Optivent

NATURAL VENTILATION STRATEGIES

A Natural Ventilation Steady-State Calculation Tool for the Early Design Stage of Buildings.

Single sided ventilation

Cross ventilation

Atria (single-cell)

Chimneys (multi-cells)

Optivent 2.0 was designed to support the latest versions of web browsers. The tool is not fully functional in older versions of IE.

PROJECT LOCATION

Project location

Project data

Project Name
Run
Date
Consultant

Location data

Latitude (decimal degrees)
Month
Hour
Prevailing mean outdoor temperature (°C)
Prevailing mean outdoor temperature was introduced in ASHRAE Standard 55-2010 as an input variable for the adaptive model. It is based on the arithmetic average of the mean daily outdoor temperatures over no fewer than 7 and no more than 30 sequential days prior to the day in question.
Meteorological Wind Speed (m/s)
Terrain data
Wind speed is only considered in Buoyancy + Wind calculations. Wind direction is assumed perpendicular to the surface of the inlets. It is recommended to consider reduced wind velocities according to the complexity of the urban layout.

Inlet (surface) Azimuth

The ASHRAE Clear Sky Model (2005) initially developed by J. L. Threlkeld in 1962, is used to estimate hourly clear sky beam and diffuse irradiance for any month of the year. This model is applicable to the United States continent and its applicability outside the USA has never been demonstrated. Optivent 2.0 extends the Clear Sky application to both northern and southern hemispheres. Note that the Clear Sky model has been updated in later versions of ASHRAE H-F.

The inlet (surface) azimuth is used to estimate solar gains in cells for the current sun position (given by latitude, month and time).

N NE E SE S SW W NW

AIRFLOW DATA INPUT
Airflow data input

Natural ventilation strategy: Single sided ventilation.

Natural ventilation strategy: Cross ventilation.

Natural ventilation strategy: Atria (single-cell).

Natural ventilation strategy: Chimneys (multi-cells).

This case represents single sided ventilation through an open window or two open vents in an isolated space.
Temperatures within the space is assumed to be the same at any given height.

This case represents cross ventilation through two open vents in an isolated space.
Temperatures within the space is assumed to be the same at any given height.

This case represents a single-cell building with an atrium connected to wide open occupied spaces.
Temperatures within the atrium and the spaces are assumed to be the same at any given height.

This case represents a multi-cell building with occupied spaces connected to a well-defined chimney, closed to the surroundings except at the inlets.
Temperatures within the chimney is assumed to be the same at any given height.

Sub-scenario

Please select a sub-scenario
Zone Description
Buoyancy driven ventilation rates depend on the difference between the internal and external air temperature among other factors. It is recommended to assume a ΔT between 1°C and 3°C for day-time ventilation and between 2°C and 4°C for night-time ventilation.

Buoyancy driven ventilation rates depend on the difference between the indoor, outdoor and tower/chimney air temperature among other factors. It is recommended to assume a ΔT between indoor and outdoor of 1 - 3°C for day-time ventilation and 2 - 4°C for night-time ventilation. Besides, indoor air temperature may be assumed to be 2°C above tower/chimney air temperature.
Outdoor T °C

Chimney T °C ?
X
Chimney T
Under direct evaporative cooling the temperature in the tower/chimney may be assumed to be 80% of the dry bulb to wet bulb temperature difference (Givoni, 1994) and this may be assumed to be 0 - 15 °C.

Indoor T °C

ΔT °C

A₁(m²)

A₂ (m²)

A₃ (m²)
A_o (m²) ?
X
A_o (m²)
The neutral plane is set at a height between the top inlet and outlet. An estimate of the outlet area required to satisfy the selected flow pattern is calculated based on this assumption and the input data.

Design considerations:

The exhaust is strongly recommended to be multi-directional.
The area in plan of the chimney must be at least equal to the outlet area.
A_o _wind (m²)

A₁' (m²)

A₂' (m²)

A₃' (m²)

A_o' (m²) ?
X
A_o' (m²)
The neutral plane is set at a height between the top inlet and outlet. An estimate of the outlet area required to satisfy the selected flow pattern is calculated based on this assumption and the input data.

