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?
The LEED EMP exception applies as long as the equipment has less than 150 tons of cooling capacity (i.e. the ground-source heat pump). This does not refer to the individual water-source heat pumps serving the zones in the building. If the cooling capacity of the ground-source water loop is less than 150 tons then the budget system can be modeled as "System 3" in Table 11.4.3A in ASHRAE 90.1-99. Although System 3 is a packaged VAV system with parallel fan powered boxes, it does not make sense to use System 9 (non-residential, single zone) because it is unreasonable to have 104 packaged rooftop heat pumps. The outside air requirements would be modeled with the VAV system, rather than as separate stand-alone units. (If there are 105 heat pumps in the building, we question whether the total cooling capacity of the ground-source heat pump is less than 150 tons. If it is not, then the EMP exception does not apply.) As for modeling the energy recovery on the outside air system, this does not need to be modeled in the budget building unless the outside air flow is 70% or more of the design supply air flow and the air flow is greater than 5000 cfm (see section 184.108.40.206 of ASHRAE 90.1-1999). With a ground-source heat pump, you should use the 13.4 EER and 3.1 COP for the water-source heat pumps serving the zones. Refer to table 6.2.1B in ASHRAE 90.1-1999. For the budget building, refer to the discussion above if the total cooling capacity of the ground-source heat pump is less than 150 tons. If there is a need to convert from SEER and HSPF to EER and COP respectively, please see references below. However, we do not think you will need these conversions for this project. The equipment capacities for the budget building shall be sized proportionally to the capacities in the proposed design. Refer to section 11.4.3.j in ASHRAE 90.1-1999. This should solve your issues with sizing the equipment and meeting loads. CONVERSION REFERENCES: SEER (From www.gard.com) The CEC method is required for compliance purposes, but many use simpler methods such as converting the SEER to EER by using a multiplier (0.85 to 0.95 are often used) and then converting the EER (95/67) to EIR using a straight unit conversion. RESNET which is the body that I've worked with on software verification methods for nationwide HERS ratings for houses uses the following equation: Cooling EIR = 0.941 * (1/(SEER/3.413)) This was developed as a simplification to more complex methods and is based on a regression analysis of numerous systems. As I'm sure you're aware, the conversion of SEER to EER is not exact and depends on the means by which the manufacturer acheived the SEER. There are then various assumptions about the part load performance of the units, but that is another topic. HSPF The industry standard test for overall heating efficiency provides a rating known as HSPF (Heating Season Performance Factor). This laboratory test attempts to take into account the reductions in efficiency caused by defrosting, temperature fluctuations, supplemental heat, fans and on/off cycling. The higher the HSPF, the more efficient the heat pump. A heat pump with an HSPF of 6.8 has an "average COP" of 2 for the heating season. To estimate the average COP, the HSPF is divided by 3.4. Applicable internationally.
Related Addenda (Corrections & Interpretations)