I will mount it). The 115 (primary) side of the transformer will be connected via spade terminals to the HUM on my funace fan board (since the furnace panel schematic shows those terminals as being on the 115 side) and the 24V side will run to those two wires shown at the top left corner of the Aprilaire manual (24V terminals on the humidistat), then I will wire the 2 wires from the solenoid valve DIRECTLY to the AA terminals on the humidistat and run the outdoor temp sensor and plug in the 115 gounded plug for the fan. By using the fan board for a 24V source, I am also gaining a current sensing relay (Instead of using the "Model 50")...right? Whew! Does that sound correct? The fan in the Aprilaire will only run when the furnace fan board tells the Humidistat via 24V connection, which in turn triggers the solenoid valve right?
Why is there even a 24V transformer built into the Aprilaire??

Another question. This Aprilaire 700 is being mounted in quite the tight space. I am concerned before I cut the plenum, about the airflow openings on the front of the aprilaire. I am assuming this is just so the power supply board can breathe (meaning the slots are passive and do not suck or blow). I am concerned because the flu pipe of my hot water heater is so close that I may be sucking CO into the plenum??

The Model 700 humidifier contains a internal 24 volt transformer that powers the solenoid water valve and a relay that turns on the fan in the humidifier when the humidifier control calls for humidity. The humidifier control does not supply power to the humidifier but acts as a switch to enable the fan and water flow. It took a while to determine this because the installation diagrams are a bit confusing.