Rules of thumb
1. Space & Volume Criteria
1a. Space allowances for office & commercial building
Item
i.
ii.
Usage
General
- Net average space
- Gross average space
- Large scale business, including
conference, canteen, storage and
work space
Specific
- Executive office
- Open plan office
- Full time professional
- Secretarial staff
- Clerical staff
- Restaurant
- Department Stores
Area (m2 per person)
10
12-14
17-18
13-20
9
6-7
6-7
4-5
1.5
1-1.5
1b. Space allowance for building services plant room
The space allowance for building service plant room is estimated below:i. Space allowance in terms of percentage of the occupied floor area:Usage
- Simple factory
- Typical building
- High technology building
- Central computer facilities
(Occupied area < 9300 m2)
Percentage of
occupied floor area
4%
6-10%
15%
3-4%
Page 1 of 12
ii. Space allowance of HVAC plant room:Usage
- Air handling plant room
• up to 10 m3/s
• 10 – 40 m3/s
• Greater than 40 m3/s
- Heating plant room
• for 100kW boiler
• for 200kW boiler
• for 1000kW boiler
• for 4000kW boiler
- Central refrigeration plant
(excluding cooling tower)
- Cooling towers
- Air-cooled condenser
Area in m2
10 m2
8 m2 per m3/s
4 m2 per m3/s
15 m2 x 3m (H)
25-30 m2 x 3m (H)
80 m2 x 4m (H)
130-150 m2 x 4.5m (H)
1–2%
of building floor area
25 + (cooling tower in kW) /66
25 + (condenser in kW) / 200
1c. Space allowance for service duct
In general, the total service duct area is about 2% of gross floor area. One service
duct serves about 15-23 m radius. For air ducting system, the service duct is
0.2 m2 per m3/s of air.
1d. Space allowance for raised floor
The raised floor design facilitates the specialist cabling system to be installed
under the raised floor for intelligent building. The optimum raised floor is
proposed below:Usage
- Traditional office
- Cellular / private office
- Open plan office
- Trading rooms or dealer desk in
finance company
- EDP rooms
- Electronic equipment
Optimum raised floor depth
in mm
150
150
200
300
600
600
In general, the raised floor to ceiling space in working area should be 2400 mm
when the raised floor is installed.
Page 2 of 12
1e. Space allowance for restaurant
The minimum area for food room is tabulated below as referred to “A guide to
application for restaurant licences”, Food and Environment Hygiene Department,
HKSARG.
i. General Restaurant
Gross floor area of the
premises in m2
Minimum kitchen
area in m2
100 or less
6
101 – 150
10
151 – 250
10
251 or above
14
Minimum aggregate area of
kitchen, food preparation room
and scullery accommodation in
m2
30% of gross floor area, but
not less than 9m2
25% of gross floor area, but
not less than 27m2
23% of gross floor area, but
not less than 36m2
21% of gross floor area, but
not less than 54m2
ii. Light Refreshment Restaurants
Gross floor area of the
premises in m2
22 or less
23 – 35
36 – 55
56 – 95
96 – 185
186 or above
Minimum aggregate area of kitchen, food preparation
room and scullery accommodation in m2
Not less than 5m2
20% of gross floor area or 7m2, whichever is the less
18% of gross floor area or 8m2, whichever is the less
14% of gross floor area or 12m2, whichever is the less
13% of gross floor area or 17m2, whichever is the less
9% of gross floor area or 28m2, whichever is the more
The above space allowance excludes the space for food refrigeration plant and
food storage area.
