If i hook my Aprilaire 400 Manual (not auto/digital) humidifier up straight to a 24v wall outlet instead of to the furnace, will it still work correctly? Or does it need to be powered by the furnace to know when to turn on and off?

The humidifier should turn on and off with the blower motor of the Furnace. This can be done by wiring the the humidifier to the HUM terminal on the furnace control board and then stepping down the power with a transformer. If the humidifier has it's own tarnsformer then a current sensing relay can be used by attaching it to the blower motors common wire.

Here is a installation guide http://electronicaircleaners.com/database/documents/aprilaire_400_installation.pdf

If your humidifier is equipped with a Digital Humidifier Control with Water

Panel change indicator, after replacing the Water Panel, turn the control

knob to the "Test/Reset" position until the "Humidifier On" light blinks to

reset its timer. (Blower must be operating and HVAC calling for heat.) Be sure

to turn the control knob back to it's original setting. If the "Humidifier On"

light continues to blink, your humidifier is in Test mode. DO NOT LEAVE THE
CONTROL IN TEST MODE OR HUMIDIFIER WILL NOT OPERATE.

Okay, I looked up the installation manual online to see exactly what you are working with.
Yes the R anc C need to be hooked to the transformer supplied with the unit. The primary 120v side needs to be hooked into the power coming into the furnace. this can easily be done were the L and N get hooked into your furnace or circuit board on the furnace. This will provide the contiuous power to the unit. Preferably you will hook this after the door switch of the furnace so that you will kill the power toit if you take the blower door off.
The ODT terminals are for the outdoor sensor if you want to hook that up it will vary the humidiy in your home according to the outdoor temperature then.
Now the W/G and Cf terminals. If you have 24v coming from the HUM terminals of your furnace you DO NOT need a second transformer. You must check your furnace manual or check the voltage to them with the furnace blower energized. The HUM terminals will only have power to them when the blower should be running.
If the HUM terminals are 120v then you need to get another transformer and hook up the primary side to the HUM terminals so it is powered only when the the blower is running. Hook the secondary side to the W/G and Cf terminals.
So bottom line is that you need to figure out what voltage your furnace is giving to the HUM terminals and then go from there. Some times the EAC terminals or the ACC terminals will be 24v on the furnace board. Either of them could be used also, as they are only energized when the blower is running also.

Heatman101

It sounds like it is not wired into the 24v fan controlon the furnace.

yes it should be bypass but solenoid could be trying to open but the voltage to it may be to low if you have 24v ac power at the 2 small wires. but to change shut down power to furnace and water to humidifier.you loosen the 2 compression nuts unwire they cant be reversed so dont worry then 2 mounting screws then reverse to install

the 760 humidifier does not need a transformer wired externally. the 760 has its' own 24v transformer inside the fan compartment. this transformer will provide the power to enrgize the water solenoid as needed. it sounds like to me the additional trasnformer is hot all the time. when the furnace comes on, the unit gets power and the internal transformer is cancelling the power from the external transformer. this would explain why it quits running when the heat is on. since the external transformer is hot all time, when the heat shuts off, the solenoid is getting energized by the external transformer. to make this run correctly, you need to get rid of the external transformer. leave the humidifier high voltage wires hooked up so they are only hot when the furnace runs. if you have a humidistat that has a r and c terminal, you will need to wire these directly to the r and c on the furnace control board and move the switch inside the humidistat to "power". make sure it is not on "bypass". then wire your 2 brown wires from the humidifier to each of the "h" terminals on the humidistat. if you have a humidistat that only has 2 terminals, wire one brown wire to each of the terminals. this should allow it to run properly.

if the funace has the 24 V you should use that but test it first with a call for heat to make sure it is 24 and not 120 coming out of that terminal!!!!!
Mount on cold air side with the bipass going to the supply.

You want the humidifier to run in heat mode which is the r-w1 terminal on the stat.