I am completely confused on the Aprilaire wiring schematic. The himidistat itself needs 24V AC which I can get from the transformer on the switch-box housing mounted to the furnace. If the Honeywell fan board on my new Heil furnace has the "EAC" and "HUM" accessory terminals (which are on the 115V AC side of the internal transformer), how do I connect the 2 leads that run from the humidifier to the humidistat?? The humidifier itself has a grounded 115V AC plug for the fan and also has a transfomer itself inside.

The humidifier is powered by the 115 volt plug. The humidistat goes to terminals or wires inside the humidifier 24-volt circuit.

You are right about the diagram being confusing as the upper left-hand side shows wires to a 2 VA minimum furnace transformer circuit. Then it says at the lower right not to use another 24 volt transformer or you can damage the unit, so the other 24-volt wires shown at the upper left to the furnace auxiliary board source should not be necessary unless you want to use the furnace to control the humidifier. The company is badly in need of a more careful diagram for that specific model; it would not take a long time to draw a good schematic and pictorial diagram for each circumstance.
http://www.djsonline.com/aprilaire_700_wiring-diagram.htm

[This message has been edited by HKestenholz (edited December 16, 2002).]

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 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)
*** The transformer in the humidifier can not be disconnected from the board, so I would say not to use the 115V HUM terminals unless a handy box with a receptacle is wired from them so you can plug in the humidifier 115-volt cord. Then the 50 relay is unnecessary.

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),
*** leave them off - It is unusual for a manufacturer to put a transformer in the device that has no function. It is probably there to provide 24 volts for the circuit and the lower right side tells you NOT to use an outside 24-volt source; so the upper left 24-volt wires are probably for the 400 series. These All-For-Every wiring diagrams are dangerous.

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.
*** The plug should power the fan and the internal circuit board through its internal transformer.

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 instructions say you don't need the 50 relay if the furnace controls the power to the humidifier, so that is correct. But you shouldn't need to connect to the 24 volts of the furnace.

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?
*** If you connect ehe 115-volt plug to the 115-volt HUM terminals.

Why is there even a 24V transformer built into the Aprilaire??
*** I would go through this wiring arrangement carefully following each and every step and drawing every circuit using only the plugged 115-volt wire and the internal transformer first, to see if that will power the solenoid from the transformer next to it. Everything else is a switch that breaks the wired path from transformer to solenoid.

Will the transformer in the humidifier control the fan relay, solenoid AND use the controls outside the humidifier so the 24-volt line at the upper left is NOT required?

Unfortunately, there is no wiring diagram or instruction that explains that clearly - one instruction negates the other.

Clues point to the use of the internal transformer for all 24-volt power without another one used (thus the wires at the upper left not used.) The 115-volt plug-in is there so the humidifier can work independently of the furnace if desired without the 50 relay OR only when the furnace fan is on, with the 50 relay in the circuit.

This can help for the installation, but it shows the separate transformer of the 600 model used: http://www.sheridanhvac.com/Aprilaire%20600%20Installation.htm

[This message has been edited by HKestenholz (edited December 18, 2002).]

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.