Since 90.1 sets the minimum requirements for energy use in buildings, it was decided that we would not require condensing boilers. Condensing boilers have special flue and condensate drain requirements that did not fit with the idea of “minimum requirements”. Since that decision was made, condensing boilers have become more and more common in buildings of all types.

In 90.1-2013 section, 7.5.3 was added. Section 7.5.3 Buildings with High-Capacity Building Service Water Heating Systems requires the total service water heating plant to have an input capacity weighted average thermal efficiency of at least 90%. This means that several boilers can be used with varying efficiencies, but the total Btuh output that heats the water divided by the total gas input in Btuh must be greater than 90%. This means that condensing boilers will be needed to meet this requirement.

The high capacity building service water heating system refers to systems with a total capacity of over 1,000,000 Btuh. This section has an effective date for compliance of July 2015. This date, after the Standard has been issued, allows the manufactures time to adjust their equipment designs and have testing done to meet the new requirements.

Since the efficiency of condensing boilers varies with return water temperatures. I personally would require the boiler to have 90% efficiency at the return water temperature as scheduled on the construction documents. This would require a return water temperature somewhere under 130°F.

Opinions expressed here are solely my own and do not express the views or opinions of ASHRAE or the 90.1 committee.