A_o' _wind (m²)

Stack Height (m) ?
X
Stack Height (m)
Stack height is the vertical distance between the centre of the inlet to the centre of the outlet.

z₁ (m)

z₂ (m)

z₃ (m)

z_o (m)

z₁' (m)

z₂' (m)

z₃' (m)

z_o' (m)

Floor Number

Enabled

Y

N
Effective apertures
Main window types and typical range of aperture sizes (after CIBSE, 2005. Table 3.2)

˂

Effective aperture:
0-90 %

Effective aperture:
0-90 %

Effective aperture:
0-50 %

Effective aperture:
0-90 %

Effective aperture:
0-50 %

Effective aperture:
0-50 %

Effective aperture:
0-30 %

˃
Inlet

1 (%)

2 (%)

3 (%)

O (%)

Outlet

1' (%)

2' (%)

3' (%)

O' (%)

The effective area of each aperture is considered for calculations of volume flow rates. It is clearly related to the way the window opens and the surrounding head, sill and jamb details. Reference should be made to actuator opening details from manufacturer’s data. ?
X
Effective aperture:

BUILDING GEOMETRY & SOLAR GAINS

Building geometry and solar gains

The calculation of solar gains (direct and conductive) is used to estimate the amount of flow rate required to remove the total heat generated within the space (required for cooling).
The solar gains are calculated for each independent cell by means of the latitude, month, hour and building orientation.

Cell dimensions

Section

Roof exposed

W (m)

H (m)

Floor plan

L (m)

Wall exposed

Area

Floor Area (m²)
Volume (m³)

Construction materials properties

Glazing

Solar Transmittance Factor (0-1)		? X Solar Transmittance Factor (0-1) The Solar Transmittance Factor is the fraction of solar radiation transmitted by the glazing, expressed as a number between 1 and 0, where 1 indicates the maximum possible solar heat gain, and zero, no solar heat gain.
Shading Proportion (%)		? X Shading Proportion (%) The Shading Proportion is the proportion of the glazing that is in the shade, expressed in percentage format, where 100% indicates full shading and 0% no shading. This is used to represent external shading.

Wall

Surface Absorptance (0-1)

The Surface absorptance is the proportion of solar radiation absorbed by a surface, expressed as a number between 1 and 0. Standardised values suggest 0.9 for dark coloured materials and 0.5 for light coloured materials.

U-Value (W/m².K)

Ext. Surf. Transmittance (W/m².K)

The external surface transmittance is the thermal transmittance of the external surface exposed to solar radiation expressed in W/m²K and it is dependent on the thickness of the construction layer and thermal conductivity of the material.

Typical values for some construction materials are:

Horizontal flow direction

100 mm brick 3.5 W/m².K

200 mm LW concrete 1.7 W/m².K

200 mm HW concrete 3.6 W/m².K

25 mm stone 5.6 W/m².K

13 mm wood 3.2 W/m².K

25 mm stucco 4.8 W/m².K

140 mm earth block 0.9 W/m².K

Downward flow direction

1 mm metal surface 4.8 W/m².K

10 mm bitumen surface 3.7 W/m².K

3 mm asphalt 4.6 W/m².K

10 mm ceramic tile 4.7 W/m².K

Roof

Surface Absorptance (0-1)

U-Value (W/m².K)

Ext. Surf. Transmittance (W/m².K)

The external surface transmittance is the thermal transmittance of the external surface exposed to solar radiation expressed in W/m²K and it is dependent on the thickness of the constructional layer and thermal conductivity of the material.

Typical values for some construction materials are:

Horizontal flow direction

100 mm brick 3.5 W/m².K

200 mm LW concrete 1.7 W/m².K

200 mm HW concrete 3.6 W/m².K

25 mm stone 5.6 W/m².K

13 mm wood 3.2 W/m².K

25 mm stucco 4.8 W/m².K

140 mm earth block 0.9 W/m².K

Downward flow direction

1 mm metal surface 4.8 W/m².K

10 mm bitumen surface 3.7 W/m².K

3 mm asphalt 4.6 W/m².K

10 mm ceramic tile 4.7 W/m².K

Building Geometry and the graphs displayed do not necessarily represent the whole building, and they could refer to a room or an interior bay of a large open space. ?

Interior bay (floor plan)

INTERNAL CONDITIONS

Internal Conditions

The calculation of internal gains is used to estimate the amount of flow rate required to remove the total heat generated within the space (required for cooling).
The internal gains refer only to a single cell and are assumed to be the same for each cell.

Occupants

The number of people is used to calculate the minimum ventilation required for the supply of fresh air (10 l/s).

at rest (76W)	people
office work (85W)	people
walking (100W)	people
exercising (120W)	people

Internal Gains

equipment W/m²
lighting W/m²

Benchmark allowances for internal heat gains in typical buildings

Building type	Use	Density of occupation (m²/person)	Sensible heat gain (W/m²)
Building type	Use	Density of occupation (m²/person)	Lighting	Equipment
Offices	General	12	8-12	15
Offices	Meeting/conference	3	10-20	5
Airports/stations	Gate lounge	0.83	15	5
Airports/stations	Circulation spaces	10	12	5
Retail	Shopping malls	2-5	6	0
Retail	Retail stores	5	25	5
Education	Lecture theatres	1.2	12	2
	Teaching spaces	1.5	12	10
	Seminar room	3	12	5
Leisure	Hotel reception	4	10-20	5
	Restaurant/dining	3	10-20	5
	Bars/lounges	3	10-20	5

CIBSE (2015). Table 6.2, Chapter 6: Internal gains.

