This is a hot water economizer and is a big preheater for domestic hot water use. This boiler plant serves a large community for heating and domestic hot water use. Domestic hot water is used for showers, washing dishes, laundry, ect, ect, ect…. The cold water supply from the city is routed through the economizer to be pre-heated before it goes through a heat exchangerand then the hot water storage tank. This system saves money because it uses the exhaust gases from the boiler to pre-heat the city water supply which is normally 50 degrees Fahrenheit. The exhaust gases from the boiler would otherwise be vented to the atmosphere and would be considered wasted heat.
Hot water boiler systems with economizers are specially engineered for this purpose and are made of special materials to handle the caustic effects of the condensation from exhaust gases when they are cooled. You should not try this at home as this will likely cause severe problems with your boiler or furnace system.