Natural Ventilation Overview

8/22/2019 Natural Ventilation Overview

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Natural Ventilation

capabilities and limitations

(comfort and energy efficiencyin domestic dwellings)


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Ventilation - a few scenariosSummer:

Cooling sensation from airflow

Structural cooling on summer nights

General (Winter or summer): How much ventilation does a healthy houseneed?

How does ventilation affect heating andcooling?

How does ventilation affect the energyneeded for heating and cooling?

How do we achieve comfort and energyefficiency together?


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Source: Natural Ventilation in Buildings, Tony Rofail, NEERG seminar,

31 Aug 2006, Windtech Consultants

In a mild summer, natural ventilation canreduce the apparent temperature(e.g.up to 80C at an airflow of 2 m/s or so)

0

2

4

6

8

10

0 0.5 1 1.5 2 2.5 3 3.5

Airflow (m/s)

Co

oling

sensation

deg.

C


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Question: If you have natural ventilation and nomechanical cooling, what is the hottest summer

temperature that allows indoor comfort?

People feel comfortable in still air at about 200C to

230C

Therefore it should be possible to have indoor

comfort up to an air temperature of maybe 300C if

you can get an indoor airflow of 2 m/s The reference below has some interesting

guidelines for achieving good natural ventilation


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Some guidelines for good airflow

cooling



Maximize air velocities in occupied rooms Two openings on opposite sides increase airflow.

Locate windows on opposite sides of the house.

An inlet window smaller than the outlet creates

higher inlet velocity (e.g. 50% smaller)

Horizontal window openings are more effective

than square or vertical openings

Vertical air shafts or open staircases or roofventilation can take advantage of stack effects

to increase airflow


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Structural cooling on summer

nights

The same kind of ventilation that gives a cooling

effect in daytime also helps to cool the structure

on summer nights

Issues: Thermal storage (e.g. concrete floors, masonry

walls) can use this structural cooling to keep

indoor temperature cooler during the next day .

Windows need to offer security while allowing

ventilation airflow


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Summer Example

time

Tem

perature(degC) 30

20

10

6pm6am 6ammidnight12noon

day night

Outsidetemperature


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Summer Example

time

Tem

perature(degC) 30

20

10


day night

Outsidetemperature

Inside

temperature


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Summer Example

time

Tem

perature(degC) 30

20

10


day night

Outsidetemperature

Inside

temperatureNormal

comfortrange

Comfort

range with

moving air


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Summer Example

time

Tem

perature(degC) 30

20

10


day night

Outsidetemperature

Inside

temperatureNormal

comfortrange

Comfort

range with

moving air

Open all

windows


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Summer Example

time

Tem

perature

(degC) 30

20

10


day night

Outsidetemperature

Inside

temperatureNormal

comfortrange

Comfort

range with

moving air

Open all

windows

Close all

windows


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Summer Example

time

Tem

perature

(degC) 30

20

10


day night

Outsidetemperature

Inside

temperatureNormal

comfortrange

Comfort

range with

moving air

Open all

windows

Close all

windowsStart

internal

fan


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Summer Example

time

Tem

perature

(degC) 30

20

10


day night

Outsidetemperature

Inside

temperatureNormal

comfortrange

Comfort

range with

moving air

Open all

windows

Close all

windows

Open all

windowsStart

internal

fan


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Summer Example

time

Tem

perature

(degC) 30

20

10


day night

Outsidetemperature

Inside

temperatureNormal

comfortrange

Comfort

range with

moving air

Open all

windows

Close all

windows

Open all

windows

Gentle forced

ventilation

overnight

Start

internal

fan


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Comments on Summer Example

Good features: Natural approach with minimum energy

consumption

Comfort level is fairly reasonableLimitations:

Poor safety margin for warmer days

Must pay attention to outside temperature

Needs lots of hands-on actions

Limited parts of the house are comfortable


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How much ventilation does a

healthy house need?We need ventilation in these areas:

Humid or smelly places (bathrooms,

kitchens) Where there are people living and

breathing (family room, bedrooms, etc.)

