ASHRAE 90.1 mandates that a Gas-Fired Boiler ? 300,000 Btu/hr & ? 2,500,000 Btu/hr must have a minimum 75% Thermal Efficiency (Et) based upon the Test Proceedure Hydronics Institute Boiler Standard.

The Hydronics Institute Boiler Standard official title is "BTS-2000 Testing Standard : Method to Determine Efficiency of Commercial Space Heating Boilers" published by The Hydronics Institute Division of Air-Conditioning, Heating, Refrigeration Institute (AHRI). Can be found at : http://www.ahrinet.org/ARI/util/showdoc.aspx?doc=1198.

The Testing Standard measures the Thermal Efficiency of the Boiler at steady state operation, fully loaded maintaining the outlet temperature at 180?F. Oddly, the inlet water temperature into the Boiler is between 35?F to 80?F which is much lower than the typical Return Water Temperature of 150?F in a Building Application. Consequently, these colder inlet temperatures are most likely overstating the Thermal Efficiency of the Boiler which is condensing even if the Boiler is a Non-Condensing Boiler. Based on various Manufacturer's data, a Boiler operating with an 80?F Return Water Temperature achieves a 10% greater Thermal Efficiency compared with operating at a standard 150?F Return Water Temperature. Then operating at a 35?F Return Water Temperature, the Thermal Efficiency should be boosted even further.

Would it be reasonable to assume that a Standard Atmospheric Boiler that meets the 75% Tc minimum based on ASHRAE 90.1 according to the BTS-2000 Testing Standard using 80?F Return Water Temperature would actually operate at 65% Tc in the field operating at 150?F Return Water Temperature?