Heating and Water Services
Design in Buildings
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3205, Australia
Chapman & Hall India, R.Seshadri, 32 Second Main Road, CIT East,
Madras 600 035, India
First edition 1996

© 1996 Keith J.Moss

ISBN 0-203-47558-5 Master e-book ISBN
ISBN 0-203-78382-4 (Adobe eReader Format)
ISBN 0 419 20110 6 (Print Edition)

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A catalogue record for this book is available from the British Library


Nomenclature 100
4.1 Introduction 101
4.2 Pressurization methods 101
4.3 Pressurization of large systems 108
4.4 Chapter closure 118
5 Steam systems 119
Nomenclature 119
5.1 Introduction 120
5.2 Steam systems 120
5.3 Steam generation and distribution 139
5.4 Chapter closure 147
6 Plant connections and controls 148
Nomenclature 148
6.1 Introduction 148
6.2 Identifying services and plant 149
6.3 Building energy management systems 160
6.4 Control strategies for heating systems 165
6.5 Chapter closure 167
7 The application of probability and demand units in design 168
Nomenclature 168
7.1 Introduction 168
7.2 Probability or usage ratio P 169
7.3 The system of demand units (DU) 183
7.4 Chapter closure 185
8 Hot and cold water supply systems utilizing the static head 186
Nomenclature 186
8.1 Introduction 186
8.2 Factors in hot water supply design 187
8.3 Design procedures 187
8.4 Chapter closure 200

is also needed to the Chartered Institution of Building Services Engineers
(CIBSE) Guide to current practice. Student membership of the Institution
currently qualifies the student to a free extract of the Guide, and application for
this grade of membership is strongly recommended.
The author is only too well aware that this book cannot address all the queries
that may arise during its study, and therefore the student is also encouraged to
seek a mentor who can advise and assist when part of the text needs more
explanation than is provided.
CIBSE will gladly advise the enquiring student of the name and number of the
secretary for the local region, who will be quite prepared to discuss the matter of
a suitable mentor.
Each chapter begins with the nomenclature used and an introduction. The
chapter closure at the end of each chapter identifies the competences that will
have been acquired after successful completion.
The text is written in a way that actively involves the reader by encouraging
participation in the solutions to examples and case studies, with some examples
being left for the reader to try. It is intended to be a learning text in practical
design.
The last chapter is entitled ‘Loose ends’, partly because it deals with topics not
covered elsewhere in the text. It also happens to be one of the author’s favourite
radio programmes.
Acknowledgements
I am indebted to Mr Shaw, Mr Sedgley and Mr Douglas, who were my principal
teachers at what used to be called the National College in Heating, Ventilating, Air
Conditioning and Fan Engineering, and is now integrated with the University of
the South Bank.
Grateful thanks are also due to Tony Barton, who preceeded me at the City of
Bath College and initially set up the courses in HVAC. He it was who introduced
a raw recruit from industry to the art of enabling students to learn.
Finally I have to thank all those students who have had to suffer my teaching

A
u
area of the unheated surfaces (m
2
)
C specific heat capacity (kJ/kg K)
d design conditions
dt temperature difference (K)
dt
t
total temperature difference (K)
E fraction of heat radiation
EAT entering air temperature (°C)
f fabric heat loss ratio Σ (UA)/ΣA, Σ(YA)/ΣA (W/m
2
K)
f
r
thermal response factor
F
1
, F
2
temperature interrelationships
F
3
plant ratio
H thermal capacity (kJ/m
2
)

LAT leaving air temperature (°C)
M mass flow rate (kg/s)
n index
n operating plus preheat hours
N number of air changes per hour
p prevailing conditions
1
Nomenclature
2 Heat requirements in temperate climates
Q
f
conductive heat loss through the external building fabric (W)
Q
p
plant energy output (W)
Q
pb
boosted plant energy output (W)
Q
t
total heat loss=Q
f
+Q
v
(W)
Q
v
heat loss due to the mass transfer of infiltrating outdoor air (W)
R
a

total thermal resistance (m
2
K/W)
t
a
, t
ai
indoor air temperature (°C)
t
b
balance temperature (°C)
t
c
dry resultant, comfort temperature (°C)
t
d
datum temperature (°C)
t
e
environmental temperature (°C)
t
ei
environmental indoor temperature (°C)
t
eo
, t
ao
outdoor temperature (°C)
t
f

v ventilation heat loss ratio=NV/3ΣA (W/m
2
K)
V volume (m
3
)
VFR volume flow rate (m
3
/s)
Y admittance (W/m
2
K)
α coefficient of linear thermal expansion (m/mK)
ρ density (kg/m
3
)
Σ sum of

