Natural ventilation-energy modeling

Per ASHRAE 90.1, Section G2.5:
Where no simulation program is available that adequately models a design, material, or device the rating authority (i.e. LEED project reviewer) may approve an exceptional calculation method to demonstrate above standard performance. Applications for approval of an exceptional method shall include documentation of the calculations performed and theoretical and/or empirical information supporting tha accuracy of this method.
Natural Ventilation is considered to be a nonstandard efficiency method.
Be sure to comply with ASHRAE 62.1 Section 5.1 when designing natural ventilation systems.

Hope this helps!

Check the 90.1 Userguide. I believe the way to handle natural ventilation is to make a baseline with mechanical ventilation and the same mechanical system into your design case model so that it kicks in when the natural ventilation does not suffice.

There is an old CIR which describes an acceptable methodology to use when attempting to show energy savings for natural ventilation. This would be a good place to start.

3/22/2007 - Credit Interpretation Request
Our project consists of two small buildings close to the ocean that will achieve substantial energy savings by incorporating a natural ventilation strategy. No mechanical heating or cooling is intended for either building, with the exception of a small electrical/server room.

The building is designed with a very narrow and long floor plate situated perpendicular to prevailing winds in the area. Ventilation openings are consistent with the requirements of ASHRAE 62.1-2004 Section 6.8. The project also meets the requirements outlined in the CIBSE Applications Manual 10: 2005 as referenced in EQc2 for Natural ventilation in non-domestic buildings. Per Title-24 2005 requirements for natural ventilation, the sum of operable windows will be greater than 5% of the floor area of each space that is naturally ventilated. The openings will also be readily accessible to the occupants of each space at all times. Outdoor airflow through the openings in regularly occupied spaces will come directly from the outdoors, not through intermediate spaces such as other occupied spaces or corridors. Openings include operable windows, through-the roof ventilators, and vents between interior spaces. Control mechanisms for the natural ventilation openings are manual. A long, tall hallway situated perpendicular to the prevailing winds will collect heated air and exhaust it the outside. The roof over much of the space is sloped allowing air to enter on the low side and exit on the high side. In all cases, the building is designed to facilitate cross-ventilation with windows low on the walls for drawing the air in, and windows and vents high in opposite walls or on the roof to draw air out. Under ASHRAE 55 definitions, the building spaces are defined as "naturally conditioned spaces, occupant controlled" where the thermal conditions of the spaces are regulated primarily by the opening and closing of windows or vents by the occupants. Since the building will have a limited number of occupants most of the time, manual control of the windows and vents has been determined the most appropriate strategy for the building to allow control over individual thermal comfort. As indicated by ASHRAE 55-2004, section 5.3, the occupants of the space will be engaged in near sedentary activities with metabolic rates ranging from 1.0 met to 1.3 met. The mean monthly outdoor temperature for the project is greater than 50 deg. F, and less than 92.3 deg. F all months of the year, as required under ASHRAE 55-2004, section 5.3 for naturally ventilated buildings.

The User's Manual for ASHRAE 90.1-2004 Appendix G states on page G-21: The proposed building default cooling system does not exclude natural ventilation from consideration. It just means that the proposed building is modeled as a hybrid system where cooling is provided by natural ventilation when conditions are acceptable and by the default mechanical cooling system when natural ventilation is inadequate to provide thermal comfort. We are requesting confirmation that the following modeling strategy conforms to the requirements of ASHRAE 90.1-2004 Appendix G modeling protocol:

1. EnergyPlus will be used to model the building since the EnergyPlus software has the capability to evaluate energy and comfort parameters tied to natural ventilation.

2. The Exceptional Calculation Methodology will be applied to calculate the natural ventilation savings.

3. The Proposed Design model will be developed to reflect the design parameters for the envelope and lighting. Operable windows will be modeled as fixed, and vents will not be included in the model. Mechanical systems will be modeled identically to the default heating, cooling and fan systems in the Baseline case, except that fans in the proposed case will be modeled as cycling on and off to meet heating and cooling loads during all hours in the proposed case, and will operate continuously during occupied hours in the Baseline Case (per the exception shown in Table G3.1.4).

4. Using the Exceptional Calculation Methodology, The Proposed Design case will be modified to include natural ventilation for all hours when the cooling and heating loads can be met. Operable windows and vents will be modeled as designed. Cooling and heating setpoint temperatures will be identical to those in the Baseline Case. Schedules will be adjusted to switch on mechanical cooling during hours when natural ventilation alone cannot meet the space temperature setpoints. The final model will meet the ASHRAE G3.1.2.2 requirements stipulating that the Proposed Design cannot exceed the Baseline Design unmet load hours by more than 50, and that unmet load hours for the Proposed Design and Baseline Design cannot exceed 300.

5. (Plan B) If the hybrid system cannot be manipulated to meet the unmet load hour requirements within the energy model, hourly output data from a natural ventilation model (having no mechanical cooling) and the Proposed Design model will be combined in a spreadsheet. Each hour where cooling and heating setpoints are met in the natural ventilation model, the hourly results for that model will be used. For all other hours, the hourly results from the Proposed Design Model will be used.

6. An Exceptional Calculation Methodology narrative provided with the EAc1 submittal will document any schedule adjustments and assumptions that were made to develop the hybrid system. The savings will also be included as a separate line item on the EAc1 submittal.

Is our proposed energy modeling strategy for natural ventilation acceptable?

3/22/2007 - Ruling
The project is requesting approval for the method of modeling natural ventilation as an energy efficiency measure and for taking credit under EA credit 1.

Submittals for natural ventilation savings will be evaluated on a case by case basis.

The tools and analysis protocol proposed is acceptable for modeling ventilation savings in this instance. Other analysis tools may also be appropriate.

To be able to adequately document the process and the results, please be sure to provide in the LEED submittal the following:

· A detailed project description
· Clear identification of the areas that are taking credit for natural ventilation
· A detailed description or references that document the modeling algorithms and/or methodology for the natural ventilation portion of the energy model
· All thermostat, fan, infiltration and other appropriate schedules for naturally ventilated areas

Also, the submitted evaluation must demonstrate that the range of unmet load hours is similar for both the proposed and baseline building, to ensure that savings are not claimed for hours outside of the control parameters. In this case, the project has proposed to meet these peak loads with a hypothetical cooling system in the proposed building.

The project will also need to clearly demonstrate that the operational schedule for the natural ventilation system as modeled aligns with anticipated occupant behavior in terms of scheduled occupancy vs. modeled operation. For example, the model cannot assume that natural ventilation will occur when no one is in the building to operate the system.

Because manual control is not addressed by the Appendix G modeling methodology, the manual control features of this project must be submitted under the exceptional calculation methodology for case by case review. The project must be prepared to demonstrate convincingly that a manual control strategy is appropriate and workable for this project.

Please also be sure to take credit for this measure as a separate item on the LEED-NC v2.2 Submittal Template.

So yes you must comply with the mandatory provisions and you cannot use eQUEST.