ID#10130 made on
EAp2 - Minimum energy performance
LEED BD+C: New Construction
We have a technical question regarding an energy model for a project that is a 450,000 sq. ft. warehouse plus 30,000 sq. ft. of office space. The warehouse is conditioned by (6) gas fired constant vol...
We have a technical question regarding an energy model for a project that is a 450,000 sq. ft. warehouse plus 30,000 sq. ft. of office space. The warehouse is conditioned by (6) gas fired constant volume 80/20 units. In addition, the warehouse contains 6 constant volume ventilation fans with low outdoor air dampers utilized in the summer to provide air movement when the temperature is above 80 degrees. According to ASHRAE 90.1 Appendix G we will need to provide heating and COOLING to this space in both the baseline and proposed with the cooling system type and parameters being equal in the proposed and baseline. We have been discussing internally what the correct path for modeling this type of system should be and believe we are at a point where we need guidance from a higher authority on the correct path. The following are different options we have discussed and the issues we see with each:
Model the baseline and proposed cooling system the same as both baseline and proposed (System 7) - Issue - the system 7 calls for a VAV reheat system. This would mean the proposed design would have VAV fans for cooling and heating. We feel this is inaccurate due to the fact the proposed design actually includes constant volume fans for heating.
Model the proposed system as constant volume heating but VAV cooling - Issue - Again we have constant volume ventilation fans in the summer and have no way of accounting for their energy usage. If we set them up as an extra utility then we are being charged on both using a VAV fan in the system as well as the constant volume fan in the summer. Essentially using twice the fan power than what is designed.
Model proposed as being constant volume all year and the baseline being VAV - Issue- the proposed is now at a big disadvantage for mechanical cooling at constant volume when we don't have mechanical cooling in the proposed design any way. Also, now the proposed and baseline cases are not held equal in the cooling season. Also, how would we model the ventilation? Are we to assume that we are now 100% outdoor air with a constant volume fan? This would put us at a big disadvantage against ASHRAE 90.1 requirements.
Another issue we are finding with all these scenarios is that we are inflating the resulting energy cost such that no savings can be realized by using a better envelope, better equipment, and reduced lighting power. Furthermore, the horsepower for the proposed ventilation fans is low due to minimal resistance in airflow (no ductwork, no coils, no filters, etc). Assuming we model these as you would a typical supply fan, the baseline system would need to follow the Appendix G calculation for allowable horsepower, and therefore the baseline horsepower would be greatly inflated compared to the proposed design. This would create an advantage for the proposed design that is inaccurate.
ASHRAE 90.1 2010 specifically covers this type of building. In the 2010 version, Appendix G has added two additional systems and provided the additional information to complete an ASHRAE 90.1 comparison for a heating only and ventilating system. When modeling after the 90.1 2010 protocol the results seemed to fall in line with where we would have expected them to be. The modeling protocol made it possible to show energy savings due to lighting, envelope, and more efficient equipment while not creating either advantages/disadvantages in the modeling technique. Would it be acceptable to the USGBC to model after the 2010 version of ASHRAE 90.1 and provide a narrative describing why we used the 2010 version to complete the analysis?
For projects containing heating-only storage buildings or spaces, it is acceptable to use system types #10 or #11 from the ASHRAE 90.1-2007 Addendum modifications to the Appendix G modeling protocol whenever applicable. ASHRAE 90.1-2007 Addendum dn establishes the Baseline system type for heated only storage buildings as System Type #10 (where the proposed case heating source is fossil fuel, fossil/electric hybrid or purchased heat) or #11 (where the proposed case heating source is electric or other). Furthermore, Section G3.1.1 Exception (e) states that thermal zones designed with heating only systems in the proposed design, serving storage rooms, stairwells, vestibules, electrical/mechanical rooms, and restrooms not exhausting or transferring air from mechanically cooled thermal zones in the proposed design shall use System type #10 or #11 in the Baseline Building design (Note: this exception would also apply for an enclosed heated-only parking garage or apparatus bay). Exception (f) states that if the Baseline system type is 10 or 11, all spaces that are mechanically cooled in the proposed building design shall be assigned to a separate baseline system determined by using the area and heating source or the mechanically cooled systems.
Note: If the project is served by a District Energy System, and the project is using Option 1 of the "Treatment of District or Campus Thermal Energy in LEED V2 and LEED 2009 - Design & Construction" document to achieve credit compliance, then the Baseline Case System Type #10 shall be revised to a single zone constant volume air handler with purchased hot water heating.
Note: this ruling does not apply to Core and Shell projects.
Related Addenda (Corrections & Interpretations)