1. ATA Melbourne Branch presentation April 2008 Jim Lambert
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 6am 12noon 6pm midnight
Outside temperature
6am
12noon
6pm
midnight
6am
day
night
time

8. Summer Example 30 20 Temperature (deg C) 10 6am 12noon 6pm midnight
Inside temperature 10
Outside temperature
6am
12noon
6pm
midnight
6am
day
night
time

9. Comfort range with moving air
Summer Example
Comfort range with moving air 30 20
Temperature (deg C)
Inside temperature
Normal comfort range 10
Outside temperature
6am
12noon
6pm
midnight
6am
day
night
time

10. Comfort range with moving air
Summer Example
Comfort range with moving air
Open all windows 30 20
Temperature (deg C)
Inside temperature
Normal comfort range 10
Outside temperature
6am
12noon
6pm
midnight
6am
day
night
time

11. Comfort range with moving air
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
6am
12noon
6pm
midnight
6am
day
night
time

12. Comfort range with moving air
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
6am
12noon
6pm
midnight
6am
day
night
time

13. Comfort range with moving air
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
6am
12noon
6pm
midnight
6am
day
night
time

14. Summer Example 30 20 Temperature (deg C) 10 6am 12noon 6pm midnight
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
6am
12noon
6pm
midnight
6am
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 AS The use of ventilation and air-conditioning in buildings Part 2 – mechanical ventilation for acceptable indoor air quality (superseded)
e.g. Australian Standard AS 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
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: 250m (floor area 100m2, ceiling height 2.5m)
Temperature difference 10oC inside versus outside
Rate of ventilation case 1 (low flow) case 2 (high flow) ACH ACH (ACH [Air Exchanges per Hour])
Heat capacity of air 3.4*10-4 kWHr/m3 oC

23. Heat/Cool power required (10oC difference): Case 1: (0.1 ACH)
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)
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)