Today, condensing boilers are becoming more popular with the intent of reducing the water temperature returning to the boiler to condense the flue gas as much as possible. There is a direct correlation between increasing condensation of the flue gas and boiler efficiency gains. Because of this, boiler construction for condensing boilers is different from non-condensing boilers. In condensing boilers, the heat exchanger has to be made of robust materials to prevent the acidic condensate from corroding and completely degrading the materials within the boiler and the flue stack. Condensing boiler heat exchangers are constructed of either stainless steel or aluminium construction.