Furnaces that operate with condensing flue products in order to achieve over 90% efficiency have occasionally experienced severe corrosion of metal surfaces in the associated condensing or recuperator areas, even when using 300 series stainless steel . Such corrosion has, in most cases, been directly attributed to the presence of acid-forming chlorides in the environment. However, experimentation on acid corrosion and analysis of corrosion mechanisms in general emphasizes the primary importance of local concentrations on corrosion of surfaces experiencing wet-dry transitions.

A study of the conditions inducing corrosion in recuperating furnaces has led to the definition of a design strategy that will provide improved corrosion resistance with inexpensive materials. A properly designed condensing furnace continuously recuperating to a degree higher than might be justified by economic payback on energy consumption alone can provide sufficient condensate for continuous backflushing of important recuperator heat transfer surfaces and the continuous wetting or cooling of transition area surfaces, all of which significantly reduce the corrosion environment and material requirements.

A domestic combination furnace and water heater has been designed with this strategy as proof of the concept and models are now being field tested . This unit contains a finned hydronic boiler combined with a recuperator in a compact package. This gas-fired module of 100,000 Btu/h (29 kW) input has outer dimensions of 2 ft x 2 ft x 1 ft (0.6 m x 0.6 m x 0.3 m) and can be applied universally to warm air and hydronic heating . One field module has operated through the 1984-85 season in Massachusetts as a hydronic unit, and another unit has operated since January 1985 in Iowa as a warm air furnace . Both units have been monitored continuously .for corrosion and heating performance.

Units: Dual