2. Learning Outcome 1: Heat and temperature, heat capacity
and heat transfer.
Learning Outcome 2: Methods used to control heating in
various situations
Learning Outcome 3: The processes and techniques used
for water, space and industrial
process heating.
Learning Outcome 4: AS3000:2007 Wiring Rules
requirements.
Learning Outcome 5: Possible causes of malfunction in
electric heating equipment and the
tests required to diagnose faults
Revision 01 2
3. Heat and temperature
What is the difference between Heat and Temperature?
Heat is a measure of the total kinetic energy of the molecules or atoms
in a body.
◦ The quantity of energy stored is measured in Joules
◦ Symbol – J
Temperature is a measure of the degree of movement of the random
oscillations of the molecules.
Alternatively, it can be defined as a measure of the hotness of a body.
No movement = No temperature. (ie. Absolute Zero)
If a body is not storing heat its temperature is absolute zero.
Revision 01 3
4. Electrical Heating
Transfer of Heat
Heat is transferred from a hotter region to
a colder region
5. Electrical Heating
Heat is Energy
Energy (W)in Joules (J) Power in Watts (W)
Time in seconds (s)
6. Electrical Heating
Temperature Scales
The common Some countries use the
temperature FAHRENHEIT scale
scale is CELSIUS
Water boils at 100oC Water boils at 212oF
Ice melts at 0oC Ice melts at 32oF
7. Electrical Heating
Temperature Scales
The temperature
scale used in science and Water boils at 373K
engineering is the
absolute KELVIN scale (K)
Zero Kelvin (0K) is “Absolute
One Kelvin “degree” Zero”
is equal to and is equivalent to
One Celsius “degree”
Zero Kelvin is “Absolute -273oC
Zero”
NO heat content; Ice melts at 273K
NO molecular motion.
The “degree” symbol o is NOT used with the Kelvin scale
12. Kelvin
◦ 0K absolute zero
◦ 273.15K ice point water
◦ 373.15K steam point of water
◦ Note 100 degrees between ice and steam
Celsius
◦ -273.15OC absolute zero
◦ 0° C ice point water
◦ 100° C steam point of water
◦ Note 100 degrees between ice and steam
Revision 01 12
13. The ability of a substance to store heat.
If equal masses absorb equal amounts of
thermal energy (heat), different substances
show a different temperature increase.
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14. Electrical Heating
Specific Heat Capacity
Specific Heat Capacity is the amount of heat energy
required to change the temperature of one kilogram of
a material through ONE KELVIN (or degree C)
Absolute Heat Energy (J) Specific Heat Capacity(J/kg.K)
Mass (kg)
Temperature change (K or oC)
16. Q = m x c x (t 2-t 1)
◦ Where:
◦ Q = Quantity of heat
◦ m = mass in kg
◦ c = specific heat capacity (tables)
◦ t 2 – t 1 change in temperature
Revision 01 16
17. Heat moves from high to low temperature
levels. The rate of heat transfer is partly
dependant on the difference between the two
temperature levels.
3 types of heat transfer
Conduction
Convection
Radiation
Revision 01 17
21. Thermal conductivity is the material’s ability
to transmit heat by conduction.
Depends on four factors:
◦ Type of material
◦ Length of transfer path
◦ Cross-sectional area of path
◦ Temperature difference
Revision 01 21
22. The frame of a motor is designed to conduct
the heat from the windings (centre of motor)
to the surface and then dissipate the heat to
the environment.
The frame of a Hot Water Service is designed
to ensure the heat is trapped in the centre of
the Service.
Revision 01 22
23. Two basic types:
◦ Open Loop Control
◦ No actual control of the amount of
heat
◦ Closed Loop Control
◦ Control over the amount of heat
(temperature)
Revision 01 23
24. Examples:
On-Off control of a switch
Set the car throttle in one position for a trip…
Simmerstat on stoves to control the hotplates
O/H fan speed control
Fixed position of valve regardless of changes to
flow requirements
Garden sprinkler
Electric toaster
Microwave oven: Power setting. Time setting
Electric Blanket
Revision 01 24
25. Three heat switching
◦ Example:
Most old Urns
Electric blankets (almost all)
Some stoves in caravans
Revision 01 25
28. Electrical Heating
Heat Control – Simmerstat
The SIMMERSTAT is an OPEN CYCLE temperature control
commonly used with stoves.
