STUDENT MATERIAL
OUTCOME 4
PREPARE A SPACE HEATING SYSTEM LAYOUT AND
SPECIFICATION NOTES
In this outcome you will study domestic space heating solutions and in particular
the technology and layout of typical wet central heating systems.
Space heating is the term used to describe the heating of rooms in buildings. In the
UK, Building Regulations require that all domestic buildings have some form of
heating in the living room to ensure comfort. This normally means that new houses
must have at least one heater providing an output of around 3kW. However, most
houses constructed nowadays have central heating as a standard feature.
Historically, most buildings were heated by open fires with chimneys using natural
fuel such as wood, coal, peat, etc. This form of heating proved very successful, and in
many cases is still used today, despite the inconvenience of arranging coal delivery
and having to prepare the fire and dispose of the remnants. In modern times the use of
more convenient fixed or portable heaters such as gas, electric and oil heaters became
very popular.
The choice of heating for houses is either local heating (often referred to as point of
use heating) or central heating.
Local heating
There are many types of local heating available and these may be either portable or
fixed type. Let us consider some of these:
Open fires: as mentioned above, these fires have been used historically over the
centuries. They provide an effective appearance. However, they are not very efficient
in converting fuel into usable heat energy. (see Figure 47) Improvements have been
made on open fires, such as air inlets controls to allow adjustment to the rate of
combustion, and better quality fuel being used resulting in less unburnt fuel remaining
in the grate.
Figure 47
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Log burning stoves: these burn fuel very slowly and are an economical method of
providing heat, as long as wood is readily available at low cost. Stoves can burn for
24 hours from one filling. Like all solid fuel appliances they do, however, require
regular stocking up and removal of residue.
Mobile propane gas heaters: these have live flames and heat by radiation and
convection. These heaters commonly provide heat outputs of 2 to 3kW. Due to the
nature of mobile heaters there is no flue or chimney to removes combustion gases and
therefore, it is important that adequate ventilation is provided in the rooms that they
are located. Ventilation is also required to ensure complete combustion. The main
disadvantage is the large quantity of water vapour released during use as this can lead
to condensation problems.
Fixed gas convector heaters: these are normally room sealed heaters using a balanced
flue. Balanced flue heaters have the heating element sealed from the room and the air
intake and gas discharge flues pass through the external wall to the open air. The flue
is balanced, as changes in wind pressures affect both the intake and exhaust flues
equally and therefore, the balance of pressure inside the heater is not disturbed. These
heaters provide outputs in excess of 2kW.
It is very important that the location of balanced flue terminals is in accordance with
current Building Regulations.
Electric heaters
There are many types of electric heaters available for domestic space heating. These
include the following:
Oil filled electric radiators: a heating element heats the oil contained within the
radiator. Oil is used in preference to water as it has a quicker ‘heat up’ period and it
reduces corrosion within the radiator. Although termed radiator, the heat output is
approximately 30% radiant and 70% convection (see Figure 48.).
Figure 48.
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Natural convector heaters: a heating element warms the air which rises and circulates
by natural convection. Nearly all heat output is by convection with little radiant heat.
The air in the room takes a considerable time to reach the desired comfortable
temperature (see Figure 49).
Figure 49.
Forced convector heaters: these have a quicker response to heating rooms. They
produce strong air movements which may cause some people to feel uncomfortable.
Noise from the fan may cause problems (see Figure 50).
Figure 50.
Night storage heaters: these contain concrete bricks or blocks that have high thermal
storage properties. The blocks are heated during the night at a time of cheap off-peak
tariffs. The blocks hold the heat and emit it slowly during the day. Modern types are
fan assisted. However, lack of control of these heaters is the main disadvantage (see
Figure 51).
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Figure 51.
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Bar fires: these provide a good source of radiant heat and have a quick response time
in heating the room. The heat from the element is radiated into the room by a
parabolic reflector (see Figure 52).
Figure 52.
All the forgoing methods of providing space heating are termed local heating.
Central heating
Central heating is the term used to describe a heating system whereby, all rooms in
the dwelling-house are heated from a single source, usually a boiler. Heat is
distributed from the source via pipes or ducts to terminals in the rooms. Central
heating systems may be controlled either centrally or locally by thermostats.
We shall now consider some types of common central heating systems used today.
