Air recirculation and outdoor air

Francesco Passerini

engineer

LEEDuser Basic Member

65 thumbs up

August 17, 2012

I have some doubts concerning air loops of the baseline model, system 5 (Packaged VAV with Reheat). Paragraph G3.1.3.13 “VAV Flow Minimum Flow Setpoints” states: “Minimum volume setpoints for VAV reheat boxes shall be 2.15 l/(s*m^2) of floor area served or the minimum ventilation rate, whichever is larger”. How shall I calculate the “minimum air ventilation rate”? And the minimum outdoor air ventilation rate? In the case that I’m modelling economizer is not requested because the climatic zone is 4a (see table G3.1.2.6A). Must I model the baseline model with air recirculation? Shall the air recirculation flow be equal to the total required supply air flow minus the minimum outdoor air ventilation rate? With Regards

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Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Fri, 08/17/2012 - 13:42

The minimum air ventilation rate is the design value for that system as shown on the mechanical schedules. The OA ventilation rate is the same as the minimum ventilation rate in G3.1.3.13. If the auto-sized supply air flow of the baseline case is greater than the minimum OA flow, then there will be recirculation. With recirculation the total supply air flow minus the OA flow will equal the recirculation air flow.

Francesco Passerini

engineer

LEEDuser Basic Member

65 thumbs up

Fri, 08/17/2012 - 15:27

Thank you, Marcus When you write “mechanical schedules” do you mean schedules of the model or of design documentation? In the proposed building most of the spaces are naturally ventilated, therefore for those spaces there is no design documentation concerning ventilation rates. To calculate the minimum air ventilation rate shall I use a specific equation, e.g. an equation that considers both the presence of people and the zone surface?

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Fri, 08/17/2012 - 15:52

Design documentation. In naturally ventilated spaces (i.e. no fan powered ventilation at all) the mechanical OA rate is zero in both models. If the spaces are natural ventilation see the example in the appendix to the Advanced Energy Modeling Guide for LEED for more detail on an acceptable modeling methodology to claim savings.

Francesco Passerini

engineer

LEEDuser Basic Member

65 thumbs up

Fri, 08/17/2012 - 18:02

Interesting, Marcus. In the building that I’m modelling there is no natural ventilation strategy: the windows are opened by the users, who can open them when they want. Therefore for the zones that in the actual building are naturally ventilated I would do in this way: - proposed building model: no ventilation - baseline model (HVAC system 5): OA ventilation rate equal to 0 (ventilation is only air recirculation). Can I consider a constant infiltration rate (the same value in both models)? On the other hand, can I neglect infiltration in both models? Thank you very much!

Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Fri, 08/17/2012 - 18:42

The mechanical ventilation still appears to be zero but the simulation should include the effect of open windows which should be identical in both models. Infiltration must be modeled identically in both models and it cannot be neglected. Always important to keep in mind that a modeler's job is to model the proposed as accurately as possible.