Active Contacts
Compensating Bimetal
Pivot Operating Bimetal
Heating
Load
Internal heater
Neutral element
29. Heater element + bi-metal strip
Main Contacts
Magnet
(to give snap
action switch)
Adjustment
Aux. Switch
Revision 01 29
30.
31. Examples:
Oven thermostat and element
Toilet cistern water level control
Car cruise control
Almost all industrial processes
HWS
Electric Iron
Electric frypan
Revision 01 31
35. Electrical Heating
Heat Control – Thermostats
A THERMOSTAT is a Closed-Cycle Control that:
• SENSES the output temperature
• COMPARES it with the pre-set value
• VARIES or SWITCHES the input energy
36. Four types are typically found in appliances.
The first three of these are totally
mechanically controlled:
◦ 1. Bimetal strip. When two metals with different
coefficients of thermal expansion are
sandwiched together, the strip will tend to bend
as the temperature changes.
In a thermostat, the bimetal strip operates a set
of contacts which make or break a circuit
depending on temperature. In some cases the
strip's shape or an additional mechanism adds
'hysteresis' to the thermostat's characteristics
Revision 01 36
37.
38. 2. Bimetal disk. This is similar to (1) but the bimetal element is in the
shape of a concave disk (like the “clicker” play toy). These are not
common in adjustable thermostats with brad spans, but are the usual
element in an over-temperature switch.
Revision 01 38
39. Electrical Heating
Heat Control – Thermostats
Bimetal Disc Thermostat
This thermostat has contacts operated by a cupped
bimetal disc.
At a pre-set temperature, the disc snaps the contacts
open.
When the disc cools to a preset value, disc returns and
the contacts snap closed.
44. 3. Fluid operated bellows. These are not that common in small appliances
but often found in refrigerators, air conditioners, stoves, and so forth. An
expanding fluid (alcohol is common) operates a bellows which is coupled
to a set of movable contacts. As with (1) and (2), hysteresis may be
provided by a spring mechanism.
Revision 01 44
45. Electrical Heating
Heat Control – Thermostats
Capillary Tube Thermostat
Bellows or
Capillary Tube
Diaphragm
Bellows Rod Bulb with volatile
moves to operate liquid
contacts
46.
47.
48.
49. Electrical Heating
Heat Control – Thermostats
Bi-Metal Thermostat
Support Stem
Invar Rod
Brazed to Stem
Brazed to Rod
Helical Bi-Metal Strip
Mounting Flange & Screw
Thread
51. Electrical Heating
Heat Control – Thermostats
Expanding Tube Thermostat
Retaining
Clips Brass Tube Tube
Tube Brazed
Expands/Contracts to Rod
Rod Free End
Tube Brazed Invar Rod
Moves to operate
to Support
contacts
52. Expanding tube thermostat
Operating rod Rod is welded
to the end of
the tube
The operating rod has a different
expansion rate than the tube
enclosing it.
Electrical
Contacts
Revision 01 52
53. Bi-metal helix
Expanding rod type
Bulb type
Revision 01 53
54. 4. Electronic thermostats. These typically use a temperature controlled
resistance (thermistor) driving some kind of amplifier or logic circuit which
then controls a thyristor or contactor.
Revision 01 54
55. Note that these terms can only apply to a
closed loop system such as thermostats. If
there is no feedback, the system cannot have:
◦ Hysteresis
◦ Differential
◦ Sensitivity
◦ Accuracy
Revision 01 55
56. Sensitivity
Is a measure of the change of output to a
change of input.
A more sensitive thermostat will have a
smaller differential.
It is a measure of how closely a unit can
maintain a given temperature.
It is better applied to temperature
measuring devices that give an analogue
output. A more sensitive device gives a
greater change of output to the change of
input (temperature).
Revision 01 56
57. Thermocouples
Resistance Temperature Detectors (RTD’s)
Diodes and semiconductor IC’s
Gas expansion system
Mercury expansion system
Coiled bimetal strip (see P&N)
Radiation Pyrometers
Revision 01 57
59. Instantaneous or tankless water heaters are small
cabinets that heat water on demand or instantly as
it passes through the heater.
They contain no significant water storage,
possessing only up to a 6 litre operating holding.
These water heaters only use energy when the hot
water outlet is turned on and shut down
immediately when the outlet is turned off.
60.
61. Mains Pressure HWS: direct heated
◦ Installed at ground level.
◦ Requires a pressure relief system.
◦ Requires an expansion control valve.
