I have a four year old HVAC system in my home, However when I turn on the A/C the outside condenser will not turn on, I can hear a humming sound, I had a friend who is an electrician check for power and that all checked out, if anyone could help I would be most appericative. Thanks everyone,

Ron

If it is humming and it has power could be a number of thing, bad capasitor. Low on freon. If it is humming then the contactor is probably pulled in. Are you 100% sure you have 220 volts out there? Check the breaker and fuses in the disconnect outside.