Heat flow into or out of a building is primarily dependent upon the prevailing
indoor and outdoor temperatures. If both are at the same value heat flow is zero,
and the indoor and outdoor climates are in balance with no heating required.
During the heating season (autumn, winter and spring), when outdoor
temperature can be low, the space heating system is used to raise indoor
temperature artificially to a comfortable level, resulting in heat losses through
1.1 Introduction
Introduction 3
the building envelope to outdoors. The rate of heat loss from the building
depends upon:

• heat flow into or through the building structure, Q

Consider Figure 1.1, which shows a section through a building. The prevailing
wind infiltrates one side of the building, where the space heating appliances must
be sized to raise the temperature of the incoming outdoor air. The heated air
moves across the building to the leeward side, where it is exfiltrated. Here the
heating appliances are not required to treat the air. Clearly, on another day the
wind direction will have changed, and therefore all heating appliances serving the
building perimeter must be sized to offset the infiltration loss Q
v
as well as the
structural heat loss Q
f
.
Theoretically, the boiler plant is sized on the basis of the total structural heat
loss plus only half of the total loss due to infiltration, as only half of the
appliances are exposed to cold infiltrating air at any instant. In practice the full
infiltration heat loss is employed in plant selection.
It follows therefore that the rate of infiltration—and hence the heat loss due to
infiltration of outdoor air are dependent upon air temperature and wind speed.
Further factors that relate to the building design may also influence infiltration
4 Heat requirements in temperate climates
rates, and include stack effect in the building resulting from stairwells, lift shafts,
unsealed service shafts and atria, and how well the building is sealed.
Thus from Q=M.C.dt, Q
v
=(V. ρ.N.C.dt) 3600, and if standard values of air
density and specific heat capacity are taken,

Q
v
=0.33 NV dt (1.3)

The revised heat loss=100×30/23=130.4 kW.
Likewise the output of a radiator varies with the magnitude of the temperature
difference between its mean surface temperature t
m
and room temperature t
i
.
Consider Figure 1.2, which shows a section through a building and the heat flow
paths expected during the winter season. When temperatures are steady a heat
balance may be drawn:

heat loss from the building=heat output from the space heater

Using appropriate equations:

(Σ(UA)+0.33NV) (t
i
-t
o
)=KA(t
m
-t
i
)
n
(1.4)

Index n is approximately 1.3 for radiators and 1.5 for natural draught convectors,
and is found empirically.


(b) Prevailing heat loss:Substituting into equation (1.4):

4286=13×2.522(t
m
-20)
1.3

From which

t
m
=62 ºC

Do you agree?
These results are summarized in Table 1.1.
The system controls must vary the radiator mean surface temperature as
the outdoor temperature varies. However, the rate of response required to
changes in outdoor climate is dependent upon the thermal capacity of the
building envelope, and this varies from lightweight structures, which have a
short response, to heavyweight structures. No margin has been added to the
radiator; a figure of 10% is frequently used in practice.Example 1.2
A natural draught convector circuit has design conditions of: t
i
=20 °C, t

i
-t
o
)∝(t
m
-t
i
)
n
∝(t
f
-t
r
)

If design (d) and prevailing (p) temperatures are put together:
(1.5)
Equating heat loss with heat output:from which

t
m
=60.7 ºC

Do you agree?
Equating heat loss with heat given up:Circuit flow temperature of 30 °C is an arbitrary value, and depends upon the type
of control device.
It is worth noting that a change in outdoor condition will not require an
immediate response from the controls to maintain a constant indoor temperature.
The response time will depend upon the thermal capacity of the building
envelope.
THERMAL CAPACITY OF THE BUILDING ENVELOPE
The thermal capacity H of the building envelope is normally measured in
kJ/m
2
of structural surface on the hot side of the insulation slab, which
may consist of a proprietary material, or it may have to be taken as the
Figure 1.3 Calibration of temperature controls.
Heat energy flows 9
air cavity if there is no identifiable thermal insulation slab within the
structure.
When the space heating plant starts up after a shutdown period of, say, a
weekend, the building envelope is cold, and heat energy is absorbed into the
structural layers on the room side of the insulation slab until optimum
temperatures are reached in the layers of material. At this point the rooms should
begin to feel sufficiently comfortable to occupy. The more layers of material there
are on the room side before the insulation slab is reached, the greater will be the
thermal capacity of the building envelope and the longer the preheat period.
Conversely, the longer is the cooldown period after the plant is shut down.
The energy equation is

H=slab thickness L×ρ×C×(t
m

+R
p
+R
i
+R
b
+R
a
+R
b
+R
so
R
t
=0.12+0.0625+0.7143+0.1613+0.18+0.119+0.06
=1.4171 m
2
K/W
As thermal resistance R∝dt:

