When gas burns it produces a great deal of-of steam, carbon dioxide, and heat. A standard efficiency gas boiler has to exhaust the steam and carbon dioxide through the flue at a high enough temperature to stop the steam condensing. This results in excess heat being lost through the flue.
High efficiency condensing gas boilers are 86% – 90% efficient (SEDBUK Band B) or 90% + efficient (SEDBUK Band A). A high-efficiency condensing boiler works on the principle of recovering as much heat as possible from the burner. This saves you money and reduces your gas consumption. This is achieved by increasing the surface area of the heat exchanger which maximizes heat transfer from the burner as well as recovering heat from the flue gases. When the boiler is in a condensing mode the flue gases give up their latent heat which is then recovered by the heat exchanger. The result of this is that the flue gases discharged on a condensing boiler are normally between 50ºc – 60ºc compared to 120ºc – 180ºc on a standard efficiency boiler. Even when the boiler is not in a condensing mode the boiler will still be more efficient than a standard efficiency boiler.
The two main problems encountered when installing condensing gas boilers compared to standard efficiency gas boilers are the flue and condense. As the flue products are so cool they tend to produce white plumbing therefore careful consideration must be given to the positioning of the flue outlet so that the plumbing does not cause a nuisance to you or your neighbours. In cold weather, this is more evident. Again as the flue products are so cool condensation is produced (approximately 1 litre per hour) which normally needs to be discharged into the drainage system which sometimes is not easy. If a drainage system is not available then the condense can either be discharged into a special soak-away (assuming suitable access to outside) or a special pump can be installed to pump the gas boiler condense long distances from the boiler.