I’m working on a LEED v4 C&S office building and there are no tenant lease agreements associated with the tenant area mechanical systems (Client does not want to impose any restrictions on obtaining a tenant). The proposed mechanical design is a 100% OA system with hydronic heating and cooling systems served by a boiler and chiller system. A 100% OA duct and a heating and cooling connection will be provided to each tenant space. As there are no tenant lease agreements regarding the tenant mechanical systems, we have received clarification from the USGBC that we can still model a 100% OA system “provided the most energy intensive terminal system is modelled, i.e. a fan-coil system with fan power being modelled according to G3.1.2.9 and minimum motor efficiencies as outlined in Table 10.8”. The equivalent baseline building HVAC system is a System 7 VAV with Reheat.
My query is regarding the calculation of the fan power associated with the fan-coil unit that I must use in the proposed design. If I follow G3.1.2.9 and select a System 3 as the closest equivalent to the proposed fan-coil system and calculate the brake horse power according to the formula CFMs * 0.00094 + A and minimum motor efficiencies according to Table 10.8, I am getting a specific fan power averaging 1.6 W/l/s (e.g. if auto-sized airflow for Tenant Area 1 = 4,165 cfm, then BHP=3.91 and Pfan=3.263 kW using a motor efficiency of 89.5%). However, the fan-coil units that are being installed in the landlord areas (which we can take credit for) have a SFP of just 0.16 W/l/s. Thus, it appears the proposed design is being severely penalised if I follow this approach. However, if I select the baseline system as being a System 1, the Pfan=1.25 kW and SFP=0.64 W/l/s based on the same airflow of 4,165 cfm. This appears to be more in line with what a fan-coil system would be. So would I be correct in using System 1 as the “equivalent” to a fan-coil system and calculate the proposed fan coil unit fan power using Pfan = CFM*0.3? Is there something else I may be missing?
Thanks in advance.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5907 thumbs up
October 25, 2017 - 4:14 pm
A system #1 can only be used for residential spaces so you cannot use that system.
It sounds like you should be using the system #3 fan power calculations. The guidance indicates that the most energy intensive system should be modeled so I would expect that you would be paying a penalty in this situation. The specific fan power will be higher for a system #7 but the energy use will probably be lower due to the use of variable speed fans.