DCB20053- PLUMBING SERVICES

HOT WATER SYSTEM

Subtopic: Hot Water Supply System In Building
 System Consideration
Design of a hot water system:
•Determine the demand of hot water - quantity and
temperature
•Selecting the type, capacity and heating surface of the
calorifier - or heat exchanger
•Selecting the water heater or boiler
•Design pipe scheme and size pipes
 System Consideration
Key factors to consider:
•Quantity (storage and flow) of hot water required
•Temperature of hot water in the system (55-65 ºC)
•Cost of installation & maintenance
•Fuel energy requirements & running costs
•Conservation of water & energy
•Safety of the user
Source: Plumbing Engineering Services Design Guide
 Design Temperature
Design temperature at around 60 – 65ᵒC
•Prevent Legionnaire Disease
•Higher temperature than 65ᵒC will be dangerous
•Consider cold water at 20ᵒC and hot water at 60ᵒC
•Mix of cold and hot water at 1:1 will give 40ᵒC
•Common for bathing and other washing purposes
•Therefore, most cold and hot water systems follow this approach of 1:1 consumption
•Pipe sizing, however, consider both at 100% consumption (i.e. only cold water or
only hot water at design flow)
 Types of water heater
Common types of water heaters
• Gas-fired water heaters
• Electric water heaters
• Water-jacketed tube heaters
• Solar water heating
• Heat pumps
 Gas-fired water heaters
Three types:
• Instantaneous
• Storage
• Circulatory
With conventional or balance flue. The heater may be supplied direct
from the water mains or from a cold water storage tank.
 Gas-fired water heaters-Instantaneous

Directly supplied heater

• Constant flow rate needed to maintain 55 ºC (minimum)
• Pressure & flow variations will affect temp. at outlets
• Only applicable for low rise buildings
Indirectly supplied heater
• Higher installation cost compared with mains-fed system
• Constant pressure from storage for shower & other fittings give more stable temp.
control
 Gas-fired water heaters-Storage

• The storage type of gas water heater is a self-contained unit and is

therefore simpler and quicker to install than a gas circulator.
• Capacities range from 75 to 285 litres.
• The smaller units are single-point heaters for supplying hot water to an
individual sink or basin. Larger, higher rated storage heaters can be
used to supply hot water to a bath, basin, sink and shower. These are
called multi-point heaters.
• They may also be installed in flats up to three storeys, with cold water
supplied from one cistern. A vent pipe on the cold feed will prevent
siphonage.
• To prevent hot water from the heaters on the upper floors flowing down
to the heater on the ground floor, the branch connection on the cold
feed pipe must be above the heaters
Gas-fired water heaters-Storage
 Gas-fired water heaters-Circulator
A gas circulator can be used to heat water in a storage
cylinder.
They are usually fitted with an economy or three-way valve.
This gives optional use of water circulation through a high
or low return pipe for variable hot water storage volume.
Domestic installations may be in the kitchen, with vertical
flow and return pipes to a storage cylinder in the airing
cupboard.
Gas-fired water heaters-Circulator
 Electric water heater
Common types:
• Instantaneous
• Cistern type
Usually power consumptions of up to 6 kW (1 phase) for cistern
type and 18kW (3 phase) for instantaneous type
May be fitted above basins, baths or sinks
Hot water pipes must be as short as possible
Immersion heater must be electrically earthed
Cable of appropriate size
 Electric water heater-Instantaneous
• Instantaneous electric water heaters are small electrically powered
units that heat water 'instantly' as it flows through the product.
• Instantaneous water heaters are compact, will not run out of hot water
and have no stand-by energy losses but have to be wired to the
electric consumer unit.
• The principal draw-back of instantaneous electric water heaters is
that the flow rate of hot water is limited by the electrical power rating;
single phase units have power ratings of up to 12kW and can provide
maximum flow rates up to 6L/min (assuming a temperature increase
from the incoming cold supply of 30ºC).
• Where three phase electricity is available (normally only in
commercial premises) instantaneous water heaters are available with
power ratings of up to 27kW which can provide flow rates of up to
12L/min.
 Electric water heater-Cistern

The cistern-type heater should be located with the water level at least
1.5 m above the water draw-off taps. If there is insufficient space to
accommodate this combination unit, a smaller pressure-type water
heater may be fitted.
These are small enough to locate under the sink or elsewhere in the
kitchen. They have two immersion heaters, the upper element of 500
watts rating is for general use supplying hot water to the basin, sink
and other small appliances.
The lower element of 2500 watts may be on a timed control to
provide sufficient hot water for baths. The pressure heater is supplied
with cold water from a high level cistern.
 Solar Water Heating
• The solar water heating systems use a renewable energy sources
or green energy to heat the water.
• The solar water heating system is the simplest system.
• It suitable for domestic or industrial use.
• The solar water heating system use solar collectors to collect heat
directly from the sun.
• There are several types of solar collectors; formed plastic, flat
plates and evacuated tube.
 Solar Water Heating - Evacuated
tube (heat pipe)

Moderately high temperature collector

Collector design varies by
manufacturer
Some tubes have moving parts
Glass tubes are fragile
Do not shed snow and frost as well as
flat plate collectors
Comparison
 Hot water supply system (Centralized Hot
Water System)
Major components:
•A boiler
•A hot water storage cylinder or calorifier
•Cold water storage tank linked by supply and circulatory
pipework
Boiler may be heated by gas, solid fuel or oil
Position of the boiler plant & pipe insulation is essential
 Centralized Hot Water System - Direct system

