We have a mixed use retail - residential project with the residential units relying on natural ventilation with the required 4% operable window area to floor area ratio. The problem is that some of the open living-dining-kitchen spaces are more than 25 ft away from the window. We do have dedicated kitchen exhaust systems towards the back of these large spaces and are hoping they qualify for what ASHRAE 62.1 calls an 'engineered' ventilation system under the 'exception to the prescriptive' at section 5.1. ASHRAE states this must be approved by a local authority. Does anyone have experience with the USGBC interpretation of this exception and any specific calculations and calc method they would require? There are no 'precedent CIR's' on the matter although I know that they have approved booster fans beyond 25 ft, etc.

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Roger Chang

Principal, Energy and Engineering Leader

DLR Group | Westlake Reed Leskosky

LEEDuser Expert

267 thumbs up

Thu, 08/25/2011 - 20:53

Take the exhaust cfm and apply a zone air distribution ventilation effectiveness (Ez) of 0.8 as per Table 6-2 (Makeup supply drawin in on the opposite side of the room from the exhaust and/or return). See if this corrected value meets the breathing zone rate calculated for the entire open living area.

Michael Wironen

Ecology and Environment, Inc.

LEEDuser Basic Member

16 thumbs up

Wed, 08/31/2011 - 13:55

Roger: we are facing a similar situation with a residential building (20% of the units have spaces more than 25' from an operable window). We have had to eliminate the proposed ventilation supply due to a structural engineering constraint. In addition to using continuous kitchen exhaust, as Michele has proposed above, we will be installing trickle vents to provide additional fresh air. Our window operable area: to floor area ratio already exceeds ASHRAE 62.1 requirements, per NYC code. Given that this design change is not being mandated by the Department of Buildings or any other authority, but rather is a response to a structural design constraint and our desire to meet ASHRAE 62.1, we will not be receiving any formal approval or review from a local authority for this proposed design. We have already satisfied code through the window operable area: floor area utilized. How critical is it that a local authority review and approve the engineered natural ventilation design when it is being proposed as best practice to meet LEED requirements, as the 'standard' design already satisfies local standards?

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Multi-family residential - ASHRAE 62.1 2007 - ch. 5 Natural Vent

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