Good afternoon. I'm getting started on a model for a county courthouse office expansion. In its pre-renovated state, the facility purchases both HW and CW through the local utility. The proposed system is to use VRF + DOAS. For the DOAS unit, the design engineer is proposing to use a HW coil that is directly linked to the utility's HW feed through a plate HX. On the cooling side, they want to use a DX-based system with a water-cooled condenser. This water cooled condenser would be linked to the utility's CW feed through a plate HX. The DX+CW system configuration has been proposed to allow the equipment to utilize a hot gas reheat for dehumidification purposes.
Long story short, I'm trying to figure out how my baseline system should be modeled so that USGBC finds it acceptable. ASHRAE 90.1 Appendix G, if I weren't dealing with DES, indicates that I would be using a System 7-VAV with Reheat because I am greater than 5 stories. The "Treatment of District or Campus Thermal Energy in LEED V2 and LEED 2009 - Design and Construction; v2.0" manual, Table 3 indicates no applicable changes to the Appendix G requirements. However, this manual states, when pursuing Option #1 for the Performance Path, in Section 2.4.1.1 (1) that the "energy source is modeled as purchased energy in both the Proposed and Baseline buildings for all air handlers [...] serviced by district or campus energy systems in order to hold the DES cost-neutral in the model." My issue is that all my loads other than those associated with ventilation air handled by an electrically-driven air-source heat pump system and only my ventilation loads use the DES. Any thoughts on how this should be modeled while still meeting LEED's baseline modeling requirements for System-7 VAV with Reheat?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
April 9, 2015 - 11:35 am
Excellent question. Somewhat difficult answer.
According to G3.1.1.1 and if you apply Addendum ai the DOAS systems must also use the purchased HW and CW in the baseline. The baseline systems should just use a HW and CW coil to condition the OA. The loads using purchased energy should do so in both models.
It is not clear whether the rest of the system should be a system 7 or 8.
If the heat source in the DOAS were gas this would be hybrid and therefore a System #7. Purchased heat is listed separately in Table G3.1.1A and there is not a hybrid version. So a case could probably be made for a System #7 since your situation is similar to a hybrid system. The added language in Addendum ai also lends some support to a System #7 since it precludes any electric heat comparison.
The rationale for a System #8 is related to the predominant condition (see note under Table G3.11A). This is normally defined by area. In your case the area served by the DOAS and the VRF is the same. Expanding the definition of predominant condition to the building loads would lead you to a System #8.
So the rationale for a System #7 extrapolates from a hybrid system and the rationale for a System #8 expands the definition of predominant condition. In both cases a connect the dots approach is necessary and therefore the correct answer is not entirely clear. I think we are leaning toward the System #8 which would be more apples-to-apples on the utilities (not comparing gas to electric). I would strongly recommend that you submit a LEED Interpretation with your recommended baseline system approach for approval.
Christopher Strunk
Mechanical EngineerStrunk-Albert Engineering
April 9, 2015 - 1:51 pm
Greatly appreciate your feedback, Marcus. I'm going to proceed with submitting a formal interpretation.