Condensing Gas Boilers
Condensing gas boilers employ either an aspirating burner with an induced draft fan, or a power burner, similar to the units described previously. However, they have an additional heat exchanger made of corrosion resistant materials (usually stainless steel) that extracts latent heat remaining in the combustion by-products by condensing the combustion products before they are exhausted. A chimney is not needed, reducing the cost of installation. Because the flue gas temperature is low, the gases are vented through a plastic pipe out the side wall of the house.
A condensing boiler can have an AFUE rating of 90% or higher. But in practice, condensing boilers in hydronic (hot water) heating systems can have difficulty achieving this efficiency. For the condensing boiler’s heat exchanger to extract all the potential latent heat effectively, the system has to run with the lowest possible return water temperatures, preferably not exceeding 45–50°C (113–122°F). Unfortunately, most radiator systems are designed to operate at significantly higher return water temperatures, which makes it difficult for the flue gas to condense. If the return water temperature is too high, actual operating efficiency may be only slightly higher than that of the better models of non-condensing boilers.