Variable Speed Fan Coil Unit

James Chueh

LEEDuser Basic Member

69 thumbs up

November 15, 2011

There are more and more companies making fan coil units with VFD motors, automatic variable supply air control rather than traditional 3-speed control, which gives them variable air volume capability similar to the typical VAV system (VAV box/terminal unit). So can variable speed FCUs be treated as VAV systems in the energy model? Under 90.1, our system should be compared with System 8 VAV with PFP box, but I think it would be unfair to us if we just model it as a constant volume system.

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Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Tue, 11/15/2011 - 14:58

Not sure I understand the question. Model the Proposed as designed and the Baseline according to Appendix G. The Appendix G Baseline system does not change based on the details of the Proposed system.

James Chueh

LEEDuser Basic Member

69 thumbs up

Wed, 11/16/2011 - 02:58

Well, What I meant is can we call these "variable speed FCUs" a VAV system then? Because normally when you say VAV it means constant SA air temp fitted with "VAV box", not a hydronic system such as FCUs.........

James Chueh

LEEDuser Basic Member

69 thumbs up

Wed, 11/16/2011 - 03:19

Normally you would treat FCU as a constant speed CAV system, and it would be disadvantageous to us if this is compared to, in the case of our baseline, a VAV system in the energy model. So I need to know if we can call our proposed system as a VAV system.

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Wed, 11/16/2011 - 13:59

You should model what you have designed in the proposed. How you model it depends on the simulation program. They are all different. In general you cannot model a FCU as a VAV as there tend to be significant differences between the two. Sounds like you are asking if modeling the variable FCU as a VAV would be an acceptable workaround in the software you are using. If that is correct I would suggest you post your question on one of the simulation software discussion groups at onebuilding.org.