When installing a smoke alarm in your home, it is important to site it correctly if it is to be effective. The best position is on the ceiling between potential sources of fire (kitchen, etc) and the access to the sleeping areas of the house, e.g. in the downstairs hallway. Try to place the alarm in the middle of the ceiling as the edges can be dead air space (i.e. slower to fill with smoke) which may stop the device working correctly.