Low temperature hot water systems (LTHW)
There are many types and configurations of LTHW systems used to heat homes, and
indeed, LTHW is by far the most popular type of central heating system used today.
LTHW systems using radiators to emit heat is the most common method of domestic
space heating.
Water is heated in a boiler and circulated around the house in pipes that are concealed
under floors passing through radiators before returning to the boiler to continue the
cycle. Radiators emit only about 20% of the total heat output by radiation and the
remainder mainly by natural convection.
The most basic LTHW central system is the one-pipe system using natural convection
(see Figure 53).
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Figure 53.
In the one-pipe system water is heated in the boiler and as the water heats up it
becomes less dense and rises to the top boiler and also expands. The heated water is
forced out of the flow pipe to radiators. The heated water enters the radiator nearest
the boiler on the flow side, exchanges heat with the radiator and discharges at the
radiator outlet. The sequence continues in series and finally the water is returned to
the boiler to be heated yet again.
The system is comparatively cheap to install. However, the main disadvantage being
the progressive and considerable drop in temperature as heat is lost during exchange
with each radiator. This may mean that the temperature of the water entering the
radiators at the end of the series is not at the desirable temperature.
The most common type of LTHW system is the two-pipe system. This system
normally uses small bore pipes of 15, 22 and 28mm diameter to circulate the water via
radiators.
The distribution pipe from the boiler is known as the flow pipe, this pipe supplies the
inlet points of all radiators with relatively hot water. As the water passes through
radiators it exchanges heat with the radiator material and the water already contained
within the radiator. This means that the water leaving the radiator will be considerably
cooler than the flow pipe water temperature. In this case the water leaving the radiator
is taken into a pipe returning back to the boiler, known as the return pipe.
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LTHW systems will also provide all domestic hot water requirements, (see Figure
.54.).
Figure 55 illustrates a typical arrangement for a micro-bore two-pipe LPHW central
heating system.
Figure 54.
Boilers can be free standing on the floor or wall mounted and can use balanced or
conventional flues to remove the gases created by combustion. Boilers used in vented
systems will be fed from a small feed cistern normally located in the roof space.
The systems can operate on heating and hot water mode or hot water only mode,
using a programmer and timer. Hot water in the heating circuit normally has a pump
to improve circulation and efficiency whilst providing versatility of boiler location.
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Micro-bore systems
Micro-bore systems use pipes of 6,8,10 and 12mm diameter from manifolds to
radiators and back. In these systems the pipework from the boiler to manifolds is
conventional mini-bore pipework of 15, 22 & 28mm diameter. The small pipes allow
greater flexibility in distribution, such as, above floor distribution, however, frictional
resistance is greater. A typical system is shown in Figure 55a.
Figure 55 (a)
A typical micro-bore manifold is shown in Figure55 (b)
Figure 55(b)
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Sealed (unvented) systems
The systems we have considered so for are known as open vented systems, i.e. they
are open to the atmosphere. Overheated water in a vented system can be
accommodated by using the vent and expansion pipe as a point of relief, and
expanded water is accommodated in the same pipe. This acts as a safety device.
Sealed systems are unvented systems. These systems have become increasingly
popular, particularly when the system uses supply water direct from the mains, as this
reduces the amount of pipework and removes the need for storage and feed tanks.
The following components are essential features of sealed systems:
Strainers:
used to prevent debris such as silt from passing into the
system with the possibility of causing problems with valves
downstream.
Pressure reducing
valves:
used to reduce the inlet pressure to the working pressure of
the equipment used, and to maintain a constant flow rate to
the draw off points. The working pressure of the system is
usually two-thirds of the test pressure of the storage vessel,
and the pressure relief or expansion valve is usually set at
this pressure. The outlet pressure of the reducing valve must
be closely related to the pressure at which the expansion
valve will open.
Check valves:
used to prevent backflow of hot water into the cold water
system. They also prevent cylinder collapse ‘implosion’
which could occur if water in the storage vessel were
siphoned out by opening a cold water tap when the stopcock
is shut down.
Anti-vacum valve:
this is a further safeguard against ‘implosion’ which admits
air into the storage vessel if the pressure inside falls below
that of atmospheric.
Expansion vessel:
used to accommodate the expansion of water in the system
when it is heated and to prevent any operation of the
expansion valve which could lead to wastage of water.
