Natural Ventilation

1
Natural Ventilation capabilities and
limitations(comfort and energy efficiency
in domestic dwellings)

• ATA Melbourne Branch presentation
• April 2008
• Jim Lambert

2
Ventilation - a few scenarios

• Summer
• 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 does ventilation affect the energy needed for
  heating and cooling?
• How do we achieve comfort and energy efficiency
  together?

3
Cooling sensation from airflow

• 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)

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

4
Cooling sensation from airflow

• 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

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

5
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 roof
  ventilation can take advantage of stack effects
  to increase airflow

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

6
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

7
Summer Example
30
20
Temperature (deg C)
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
8
Summer Example
30
20
Temperature (deg C)
Inside temperature
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
9
Summer Example
Comfort range with moving air
30
20
Temperature (deg C)
Inside temperature
Normal comfort range
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
10
Summer Example
Comfort range with moving air
Open all windows
30
20
Temperature (deg C)
Inside temperature
Normal comfort range
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
11
Summer Example
Close all windows
Comfort range with moving air
Open all windows
30
20
Temperature (deg C)
Inside temperature
Normal comfort range
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
12
Summer Example
Close all windows
Start internal fan
Comfort range with moving air
Open all windows
30
20
Temperature (deg C)
Inside temperature
Normal comfort range
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
13
Summer Example
Close all windows
Open all windows
Start internal fan
Comfort range with moving air
Open all windows
30
20
Temperature (deg C)
Inside temperature
Normal comfort range
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
14
Summer Example
Close all windows
Open all windows
Start internal fan
Comfort range with moving air
Open all windows
Gentle forced ventilation overnight
30
20
Temperature (deg C)
Inside temperature
Normal comfort range
10
Outside temperature
6pm
6am
6am
midnight
12noon
day
night
time
15
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

16
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

17
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)

18
How much ventilation does a healthy house need?
(continued)

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

19
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

20
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

21
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?

22
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 3.410-4 kWHr/m3 oC

23
Effect of ventilation on energy for heating or
cooling Sample calculation

• 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)

24
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

25
How do we achieve comfort with energy efficiency?

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

26
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)
27
Winter heating example(Solar Venti)
28
Winter heating example
29
Summer cooling example
30
Review

• Summer
• 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)