The construction and working principle of boiler economiser are simple. At the bottom part, it has a horizontal inlet pipe through which we feed water with normal temperature to the economizer. There is another horizontal pipe fitted at the top of the economiser. These two horizontal pipes, which are bottom and top pipes connected through a group of vertical pipes. There is an outlet valve fitted on the top horizontal pipe to supply hot water to the boiler. The flue gases from the boiler furnace flow through the vertical pipes of the economizer. Here, the flue gases transfer remain heat to the water through the surface of the vertical pipe when the water goes up through the vertical pipes to top horizontal pipe. In this way heat of the flue gases gets utilised through the economizer for heating up the water before entering into the boiler for producing steam.