Potential of using passive/active stacks to enhance the

natural ventilation in HDB flats

This study investigates the feasibility of applying passive and active stack systems to enhance

the natural ventilation in public housing. The primary objective is to assess the status of natural

ventilation in a typical 4-room HDB flat using scaled model in the wind tunnel, and to develop an

effective passive or active stack system to enhance natural ventilation in the flat. CFD simulations

were also conducted to have a better understanding of the airflow inside the HDB unit using the

active stack systems and parametric studies were done to identify the various critical design

factors such as stack location, stack size etc.

It was observed that the active stack provides substantial increase in the internal air velocities. It

was noted that the velocities increase with the increase in fan velocities and also with the

increase in the stack size. It was also noted that, even though the enhancement in the velocity is

much higher when the doors are closed, they are more or less localized. But when the doors are

kept open, the velocity increase is more evenly distributed. Since our main concern is the

nighttime when bedroom doors are closed, more attention is given towards such situation. The

percentage increase in the velocity is up to 550% and the maximum velocity achieved was 0.67

m/s. A velocity of 0.26 m/s was achieved even with the smallest stack. For larger stacks high

velocities were achieved even when the wind tunnel was turned off. This indicates that on a calm

evening thermal comfort could be achieved. It was also observed that for larger stacks, the

increase in the fan speed has no significant effect on the air velocities. The maximum size of the

stack used is 0.4 m X 0.4 m, which is not much compared to a typical column of width 0.30 m of

an HDB flat and therefore the economic feasibility is well validated.

10.5

2

3.22

3.22

2.951.81 1.76

2.28 Bath

2.08 1.60

Service

2.28Bath

2.95

Master

3.12

1.22

Bed Room

Store

3.58

Bed Room

4.34

Living Room

Kitchen

3.68

10.55

10.55

3.00

3.00

Section A

Section B

Floor Plan

Elevation

10.20

A

BBed Room

Bed Room Living Room

Bed Room Bed Room KitchenMaster

Figure 1: Plan and sectional details of the residential unit under investigation

Figure 2: Physical model for the wind tunnel study (scale 1:5)

Figure 3: External wind velocities measured in the wind tunnel experiments

Air Velocity Increment Analysis

0%

20%

40%

60%

80%

100%

Open Close Open Close Open Close Open Close

15x15-Loc.1 40x40-Loc.1 15x15-Loc.2 40x40-Loc.2

Perc

enta

ge In

crem

ent (

%)

WT WT+IF IF

Figure 4: Comparison of the air velocity increment due to wind effect (WT), combination of wind

effect and stack fan (WT+IF) and stack fan (IF) alone

C

1Bath

B

0

A

Bed Room Master

Bed Room2

D

EBath

X

Kitchen

Store

Bed Room Living Room

Y F G H

Figure 5: Velocity vectors of Case 30: stack size 40x40, location 2-doors closed

F

Figure 6: Section of velocity vectors for Case 30: stack size 40x40, location 2-doors closed