Vapor and combustion by-products combine to create highly acidic condensate. This restricts the types of metal used in condensing appliances for hydronic heating. Prior practice was to use building location, lighting design and other factors and 20°F ΔT calculations for designing hydronic heating systems. This led to systems designed for worst case scenarios, resulting in overcapacity and wasted energy. With the emphasis on Green design, systems should and can be designed to building load. New technology in controls, segmented systems and lead lag capability allow systems to only use the energy needed, to provide the appropriate amount of heat.