I need to install RPZ device for my lawn sprinkler system. Can this device be installed vertically or it must be installed horizontally. I am working with the tight space where the pipe is going up and then horizontally right by the celling so it is almost impossible to install the valve horizontally. I live in Cook County, Illinois.

Assuming that this is a residential lawn irrigation system you have numerous options available for a vertical installation depending on the manufacturer you choose for your backflow prevention assembly. The ASSE website asse-plumbing.org can provide you with all of your available options for an approved backflow prevention assembly. Many residential lawn irrigation systems have the backflow prevention assembly installed outside so this may be an option, an outside installation would also prevent a possible flood situation in the event the backflow assembly discharges inside your home. As with all plumbing modifications it is best to seek the advice of a State of Illinois Registered Plumbing Contractor.