Most brands of condensing gas boiler are in the highest available categories for energy efficiency. These boilers typically receive an Eco Logo and/or Energy Star Certification. In the United States, all residential boilers (of foreign or domestic origin) are tested and rated by the US Department of Energy (D.O.E) to an Annual Fuel Utilization Efficiency (AFUE) rating. All residential condensing boilers currently available have an AFUE of 90% or more. The vast majority are now rated at 95%+. All condensing boilers in the U.S. are fitted with microprocessors to modulate output, and capable of operating on outdoor reset.
Boiler performance is based on the efficiency of heat transfer and highly dependent on boiler size/output and emitter size/output. System design and installation is critical. Matching the radiation to the Btu output of the boiler and consideration of the emitter/radiator design temperatures determines the overall efficiency of the space and domestic water heating system.
One reason for an efficiency drop is because the design and/or implementation of the heating system gives return water (heat transfer fluid) temperatures at the boiler of over 150F, which prevents significant condensation in the heat exchanger. Better education of both installers and owners could be expected to raise efficiency towards the reported laboratory values.
Most non-condensing boilers could be forced to condense through simple control changes. Doing so would reduce fuel consumption considerably, but would quickly destroy any mild steel or cast-iron components of a conventional high-temperature boiler due to the corrosive nature of the condensate, and is the reason why most condensing boiler heat-exchangers are made from stainless steel or aluminum/silicon alloy.
The lower the return temperature to the boiler the more likely it will be in condensing mode. If the return temperature is kept below approximately 130F the boiler should still be in condensing mode making low temperature applications such as radiant floors and even old cast iron radiators a good match for the technology.
Most manufacturers of the new domestic condensing boilers produce a very basic "fits all" in-built control system that ends up with the boiler running in condensing mode only on initial heat-up, after which the efficiency drops off, although it should still exceed that of older models.