◦ New houses require a tempering valve for warm
water to the bathroom.
Revision 01 61
62. Mains Pressure HWS
-Direct heated Insulation
Hot water Out
Note: The tank operates
at mains pressure.
Cold water In Water Heater +
thermostat
L/O 3.1
Revision 01 62
63.
64. If both have the
same colour tags,
then this wont be
a problem
1400kPa
Revision 01 64
65. Bottom Cold Water
Expansion Valve
must be
200kPa lower than
the top pressure
relief valve.
1200kPa
Revision 01 65
67. • Hot water (73°C max.) to
laundry and kitchen. Pressure
Relief Hot Water
• Warm water (50°C max.) to Valve Outlet
bathroom. (73°C max.)
• If major renovations are
Tempering Warm Water
carried out in the bathroom, Valve Outlet
then a tempering valve must (50°C max.)
be added.
• The house owners can sign a Cold Water
form saying they don’t want it Inlet
(as only adults will be using Cold Water
Cold Water
Expansion
it), and the plumber will not Valve
Tap
be responsible for any
consequences.
Revision 01 67
68. Heat exchange Storage HWS
Small Storage HWS designed
for under sink operation
Revision 01 68
69. Must be mounted above taps.
Low pressure hot water only.
More to go wrong.
◦ If float valve sticks…
Revision 01 69
70. Low Pressure HWS
Toilet cistern type
water level sensor
Element
and electrical Cold
connection Water
In
Hot Tank fills from
Water Bottom
Gravity
Out
Feed
Revision 01 70
71. Faults:
◦ Element goes open circuit.
Replace element.
◦ Thermostat either stays on, or stays off
Replace thermostat
Revision 01 71
72. • Solar
– Still requires booster
element
– 8-10 year pay back
period
– May require extra roof
support.
– Does the roof face the
required direction?
Revision 01 72
73. In solar systems cold water travels through the
roof-mounted solar collector where the water
absorbs heat from the sun.
Water heating using solar energy occurs during the
day and the solar involvement varies significantly
throughout the year depending on the climatic
conditions.
The apparatus of solar heaters includes the solar
collector, insulated storage tank and, if required,
pump and control valves.
74. Flat-plate collectors are the most common
collector for domestic water heating.
A typical flat-plate collector is an insulated
rectangular-type metal box with a transparent
cover (similar to a greenhouse) and a black
absorber plate.
75.
76. The evacuated-tube collectors consist of rows of
parallel transparent double glass tubes, each
containing an electromagnetic energy absorber and
covered with a solar-sensitive coating.
Sunlight enters the tube, strikes the absorber and
heats the water flowing through the collector.
77.
78.
79.
80. Calorifiers are cylinders with an internal coil which
allows the use of any type of boiler for hot water
production.
The calorifier can be either mains-pressure or low-
pressure hot water storage systems.
A significant amount of heat energy can be
transferred to the calorifier, allowing a large
production of hot water from a relatively small
cylinder.
81.
82. Heat pump HWS
◦ More expensive than conventional HWS
◦ Smaller than Solar HWS
◦ Can operate with or without sunshine
◦ Operates as a split system
Revision 01 82
83. A heat pump water heater absorbs heat from the
surrounding environment and pumps the acquired
heat energy into a hot water storage tank.
The heat pump serves as a heater by absorbing
heat from the surrounding environment and
pumping it into a closed-system heat-exchanger
water storage tank.
84. The compressor compresses cool refrigeration gas, causing it to become hot, high-
pressure refrigeration gas
This hot gas runs through a set of coils so it can dissipate its heat, and it condenses
into a liquid.
The refrigeration liquid runs through an expansion valve, and in the process it
evaporates to become cold, low- pressure refrigeration gas
This cold gas runs through a set of coils that allow the gas to absorb heat and cool
down the air inside the building
A solar heat pump works on the same principle only in reverse i.e the coils carrying
the hot gas are used to heat the water.
Revision 01 84
85.
86. If the water heater’s thermostat, which controls the
resistive heating element, malfunctions the
pressurised water in the tank could continue to
heat and superheat (beyond 100 °C).
This will cause two problems:
First, since water expands when heated, the water
pressure in the tank will increase as the water is
superheated.
If the pressure exceeds the vessels maximum
pressure threshold the tank could rupture or even
explode.