(1.7)
10 Heat requirements in temperate climates
Thus
(1.7)

Substitute:from which


2

For wall (b), total thermal resistance R
t
remains the same, but the slabs of
material are arranged so that now the material on the hot side of the
insulation includes the plaster and the inner leaf of the wall.
Figure 1.4 Two similar walls with insulation in different locations.
Heat energy flows 11
Inside surface temperature t
1
and temperature t
2
at the interface will have
the same value. Interface temperature t
3
needs calculation, and from
equation (1.7):Substituting:from which

t
3
=14.9 ºC

The mean temperature of the inner brick leaf of the wall therefore will be

of heat energy from the space heating plant. The following analyses may
be made.

1. Slab density has a significant effect on thermal capacity on the hot side
of the thermal insulation slab.
2. It will take longer for comfort conditions to be reached in wall (b) than
in wall (a). Thus the preheating period will need to be longer.

12 Heat requirements in temperate climates
Conversely, cooling will take longer, allowing the plant to be shut down
earlier.
3. It will take longer for the inside surface temperature of 17.3 °C to be
reached in wall (b).
4. The thermal transmittance coefficient (U) is the same for both wall (a) and
wall (b).
5. The thermal admittance (Y) is lower in wall (a) than in wall (b) (see
Example 1.4).
6. The location of the thermal insulation slab in the structure dictates the
thermal response f
r
for that structure (see Example 1.4). It can
effectively alter a sluggish thermal response (heavyweight) structure to a
rapid thermal response (lightweight) structure when the insulation slab is
located at or near the inside surface.
7. The inner leaf of the wall in composite wall (b) should consist of
lightweight block if a faster response is required.
8. The air cavity may be taken as the insulation slab in the absence of
insulation material in a composite external structure when identifying the
slabs on the hot side.
9. If insulated lining is located on the inside of an external wall during

2
during the building process. There are clearly environmental penalties
and benefits here, which lead to a consideration of an environmental cost-
benefit analysis. This extends beyond the scope of this book, but
environmental control by building structure is nevertheless an important
associated topic.
VAPOUR FLOW
Air at an external design condition of 3 °C during precipitation (rainfall) can be
at saturated conditions (relative humidity of 100%). If it is then sensibly heated
to 20 °C dry bulb by passing it through an air heater battery, its relative humidity
drops to 32%. At both dry bulb temperatures the vapour pressure remains constant
at 7.6 mbar. This is because moisture in the form of latent heat has not been
absorbed from or released to the air.
The partial pressure of the water vapour in the air is therefore altered
only by adding or removing latent heat through the process of evaporation
or condensation. Indoor latent heat gains are incurred immediately the
building is occupied, owing to involuntary evaporation from the skin
surface, exhalation of water vapour from the lungs, and sweating. In the
winter, therefore, latent heat gain indoors is inevitable during occupancy
periods. Cooking, dishwashing and laundering add to the latent heat gains.
If the building is heated, the air is usually able to absorb the vapour
production, with a consequent rise in vapour pressure. In an unheated and
occupied building condensation may occur on the inside surface of the
external structure because the air is unable to absorb all the vapour being
produced.
Vapour pressure in heated and occupied buildings is inevitably higher than the
vapour pressure in outdoor air. Vapour therefore will migrate from indoors to
outdoors. In highly ventilated buildings or indoor locations this may well take
place via the ventilating air.
Otherwise it will migrate through the porous elements of the building

W/m
2
K) and relatively high rates of infiltration (above 1 air change per hour)
require higher levels of convective heating than radiant heating to maintain
indoor design conditions. This is based on the knowledge that, living in the
natural world as we do, we are quite comfortable in outdoor climates of
relatively low air temperature and velocity if solar radiation is present with
sufficient intensity.
A similar response is to be found indoors when a significant proportion of
appliance heat output is in the form of heat radiation.
The calculation of plant energy output Q
p
using temperature ratios F
1
and F
2
accounts for the varying proportions of radiant and convective heating offered by
different heating appliances in such building envelopes.
However, it will be shown that for buildings subject to current thermal
insulation standards (average U value 0.5 or less) and with infiltration rates below
1 per hour, plant energy output is about the same value for both highly radiant
and highly convective systems.
The question then arises as to which system is appropriate for a particular
application. This has in the past generated considerable discussion in the
technical press. The answer for modern factories and workshops is not now to
be found in the building type and shape but in the use to which the building will
be put. High-tech dust-free environments will generally benefit from radiant
systems. Processes producing dust and fumes will require heated make-up air,
dictating an air-heating system. Areas of high occupancy will require the
1.3 Plant energy