I have a good downdraft coming down one of the chimeny tubes which is where my hot water heater and boiler exhaust through and it is keeping the exhaust from going up and out of the house , there is an attic fan that comes on by itself , help ,

You have a very high hazzard situation. You need local help right away. Call your local HVAC company and have them do a On Site Inspection and repair of your problem. Sorry but we cannot help you hear due to the nature of your problem since it is considered an emergency situation that involves your health and welfare.