87. Secondly, the release of superheated water (water
heated above 100 °C up to its critical temperature
of 374 °C without boiling) causes the water to burst
into steam (1 litre of water can produce about 3
litres of steam), causing a sudden increase in
volume and release of energy.
Lowering the pressure of water lowers the boiling
point. There is less pressure above the water to
overcome. The superheated vapour plume expands
until its pressure equals that of the surrounding
atmosphere.
88. Types:
◦ High Temperature radiators
◦ Low temperature panels and convection units
◦ Thermal storage systems
◦ Heat pumps (reverse cycle air conditioners)
Revision 01 88
89. Types:
◦ Low temperature panels and convection units
Under-carpet / under concrete heaters (MIMS in
concrete slab)
Can be operated using cheaper power at night
Blower heaters
Oil filled floor heaters
Revision 01 89
90. Stoves (ranges):
◦ Four types of cooktops:
Coiled element
Solid element
“Ceramic” cooktop
Induction cooktop
Revision 01 90
96. • Stoves:
– Wiring: Half the elements
with their controls
Other half of the elements
with their controls
A A N Connection Box
Revision 01 96
97. • Microwave ovens bombard food with
electromagnetic radiation at 2.45GHz
• Water absorbs the energy. The molecules vibrate
and get hot.
• The oven will dissipate the same energy in the
cavity no-matter what. (eg. 800W)
• Small quantities will cook faster. Large quantities
cook slower.
• Metal reflects the microwaves
• If a microwave oven is left empty, the microwaves
will reflect back into the magnetron and heat it up.
This destroys the magnetron.
Revision 01 97
98. There are four (4) process heating methods
available for converting the electric energy to
heat energy.
1. Resistance
2. Infra-red
3. Induction
4. Dielectric
Revision 01 98
99. Resistance process heating
All the heat generated by an element is transferred by either
convection or conduction
The elements used may be either wire, strip or solid rods.
Typical applications include; duct heaters, furnaces, refrigerators,
space heaters, greenhouse heating and trace heating.
In all cases their temperatures are controlled by thermostats
Revision 01 99
100. Infra Red heating:
◦ Spray painting booths for cars
Induction Heating:
◦ For directly heating small steel parts.
◦ Similar to locking the rotor of a motor… it gets hot.
◦ Usually the work piece has currents induced in it
directly.
◦ Frequencies between 50Hz and 5MHz used.
10
Revision 01 0
101. Dielectric Heating:
◦ Used to heat non-conducting material.
◦ If an insulator is placed between two electrode
plates, and AC is applied to the plates, the
molecules are agitated and heat up.
◦ Used in plywood manufacture
◦ Used to dry breakfast cereal and dog biscuits
Electric Arc
◦ Used in the steel industry up to 150 tonnes
◦ Used in glass furnaces. eg. Bradford pink batts.
◦ Arc welders fall in this category.
10
Revision 01 1
102. Demonstrate knowledge of the possible
causes of malfunction in electric heating
equipment and skills the testing and fault
finding.
5.1 List the possible causes of faults in a
malfunctioning electric heating
device/circuit.
5.2 Conduct tests and locate a fault in a
malfunctioning electric heating
device/circuit.
10
Revision 01 2
103. Open circuits
-physical breaks in the element
-breaks in wiring
Short circuits
-resistance reduced to 0Ω
Partial open circuits
-loose connections etc
10
Revision 01 3
105. Element Testing
To test an element for continuity the appliance should first be disconnected
from power. After the appliance has been made safe to work on, the
element needs to be isolated from the rest of the electrical circuit by
removing at least one of the connecting wires. Once that is done, an ohm
meter or continuity tester's leads can be held against each terminal of the
element.
The exact resistance of an element is often not important as it will not
usually change over its life span except to become totally open (show
infinite resistance) when defective or becomes shorted to ground (see
below). In case you're curious, a large cooktop surface burner is usually in
the area of 27 ohms, a small 45 ohms. A griller element's resistance may
be in the area of 20 to 40 ohms depending on its wattage.
10
Revision 01 5
107. Short to Earth
An element can also become partially shorted to ground.
While this may not be enough to create a dead short and
cause the element to fail outright, it can create a shock
hazard. To test an element for a short to ground, an
ohmmeter should be set on its highest ohm scale (1K or
10K) and tested from one of the element's terminals to the
element's metal sheath. It may be necessary to rub the
outer element surface with the meter probe to make a
good contact. If anything other than infinite resistance is
shown, replace the element.
10
Revision 01 7
