I have an Aprilaire humidifier 400 and a Carrier furnace/air conditioner with an Infinity Touch Control unit and am now confused as to what I should set it in Summer. Help

Your humidifier is a bypass type. The only voltage needed is 24vac to open the water valve. Air only flows through the humidifier when the furnace blower is running, and the wet pad will add very little humidity if the furnace is not running and heating the air before flowing through the wet pad in the humidifier.
It is best the water valve is only energized when the furnace is running. Some furnaces have a connector on their control boards labeled HUM for this purpose. Test it's voltage when the furnace is running because some will output 110vac instead of 24vac you need.

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.

hi the water runs down the pad the fan blows air across is , this controled by the stat which should control the water flow / on or off, this should be under warranty in the first year

Try closing damper slightly between humidifier and furnace????

if you mean your aprilaire humidifier isn't working then open the cover and remove the 4 screws on the fan motor mount inside and check for voltage at the transformer input 120 volts and output 25.5 volts a.c., then check the control board wherever you have it located on the furnace or upstairs and make sure yhe humidity level is set high enough to make the unit run(you will hear the water solenoid click on the furnace below the aprilaire unitwhere the water line is if its working correctly),then check the humidistat on the outside of the building as they can go bad

No water should not just sit there in the system. It is supposed to recirculate itself. You should really call a tech out and have them check your system.

You may be having problem with air scaping from the base of the hum. it will probably may have to put silicone around the base of the hum. to stop the air scaping.

We recently had the same problem. After cleaning both the furnace filter and the humidifier water panel, the clicking noise (electric air filter) and the loud bang went away.