RESULTS

Results

Cell - Heat Gains
Number of people
occupant gains (W/m²)
Equipment gains (W/m²)
Lighting gains (W/m²)
Total internal gains (W/m²)
Total Solar Gains (W/m²)
Cell 1
Cell 2
Cell 3
Total heat generated (kW)
Cell 1
Cell 2
Cell 3

Apertures Data
	Effective	Height	Airflow
	Area	Zn	Rate
	(m²)	(m)	(m³/s)
			B	B+W
Inlet 1
Inlet 2
Inlet 3
Inlet' 1
Inlet' 2
Inlet' 3
Inlet O
Outlet 1
Outlet 2
Outlet 3
Outlet O	-

Download results in PDF:

Buoyancy + Wind driven

Adaptive comfort band

To obtain reliable results, the outdoor temperature in Air Flow Data Input must be consistent with the prevailing mean outdoor temperature in Project Location.

The calculation of indoor air temperatures over short time intervals requires an intensive calculation process.

This tool uses simple calculation methods by means of external temperature, total heat gains and total heat loses achieved with natural ventilation to give a useful indication of the predicted indoor air temperature. No heat storage is considered.

Equation 1.2 form CIBSE, 2015 is used to calculate operative temperatures. For simplicity reasons, mean radiant temperature is assumed equal to the predicted indoor air temperature.

The adaptive comfort band is only applicable in spaces that meet the following criteria:

No mechanical heating or cooling system is in operation.
Metabolic rates ranging from 1.0 to 1.3 met.
Occupants are free to adapt their clothing levels within a range of 0.5 to 1.0 clo.

In downdraught evaporative cooling the effect of wind should be inhibited to prevent negative effects (overpressure or suction).

Buoyancy driven

Adaptive comfort band

No mechanical heating or cooling system is in operation.
Metabolic rates ranging from 1.0 to 1.3 met.
Occupants are free to adapt their clothing levels within a range of 0.5 to 1.0 clo.

OptiVent 2.0 is a simple natural ventilation steady-state tool which was initially developed as an in-house tool by a practice of architects and environmental design consultants in the UK.
The tool has been developed to expand the range of generic airflow strategies that can be explicitly evaluated and support strategic decisions regarding the feasibility of natural ventilation during the early design stage.

References:

ANSI/ASHRAE (2013). Standard 55-2013, Thermal Environmental Conditions for Human Occupancy. American Society of Heating Refrigerating and Air Conditioning Engineers.
ASHRAE (2005). Handbook: Fundamentals - SI edition. American Society of Heating Refrigerating and Air Conditioning Engineers.
CIBSE (2015). Environmental Design. Guide A, 8th Edition. Chartered Institution of Building Services Engineers, London.
CIBSE (2005). Natural Ventilation in Non-Domestic Buildings. Applications Manual AM10. Chartered Institution of Building Services Engineers, London.
Etheridge, D. (2011). Natural Ventilation of Buildings: Theory, Measurement and Design. Wiley.
Ford, B., R. Schiano-Phan, E. Francis (2010). The Architecture & Engineering of Downdraught Cooling. PHDC Press.
Givoni, B. (1994). Passive and Low Energy Cooling of Buildings. Van Nostrand Reinhold.

This tool has been developed by Juan VallejoJuan Vallejo
Dipl. Arch, MSc, PhD candidate.
Environmental Design Analyst. Natural Cooling Ltd, UK. and Pablo AparicioPablo Aparicio
Dipl. Eng, MSc, Industrial Organization and Business Management, Ph.D.
University of Seville, Spain. , coordinated by Brian FordBrian Ford
Emeritus Professor University of Nottingham, UK. . Acknowledgements to the University of Nottingham (United Kingdom) and Universidad de Sevilla (Spain).

The authors wish to thank Camilo DiazCamilo Diaz
PhD, Environmental Design.
Associate Director at WSP/Parsons Brinckerhoff, UK. for the development of the original version of Optivent in 2003, to David Etheridge for his helpful comments during the peer review process, and the postgraduate students from University of Westminster, University of Nottingham and University College of London (UCL) for providing useful feedback when approaching to the last stage of development.

Project location

Airflow data input

Chimney T

Ao (m²)

Ao' (m²)

Stack Height (m)

Effective aperture:

Building geometry and solar gains

Solar Transmittance Factor (0-1)

Shading Proportion (%)

Surface Absorptance (0-1)

Ext. Surf. Transmittance (W/m².K)

Surface Absorptance (0-1)

Ext. Surf. Transmittance (W/m².K)

Interior bay (floor plan)

Internal Conditions

Results

Adaptive comfort band

Adaptive comfort band

A_o (m²)

A_o' (m²)