Natural Ventilation – capabilities and limitations

(comfort and energy efficiency in domestic dwellings)

ATA Melbourne Branch presentation

April 2008

Jim Lambert

Ventilation - a few scenariosSummer:• Cooling sensation from airflow• Structural cooling on summer nightsGeneral (Winter or summer):• How much ventilation does a “healthy” house

need?• How does ventilation affect heating and

cooling?• How does ventilation affect the energy

needed for heating and cooling?• How do we achieve comfort and energy

efficiency together?

Cooling sensation from airflow

Source: Natural Ventilation in Buildings, Tony Rofail, NEERG seminar, 31 Aug 2006, Windtech Consultants

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

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Airflow (m/s)

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Cooling sensation from airflow

Source: Natural Ventilation in Buildings, Tony Rofail, NEERG seminar, 31 Aug 2006, Windtech Consultants

Question: If you have natural ventilation and no mechanical 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

Some guidelines for good airflow cooling

Source: Natural Ventilation in Buildings, Tony Rofail, NEERG seminar, 31 Aug 2006, Windtech Consultants

• 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 roof

ventilation can take advantage of “stack effects” to increase airflow

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

Summer Example

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6pm6am 6ammidnight12noon

day night

Outside temperature

Summer Example

time

Tem

pera

ture

(de

g C

) 30

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10

6pm6am 6ammidnight12noon

day night

Outside temperature

Inside temperature

Summer Example

time

Tem

pera

ture

(de

g C

) 30

20

10

6pm6am 6ammidnight12noon

day night

Outside temperature

Inside temperature

Normal comfort range

Comfort range with moving air

Summer Example

time

Tem

pera

ture

(de

g C

) 30

20

10

6pm6am 6ammidnight12noon

day night

Outside temperature

Inside temperature

Normal comfort range

Comfort range with moving air

Open all windows

Summer Example

time

Tem

pera

ture

(de

g C

) 30

20

10

6pm6am 6ammidnight12noon

day night

Outside temperature

Inside temperature

Normal comfort range

Comfort range with moving air

Open all windows

Close all windows

Summer Example

time

Tem

pera

ture

(de

g C

) 30

20

10

6pm6am 6ammidnight12noon

day night

Outside temperature

Inside temperature

Normal comfort range

Comfort range with moving air

Open all windows

Close all windows

Start internal

fan

Summer Example

time

Tem

pera

ture

(de

g C

) 30

20

10

6pm6am 6ammidnight12noon

day night

Outside temperature

Inside temperature

Normal comfort range

Comfort range with moving air

Open all windows

Close all windows

Open all windows

Start internal

fan

Summer Example

time

Tem

pera

ture

(de

g C

) 30

20

10

6pm6am 6ammidnight12noon

day night

Outside temperature

Inside temperature

Normal comfort range

Comfort range with moving air

Open all windows

Close all windows

Open all windows

Gentle forced ventilation overnight

Start internal

fan

Comments on Summer Example

Good features:

• “Natural approach” with minimum energy consumption

• Comfort level is fairly reasonable

Limitations:

• 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

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

How much ventilation does a “healthy house” need? (continued)

There are Australian Standards about minimum ventilation

• e.g. Australian Standard AS1668.2 1991 The use of ventilation and air-conditioning in buildings Part 2 – mechanical ventilation for acceptable indoor air quality (superseded)

• e.g. Australian Standard AS1668.2 2002 The use of ventilation and air-conditioning in buildings Part 2 – mechanical ventilation for acceptable indoor air quality (plus 2 amendments and 1 supplement)

• BUT people say that (for comfort) you really need MORE ventilation than the statutory minimum

How much ventilation does a “healthy house” need? (continued)

How does (natural) ventilation affect home heating and cooling?

Summer (hotter outside than inside)• Good effects:

– Movement of air around people (helps with cooling effect – already covered)

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

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

fresh air flow for health requires energy for cooling

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 for

health requires energy for heating– If roof space is ventilated, then potentially

useful heat may escape to the atmosphere

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?

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 inside versus

outside• Rate of ventilation

case 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/m3 oC

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

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

How do we achieve comfort with energy efficiency?

A solution –

HRV (Heat Recovery Ventilation)(will be covered in Bernard Desormeaux talk)

Other Ideas(from Danish experiment - see ref below)

• In cold weather, draw incoming air from under PV panel 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 load

Guidelines 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)

Winter heating example(Solar Venti)

Winter heating example

Summer cooling example

ReviewSummer:• Cooling sensation from

airflow• Structural cooling on

summer nights

General (Winter or summer):• How much ventilation does

a “healthy” house need?• How does ventilation affect

heating and cooling?• How do we achieve

comfort and energy efficiency together?

• These can limit the need for air conditioning

• They could benefit from mechanical ventilation

Quite a lot (Bernard will give a better answer)

Natural ventilation “wastes” energy

(Bernard will cover this)