Figure 4.35 Airflow analysis

Figure 4.14Airflow analysis29

It should be considered that in case of undesirable and strong winds, wind protection can be provided by wind-breaks and orientation of buildings, which is covered in other section. Research shows that in an unprotected building exposed to winds speed of 9m/s have up to 2.4 larger heating demand compared to buildings exposed to wind speed of 2-2.5 m/s.

Natural ventilation integrated with thermal mass is an effective passive cooling system that can be used to adjust the indoor environment to ensure indoor thermal comfort and maintain acceptable indoor air quality (IAQ). Natural ventilation can be applied for comfort ventilation during day (comfort ventilation) and for cooling the building at night (night ventilation).

Nighttime ventilation reduces the temperature of the internal mass of the building at night so that the mass will absorb heat during the day. The mass temperature is reduced by “flushing” the building with cool (low humidity) night air. The air reduces the temperature of the internal mass sufficiently to keep the building cool during much of the day.

Natural ventilation can be utilized by:

 the orientation of the building (not necessarily perpendicular to the wind direction)  use of surrounds to create low and high pressure zones  locating inlets in high, outlets in low pressure areas  small inlet and large outlet sizes  inlets which direct the flow  undisturbed inside flow, open plan

The forces producing natural ventilation in buildings are:

 air movement produced by pressure differences - wind and  air change caused by difference in temperature- stack effect (buoyancy).

Therefore, the impact and effectiveness of Natural Ventilation in the building depends on:

 Prevailing wind velocity and direction  Building conditions and orientation  Window sizing, location, and functioning  Surrounding environment  Outdoor temperature and moisture

Infiltration

The infiltration in a building is measured with the n50 test, also named as, the blower-door test, which combines both under and over pressurization tests. The resultant air leakage at 50 Pascal's pressure must be no greater than 0.6 air changes per hour (0.6 ac/h @50 Pa).

There is a difference between ACH50 and ACHnat and it is recommendable to reach an ACH50 <3. An ACH50<1.5 means that the building requires mechanical ventilation. Since the building in its regular occupancy is not decompressed at 50 Pa, in order to convert the ACH50 value obtained from the blower door test into the air exchange under natural settings, a rule of thumb is to divide the ACH50 by 20. Energy efficient houses have an average ACHnat of 0.5 h-1 (with a range of 0.02 h-1 to 1.63 h-1), compared to 0.9 h-1 for ‘normal new construction houses’. The energy loads caused infiltration and ventilation are related with the the amount of air changes per hour:

Qinfiltration= (ACH * volume * c x*ρ) * (Tinside-Toutside)

Where:

ρ = Density of Air 1.2 kg/m3

c = Specific Heat Capacity of Air (20°C) ~ 1000 J/kg K

The heat losses due to infiltration are similar to the heat losses by conduction through the wall, since the temperature difference between the inside and the outside is causing them.

Wind and buoyancyventilation

Natural ventilation can be caused by wind or buoyancy.

 The wind is air movement induced by pressure difference and depends on the wind speed, the window size and placement.

 The buoyancy (stack effect) is caused by a temperature difference in a space and depends on the height difference between two openings (windows) in a same room.

The air flow or air movement is usually expressed through a volume flow rate, q. Typical units used are cubic meter/hour (m3/h) and liters per second (L/s). Sometimes, the ventilation rate is also expressed on a per person or per unit floor area basis.

q = area * velocity

Air flow is caused by a pressure difference. Air will flow from a zone of high pressure to a zone of low pressure. The bigger your window, the better for natural ventilation. But that has to be the upper part of your window.