Page 3 of 12
2. Thermal Transmittance
It is difficult to produce common U-values for wall and for roof as the U-value is
dependent on the combination of materials of a building. In general, the following
U-values can be used as reference for heat gain calculation applicable in Hong
Kong:•
U-value for composite wall panel = 2.32 W/ m2°C (non-structural wall)
(For 5mm white mosaic tiles + 10mm cement/sand render + 100mm concrete
panel + 10mm gypsum plaster)
•
U-value for composite structural wall panel =1.91 W/ m2°C
(For 5mm white mosaic tiles + 10mm cement/sand render + 300mm concrete
panel + 10mm gypsum plaster)
•
U-value for composite curtain wall panel = 0.41 W/ m2°C
(For 8mm black glass + 75mm mineral wool felt insulation + 50mm air space
resistance + 2mm pressed steel panel)
•
U-value for roof panel = 0.37 W/ m2°C
(For 25mm concrete tiles + 20mm asphalt + 50mm cement/sand screed +
50mm polystyrene insulation + 600 mm re-enforcement concrete + 10mm
gypsum plaster)
Regarding the glass panel, the following shading coefficient can be used as
reference for heat gain calculation:•
•
The shading coefficient of tinted glass = 0.7
The shading coefficient of single clear glass = 0.95
2a. Overall Thermal Transfer Value (OTTV) requirements of Hong Kong
The external walls and roofs of a building should be designed and constructed to
the following requirements:•
•
For a building tower, the OTTV should not exceed 30W/m2 (revised in 2000,
previously 35W/m2)
For a podium, the OTTV should not exceed 70W/m2 (revised in 2000,
previously 80W/m2)
The following can be exempted from the OTTV requirement:•
•
•
•
an external wall of a refuge floor
an external wall or roof of a carparking floor
an external wall of a lightwell having an area on plan not exceeding 21 m2
any wall on any roof
Page 4 of 12
3. Environmental Aspects
3a. Indoor condition
Based on the Code of Practice for Energy Efficiency of Air Conditioning
Installations,
Summer
Office and Classroom
Minimum dry bulb temperature
Minimum relative humidity
23°C
50%
Other Applications except Office and Classroom
Minimum dry bulb temperature
22°C
Minimum relative humidity
50%
Winter
Hotel
Maximum dry bulb temperature
Maximum relative humidity
24°C
50%
Other Applications except Hotel
Maximum dry bulb temperature
Maximum relative humidity
22°C
50%
3b. Outdoor condition
Based on the Code of Practice for Energy Efficiency of Air Conditioning
Installations,
Summer
Maximum dry bulb temperature
Maximum relative humidity
33.5°C
68%
Winter
Minimum dry bulb temperature
Minimum relative humidity
7°C
40%
Page 5 of 12
3c. Occupancy gain
Activity level at 22°C
Seated at rest
Office work
Standing
Eating in restaurant
Factory
Dancing
Sensible gain
80W
90W
95W
100W
115W
125W
Latent gain
35W
50W
55W
60W
120W
140W
Total gain
115W
140W
150W
160W
235W
265W
3d. Lighting gain
For fluorescent lighting fitting = Total watts x 1.25
For Incandescent lighting fitting = Total watts
In general, lighting load is assumed to 33W/m2 for producing a lighting standard
of around 540 Lumen/ m2. The minimum assumption of lighting load is 20W/ m2
for low lighting standard.
3e. Equipment gain
The office machinery = 20 – 50 W/ m2
4. Ventilation requirements
4a. Outside air supply
Building Application
Office
Hotel
Restaurant
Smoking lounge
Toilet
Classroom
Theatre
Shopping mall
Corridor & utilities
Outside air supply rate (l/s per person)
8 – 10
8 – 10
10
30
25
8 – 10
8 – 10
1 l/s per m2
0.25 l/s per m2
4b. Air infiltration
Building Application
Office
Hotel room
Restaurant
Shopping mall
Classroom
library
Air infiltration rates
1 air change per hour
1 air change per hour
1 air change per hour
0.5 - 1 air change per hour
2 air changes per hour
0.5 – 0.75 air change per hour
Page 6 of 12
4c. Air ventilation
The air change rate of different building applications are as follows:Building Application
for air-conditioned space
General Office
Conference Room
Factory
Restaurant
General sport hall
Squash courts
Theatre
Shopping mall
Classroom
library
Building Application
for mechanical ventilation
Office toilet
Pantry or print room
Pump room
Refuse collection chamber
Kitchen (Chinese)
Generator Room
E&M Plant Room
Air change rates
4 – 6 air changes per hour
6 – 10 air changes per hour
6 air changes per hour
10 – 15 air changes per hour
1 air change per hour
4 air changes per hour
6 - 10 air changes per hour
4 - 6 air changes per hour
2 air changes per hour
3 – 4 air changes per hour
Air change rates
15 air changes per hour
15 air changes per hour
10 – 15 air changes per hour
15 air changes per hour (Exhaust)
10 air changes per hour (Supply)
60 air changes per hour (Exhaust)
50 air changes per hour (Supply)
10 air changes per hour
10 air changes per hour
5. Comfort Air Movement
Maximum recommended mean air velocities in the occupied space:In winter
In summer
at 21°C
at 22°C
at 23°C
at 24°C
on back of neck
0.15 m/s
0.25 m/s
0.15 m/s
0.2 m/s
0.25 m/s
0.3 m/s
0.2 m/s
If the mean air velocity exceeds 0.8 m/s, papers will be lifted on the table.
Page 7 of 12
6. Building Global Loads
Building Application
Office
Recommended cooling load (m2/kW or ft2/TR)
4.9 – 5.7 m2/kW or 190 – 220 ft2/TR
Shop
3.9 m2/kW or 150 ft2/TR
Shopping arcade
(Common areas)
Lift lobby
6.5 m2/kW or 250 ft2/TR
Hotel
(Function areas)
Hotel
(Guest room)
Classroom
Residential
Auditorium/theatre
Restaurant
(Chinese)
Restaurant
(Western)
3.9 m2/kW or 150 ft2/TR
6.5 m2/kW or 250 ft2/TR
7.8 m2/kW or 300 ft2/TR
3.9 m2/kW or 150 ft2/TR
5.7 m2/kW or 220 ft2/TR
3.9 m2/kW or 150 ft2/TR
2 – 2.6 m2/kW or 80 – 100 ft2/TR
2.6 – 3.1 m2/kW or 100 – 120 ft2/TR
Page 8 of 12
7. Ductwork System
In general, the pressure drop of straight duct is around 1 Pa/m and 8 Pa/m for low
velocity system and for high velocity system respectively.