Expansion valve:
used to relieve the pressure in the system due to the
expansion of water if for any reason it is not accommodated
in the expansion vessel.
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Other safety devices:
(i) thermostats
(ii) temperature- operated cut outs
(iii) temperature relief valves
Boiler types
As mentioned previously there are many types of domestic boilers available today.
Basically these may be classified as conventional, condensing and combination
(combi) boilers. Boilers are designed to run on any of the following fuel types:
•
•
•
•
•
Natural gas
Propane gas
Oil
Solid fuel
Electric
Conventional boilers
These are the most common type of boilers to be used since central heating became
popular. Fuel is burnt in the boiler and the heat is exchanged with water passing
through. Combustion gases are expelled via vertical (conventional) flues or balanced
flues (see Figure 56).
Figure 56.
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Combination boilers (Combi)
These are instantaneous water heaters, that is, they heat domestic hot water and
central heating water on demand. The principle of boiler operation is similar to the
conventional type. However, most come as sealed package systems and this provides
great savings in installation time and costs. In such cases, no storage provision is
required for cold or hot water. (see Figure 57 (a))
Figure 57 (a).
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A typical ‘combi’ boiler system is shown in Figure 57(b)
(Figure 57 (b))
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Condensing boilers
These are the most energy efficient type of boilers, as they recover heat from the flue
gases. This is done by increasing the surface area of the heat exchanger which
recovers extra sensible heat whenever the boiler is in operation (see Figure 58).
Condensing boilers do however, have to discharge the condensate collected into the
drainage system. Combination condensing boilers are available on the market.
Figure 58.
Solar heat supplement
In countries where there is a substantial amount of sunshine available throughout the
year it is possible to utilise solar energy as the primary heating and hot water source.
In Scotland and the rest of the UK, there is not sufficient sunshine to provide the
required heat for buildings throughout the year. However, solar energy can be
harnessed to supplement hot water and heating systems, particularly in the summer
months.
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The principle used in solar heating systems is to place glass fronted plates known as
solar panels, at an angle most suited to maximise heat gain. Water is pumped through
pipes welded to black metal plates. The heat collected is stored in a hot water storage
vessel to supplement the hot water or heating system. (see Figure5 (a) & (b.))
In Scotland most solar gain is acquired through south facing fenestration. This can
contribute a considerable amount of heat energy directly into rooms throughout the
year, but, of course, the amount of solar gain is weather dependant.
Figure59 (a)
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Figure59 (b).
Warm air central heating systems
These systems maintain desired temperatures by the controlled circulation of warm
air. Air is heated by exchanging heat with either water in a boiler or electric elements
in an electric heater and then forced out of the heater by a fan. The heated air is then
distributed into rooms via a system of ductwork terminating at grilles known as
diffusers at floor level. The cooler air that is displaced by the warm air may be
recirculated via return ducts where it mixes with fresh air extracted from outside
before passing through the heat exchanger once more.
A diagram showing a gas warm air system is shown in Figure 60 (a).
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Figure 60 (a).
It is important that diffusers are correctly positioned to ensure an efficient warm air
system. The aim is to produce maximum comfort by maintaining an even temperature
across each room by gentle, draughtless, air circulation.
Floor perimeter warm air diffusion is the most popular location for domestic buildings
and produces good results.(see Figure 61.) Low side-wall and high level may also be
used successfully if carefully designed. Most diffusers are capable of being closed off.
The positioning of diffusers is often influenced by the structure of the building and by
economics as well as air distribution considerations (see Figure60 b).
Figure 60 (b)
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Typical warm air ducting systems are shown in fig 60 (c)
Figure 60 ©
Advantages of warm air systems include the following:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Space saving
Fast response
Energy savings due to short heat-up period
Improved ventilation
Reduced condensation
No risk of water leaks.
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SPACE HEATING -TUTORIAL
1.
Describe the difference between local and central heating.
2.
Outline various types of domestic local heating.
3.
Describe factors that should be considered when selecting a heating system.
4.
Describe with the aid of a sketch an LTHW central heating system.
5.
Outline control instruments used with the above system and state the purpose
of each.
6.
Describe with the aid of a sketch an electric off-peak storage heater and show
the electric arrangements for such a system.
7.
Outline other suitable domestic central heating systems stating the benefits of
each.
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