•Water through the boiler can be drawn off from the taps
•Saves the cost of a storage and expansion cistern and
pipework
•Heated quicker
•Adequate pressure on the main
•Sealed primary circuit can be pumped or can circulate by
natural convection
 Centralized Hot Water System - Indirect system
• Separate pipe line for the water drawn off at taps
• Used in hard water areas to prevent scaling of boiler and
pipes
• Used when heating is combined with the system
• It costs more than direct system but requires less
maintenance
• An expansion vessel in primary pipework to eliminate the
need for an expansion cistern, expansion pipe and boiler
feeder pipe
Centralized Hot Water System - Systems for high-rise buildings

• More economical to pressurise water in a sealed system

• Proper zoning is required (e.g. 30 m)
• Boiler & calorifiers to withstand water pressure
• Sealed primary circuit saves on pipework and the expansion and feed
tank
• Expansion vessel takes up the expansion of water in the primary
circuit
• The pipes, calorifiers, head tanks & boiler must be well insulated
 Centralized Hot Water System - Systems for high-
rise buildings
Prevent dead legs in hot water systems
‘Dead legs’ occur in hot water systems where water does not flow for a
period of time
Such as at night when hot water is not used and the contents of the pipes
and appliances cools down
Water cooled to 20 to 45 ºC becomes more susceptible to bacteria growth,
and overnight gives adequate time for possible bacteria to multiply
Two common approaches to avoid dead legs:
1. Install a secondary return pipe to the furthest draw off point to minimize
the length of the dead leg
2. Maintain the water temperature at all times
 DCB20053- PLUMBING SERVICES
HOT WATER SYSTEM
Subtopic: Design Features of Boilers and Cylinders
 Boiler
• A boiler is an encased vessel that provides a means for combustion heat
to be transferred into water until it becomes heated water or a steam. The
steam or hot water is then usable for transferring the heat to a process.
• The two most general types of boilers are hot water and steam boilers.
Most small and large commercial buildings, manufacturers and business
use hot water boilers.
• Hot water boilers are preferred because they normally do not need
operators or major water chemistry.   Since they operate at lower
temperatures, hot water boilers can operate at higher fuel conversion
efficiencies than steam boilers.
• Steam boilers are used in many different applications, their one purpose:
to use water and transform it into steam by   heat. Some boilers heat the
steam even hotter than the boiling point temperature. This is referred to as
superheated steam.
Component of boiler
 Heat Up Period
• A time taken to change the cold water to hot water.
• To help reduce the effect of circulating cold water to the
zones when the boiler has not run for an hour.
• The heat-up period for every plant will be different and will
depend on many factors. A small low-pressure boiler in a
compact plant such as a laundry, for example, could be
brought up to operating pressure in less than 15 minutes.
• A large industrial complex may take many hours. The
starting point, when safely bringing a small boiler on line,
is the main stop valve, which should be opened slowly.
 Heat Up Period
It is essential that when a boiler is brought on line, it is done in a slow,
safe and controlled manner to avoid:

• Water hammer - Where large quantities of condensate lie inside the

pipe and are then pushed along the pipe at steam velocities. This can
result in damage when the water impacts with an obstruction in the
pipe, for example a control valve.

• Thermal shock - Where the pipework is being heated so rapidly that

the expansion is uncontrolled, setting up stresses in the pipework and
causing large movement on the pipe supports.
Priming - Where a sudden reduction of steam pressure
caused by a large, suddenly applied load may result in boiler
water being pulled into the pipework. Not only is this bad for
plant operation, the boiler can often go to 'lock-out' and it will
take some time to return the boiler to operating status. The
discharged water can also give rise to water hammer in the
pipework.
 Operation Of Boiler’s Components
Pressure & Temperature gauge
• Monitor water temperature and pressure.
• If water pressure is low (below 12 psi) the system needs to have
water added
Pressure Relief Valve
• Relieves high water pressure
• The boiler's automatic filling system using the pressure relief valve
should maintain proper water level by maintaining a 12-15 psi
pressure.
• Placed at the outlet to operate when the pressure in the tank exceeds
the value for which it was designed to operate safely.
 Operation Of Boiler’s Components
Anti Vacuum Valve
• These allow oxygen to enter the boilers until pressure builds in the
boiler (i.e. as the temperature rises in the boiler the steam creates
pressure). Once pressure builds, it pushes the valve up and shuts the
valve
Temperature cut-Off Sensors
• The second part is typically called a thermal cut-off switch. This is a
temperature sensor located near the burner that opens to shut the gas
off.
• This will usually occur when adequate combustion air is not available.
this will cause the burner flames to droop and the cut-out to open.
 Calculation of Boiler

Boilers are rated in kilowatts, where 1 watt equates

to 1 joule of energy per second, i.e. W J/s. Many
manufacturers still use the imperial measure of
British thermal units per hour for their boilers.
For comparison purposes 1 kW equates to 3412
Btu/h.
 Sample Calculation
By referring to Table 1, calculate the boiler power required
for hospital hot water storage using the data below.
• Sanitary appliances 25WB used 4 times, 15 bath used
twice, 15 shower used 3 times and 10 sinks used 3 times
• Temperature rise of water = 55ºc
• Heating up time or recovery period = 2 hours
• Efficiency of plant = 60 percent
• Specific heat capacity of water = 4.2 kJ/kg K
Step 1:

25 wash basins used four times = 25 x 4 x 1.5 = 150

15 bath used twice = 15 x 2 x 70 = 2100
15 showers used three times = 15 x 3 x 13 = 585
10 wash up sinks used three times = 10 x 3 x 15 = 450
Total : 3285
Step 2:

Capacity of each calorifier = 3285 x 2/3 =2190 litres

Boiler power = s.h.c x kg x temperature rise (c)

Heating up time in seconds x efficiency

Boiler power = 4.2 x 2190 x 55

2x 3600 x 0.6
= 117.10 kW.
The 118kW boiler would be satisfactory.