How much ventilation do we need? This question does not seem to have a

simple answer


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healthy house need? (continued)

There are Australian Standards aboutminimum ventilation

e.g. Australian Standard AS1668.2 1991 The

use of ventilation and air-conditioning inbuildings Part 2 mechanical ventilation foracceptable indoor air quality (superseded)

e.g. Australian Standard AS1668.2 2002 The

use of ventilation and air-conditioning inbuildings Part 2 mechanical ventilation foracceptable indoor air quality (plus 2amendments and 1 supplement)


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BUT people say that (for comfort) you

really need MORE ventilation than the

statutory minimum


healthy house need? (continued)


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How does (natural) ventilation

affect home heating and cooling?

Summer (hotter outside than inside)

Good effects:

Movement of air around people (helps withcooling effect already covered)

Venting of roof space (removes heating effectof hot air above ceiling)

Bad effects: When external air is too hot for airflow cooling,

fresh air flow for health requires energy forcooling


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How does (natural) ventilation

affect home heating and cooling?(continued)

Winter (colder outside than inside)

Good effects: None (although you do need ventilation for health)

Bad effects:

When external air is cold, fresh air flow forhealth requires energy for heating

If roof space is ventilated, then potentially

useful heat may escape to the atmosphere


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How does ventilation affect the energy

needed for heating and cooling? Ventilation replaces inside air with outside air

In winter, you need to heat the new air

In summer you need to cool the new air(if outside air temperature is more than say 30oC)

How much energy does this take?


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Effect of ventilation on energy for heating

or cooling

Sample calculation Assumptions: Size of ventilated space: 250m3

(floor area 100m2, ceiling height 2.5m)

Temperature difference 10oC insideversus outside

Rate of ventilationcase 1 (low flow) case 2 (high flow)

0.1 ACH 3.0 ACH

(ACH [Air Exchanges per Hour])

Heat capacity of air 3.4*10

-4

kWHr/m

3 o

C


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Heat/Cool power required (10oC difference):

Case 1: (0.1 ACH)Power required to maintain indoor temperature

85W (like 1 conventional light globe)

Case 2: (3.0 ACH)Power required to maintain indoor temperature

2.55 kW (like 1 hefty radiator)

Effect of ventilation on energy for heating

or cooling

Sample calculation


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How do we achieve comfort with

energy efficiency?The problem:

Comfort/health needs ventilation

Ventilation introduces outside air

Outside air is often at the wrong

temperature

Heating or cooling the air needs energy


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How do we achieve comfort with

energy efficiency?A solution

HRV (Heat Recovery Ventilation)

Oth Id


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Other Ideas(from Danish experiment - see ref below)

In cold weather, draw incoming air from under PVpanel to get solar pre-heating of the air

In hot weather, vent the roof space to atmosphere,

draw external air over the ceiling to minimise heat

loadGuidelines used in experiment:

Efficiency of heat recovery at least 80-90%

Power consumption of ventilation 30-40W for a household

Building completely airtight (natural infiltration 0.1 air exchanges/hour)(assumes optimised insulation, low energy windows)

Noise level less than 25dB

(ref Cost effective PV assisted energy efficient ventilation systems for

housing Pederson, Cenergia Energy Consultants, Denmark)


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Winter heating example(Solar Venti)

Wi h i l


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Winter heating example

S li l


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Summer cooling example


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The following air flow diagrams demonstrate

several natural ventilation design devices:


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Natural ventilation may also be supplemented by use

of electric fans and mixed-mode cooling, which use

electricity, but far less than as for total mechanical

cooling.

There are several mixed-mode types that are

differentiated by their operating strategies:

Concurrent uses mechanical cooling and naturalventilation in the same spaces at the same time

Changeover switches between mechanical

cooling and natural ventilation on a daily or season

basis

Zoned uses mechanical cooling and natural

ventilation in different zones of the building

Any combination of the above three may be utilized.


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Established Techniques

Set building orientation to receive prevailing

breezes. Cooling of breezes by vegetative shading

and water cooling of air flow to building

spaces. Balance use of passive solar, daylighting,

sun shading, and landscaping strategies to

optimize natural ventilation.

Use of an integrated design process to

enhance performance.


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Emerging Trends

Greater emphasis on providing natural

ventilation in public and commercialbuilding within temperate climate zones.

Modern adaptation of traditional

architectural devices such as windchimneys, atria, courtyards, windows, and

operable blinds to induce natural air flow.

Electronic modeling of natural ventilationand building form.

Documents

Natural Ventilation Overview