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CS-2009 EAp2:Minimum Energy Performance

Modeling fans for tenant areas for a C&S Project

Hello, I did some research to find a similar question on this forum before posting it, but even though there were several questions related to the topic, I did not find the exact situation: We are modeling a Core&Shell office building, the owner will install an HVAC system and will only deliver chilled water and outdoor air to the tenant spaces. The design guidelines will specify that Fan&coil units should be installed for all tenant spaces. For common areas, such as lobbies, the design includes the capacity and model of the corresponding fan&coil unit. The question is how to model the fans for the tenant areas, should we model them exactly as the baseline model? If not, assuming we model fan&coil units with auto-size capacities, the simulation results show a lot of energy savings in fan energy consumption (more than 70%). The pressure drop for the tenant fan&coil units, using the value of 0.35 W/cfm (same as the PFP boxes of the baseline model), gives us the same fan power for each tenant area, but for the baseline model there are also the air loop fans which deliver the air for each floor, and here is where the main difference in energy consumption comes from, since in the proposed model chilled water is delivered directly to each space. The power, pressure drop and fan efficiency of the baseline air loop fans are defined using the Appendix G Fan Power Calculator. I am not sure how to model the proposed model in order to obtain a reasonably fan energy savings. Any suggestions would be greatly appreciated. Best Regards, Santiago

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Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4454 thumbs up

Wed, 07/01/2015 - 17:16

To claim energy savings you will need to have something required by the tenant lease agreement. Otherwise the parameter should be modeled identically. You can't just auto-size the fans and use only the allowance for the PFP boxes. Requiring the installation of a fan/coil unit should be augmented with specific performance parameters relative to fan power (perhaps a W/cfm limit) in order to claim any energy savings. You will need to develop these requirements, require that tenants follow them, and justify the methodology you use to model it to the reviewer.

Thu, 07/09/2015 - 11:52

I have a similar problem. There won't be any lease agreement signed regarding fancoil units that the tenants will install in their spaces. The reference model has a VAV system with reheat so I can't model the proposed case (CAV with fancoils) in the same way as the reference case. Does it mean that we have to calculate the fan power of fancoil units based on the guidelines of section G3.1.2.9 (but without inclusion of fans in AHUs) assuming some level of pressure drop of the fancoil unit?
Profile picture for user sheffer
Marcus Sheffer
LEED Fellow
7group / Energy Opportunities
LEEDuser Expert
4454 thumbs up

Thu, 07/09/2015 - 15:41

I am assuming you have a system #5 in the baseline but that is not clear. There are two modeling requirements at play - selecting the baseline system according to Table G3.1.1A versus modeling the proposed as designed according to Table G3.1.10 (b)(Proposed). You will need to violate one of them. The question is, which one? You should always choose the one that produces a logical comparison and the most conservative results. The other issue is that the HVAC in the tenant spaces that has not been designed must be modeled identically. In order to do so it makes the most sense to change the baseline system type to a system 7 or 8. You can then do a more valid comparison which concentrates on any savings from the central plant while keeping the system level neutral. Your suggestion of modeling the fan power in both according to G3.1.2.9 would not be allowed as the calculations differ for a VAV system with reheat and a CAV with fan coils. I did not see any LEED Interpretations on this issue so you might want to submit one to be sure your approach will be acceptable.

Fri, 07/10/2015 - 08:56

I have a system #7 in the baseline. The proposed model is a CAV system with 4-pipe fancoil units. AHUs in the proposed case can be modeled without a problem because we know all its parameters (fan powers, heat recovery efficiencies etc.). So the only problem is with terminal units - fancoils (fan power) in tenant spaces. Could you tell me what kind of comparision you had in mind if the reference system is #7?

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