•
Low pressure system is the fan pressure up to 1000Pa.
•
Medium pressure system is the fan pressure between 1000Pa and 1700Pa
•
High pressure system is the fan pressure between 1700Pa and 3000Pa
•
Low velocity system is the air velocity up to 12.7 m/s
•
High velocity system is the air velocity above 12.7 m/s
•
In general, the low pressure and low velocity system operates at 10 m/s or less.
•
The high pressure and high velocity system operates at 10 – 20 m/s.
Duct velocities
Building application
Quiet public buildings, hotel
Busy public buildings
Factories
For quiet situations
Cinema, assembly halls
Office, shops, exhibition areas
Main Duct (m/s)
5–8
8 – 11
12 – 20
5
7–6
9
Branch Duct (m/s)
4–6
4–6
10 – 20
3
5
6
•
The pressure loss of louvre is around 50Pa.
•
The pressure loss of air filter at rated capacity, reasonably clean is around
70Pa.
•
The pressure loss of air heater, two rows of grilled tubes, is around 50Pa.
•
The pressure loss of multi-blade damper operating its face velocity of 5m/s
and 10m/s is maximum of 250Pa and maximum of 1000Pa respectively.
•
The pressure loss of silencer is around 100Pa.
•
The pressure loss of cooling coil is 120Pa maximum.
•
The outlet velocity of supply air louvre is 2.5 m/s through free area
•
The inlet velocity of return air louvre is 2 m/s through free area.
Page 9 of 12
8. Piping System
In general, the minimum velocity of the flow of water is 0.75 m/s.
• For 15 – 50mm pipe diameter, the water velocity is 0.75 – 1.5 m/s
• For over 50mm pipe diameter, the water velocity is 1.25 – 3 m/s
The frictional loss of straight pipe is maximum of 990Pa/m (10 ft of water per 100
ft of straight pipe) in large air conditioning system, in general, 500Pa/m is used
in the frictional loss of straight pipe.
For central heating system is maximum of 360Pa/m, in general, 280Pa/m is used
for the frictional loss of straight pipe in consideration of expansion of water
volume.
The frictional loss of cooling coil for air-conditioning plants is 20-30kPa.
Recommended water velocities
Service
Velocity Range (m/s)
Pump discharge
2.5 – 3.6
Pump suction
1.3 – 2.1
Drain line
1.3 – 2.1
Header
1.3 – 4.5
Riser
1–3
General service
1.5 – 3
City water
1 - 2.1
In general, the allowable flow rates of black steel pipe BS1387 operating at a
nominal 75°C in the heating system are as follows:Diameter (mm)
15
20
25
32
40
50
65
80
100
125
150
Water (kg/s)
0.1
0.2
0.4
0.8
1.25
2.5
4
7
15
25
40
Page 10 of 12
9. Piping System (Cont’d)
The expansion of water in the heating system is tabulated below:Temperature °C
38
52
66
79
93
107
121
135
149
163
177
191
205
Volume increase (%)
0.6
1.2
1.8
2.8
3.5
4.5
5.6
6.8
8.3
9.8
11.5
13.0
15.0
The expansion of water is in % of increase of total volume of water in the system.
Page 11 of 12
10. Noise & Vibration Control
The comfort noise levels are shown below:Recommended maximum background noise levels:Building Application
Very quite rooms, e.g. concert
halls, sound studios
Quiet rooms, e.g. bedrooms,
theatre, churches
Private rooms, e.g. small
offices, libraries, living rooms
Public rooms, e.g. general
offices, classrooms, small shops
General areas, e.g. reception
areas, restaurants
Work areas, e.g. kitchens,
computer rooms, large shops
Industrial areas, e.g. factories
Noise Rating (NR)
NR 20
NR 25
NR 30
NR 35
NR 40
NR 45
NR 50 or above
Influence of external noise levels:Criterion for new noise
If more than 10 dBA above
background noise
If less than 5 dBA above
background noise
Comments
Compliant expected
Marginal
11. Electrical Load for air-conditioning equipment
In general, the electrical load for air-conditioning installation in an office building
is around 130 – 150W/m2. Such electrical load includes the power consumption of
chiller, chilled water pump, air handling unit and control system.
References : - N. Pavey, BSRIA, Technical Note TN 17/95 Rules of Thumb, 2nd Edition
- EMSD, Code of Practice for Energy Efficiency of Air Conditioning Installations,
2002
- ASHRAE, Fundamentals Handbook
- Carrier Air Conditioning Company, Handbook of Air-conditioning System Design
Page 12 of 12