Rating System

LEED BD+C: New Construction

Rating System Version

v2 - LEED 2.2

Inquiry

This credit interpretation request is being submitted regarding the baseline budget modeling of our system using the exception to the ECB method taken by LEED, allowing for the modeling of the budget HVAC systems for units under 150 tons with air cooled condensers. Our project is for a college in Illinois which contains a ground source closed water loop. Outside air is provided by two 100% OA units with water-cooled dx cooling coils and enthalpy wheels providing a constant discharge temperature. The OA units provide outside air to 104 water cooled heat pumps throughout the building, some of which have supplemental electric heat. First off, does the exception taken by LEED to ASHRAE 90.1 allow for the budget modeling of an air cooled unit during cooling only or for both heating and cooling? We are modeling our system in Equest DOE2 and I do not believe it is possible to model a heat pump which is water cooled during the heating season but air cooled during the cooling season. The intent of the exception is to encourage the use of water cooled equipment and therefore it seems appropriate to provide the benefit for heating as well to further encourage the use of water cooled equipment. Is this a correct interpretation? LEED allows us to model the budget system as air cooled with an efficiency equal to the minimum efficiency in ASHRAE 90.1 table 6.2.1B. Most of our heat pumps are small, falling in the <65,000 Btu/h range, and given that our actual system uses single package units, the budget system would call for an cooling efficiency of 9.7 SEER and a heating efficiency of 6.6 HSPF. We are modeling our building using Equest DOE2.2 and there is no way to directly input a seasonal efficiency into the program, nor do I know of any program that would allow for the direct use of a seasonal efficiency. There is no way to directly calculate EER and COP from SEER and HSPF, which makes the efficiencies given in table 6.2.1B impossible to use when using the ECB method. We propose that all air-cooled heat pumps <65,000 Btu/h use the EER and COP efficiencies listed for units >65,000 Btu/h and < 135,000 Btu/h. Is this acceptable? The ECB method requires that the proposed and budget system have the same capacity. This will result in some of the HVAC units in the budget system being unable to meet room setpoints. Reasons for this would be the difference between the capacity curves for an air-cooled and a water-cooled system, as well as the fact that our proposed system has a 78% effective enthalpy wheel, but the budget system will only have a 50% effective wheel. Is it acceptable to have rooms not meeting the required temperature for comfort in the budget system? One way to avoid the problem with the heat pumps would be modeling the systems as constant capacity whenever possible (although this will distort the values for any heat pumps with supplemental heat) but what about the decreased effectiveness of the heat wheel resulting in some of the rooms not meeting setpoint? We propose the modeling of an electric heating coil in the MAU with a capacity equal to the difference between the heat provided by the proposed and budget enthalpy wheel to appropriately model the benefit we are receiving from using more effective energy recovery. Is this acceptable or are actual energy savings to be lost in the model due to the budget system not having to meet room setpoints?