[Bldg-sim] Actual Thermal Efficiency of Boilers

[Doebber, Ian]

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?

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