Selecting the emitters for water central heating
Radiators, as the term is normally used, are simple heat exchangers which distribute the heat by natural air circulation (hot air rises, so the heated air next to the surface of a radiator rises pulling cooler air up from the floor level). They are simple (very little can go wrong), easy to install and operate.
It is worth remembering that any room can have more than a single radiator, with rooms greater than 6 metres (18 ft) in any one direction, it is worth considering distributing a number of radiators to minimise the thermal gradient within the room.
Normally the manufacturer’s data sheet will quote the output for when there is a temperature difference of 56 °C (100 °F) between the water in the radiator and the air in the room. Where the temperature difference is not 56 °C, the following correcting factors are necessary to determine the actual anticipated output from the radiator.
Design temperature difference

Correcting
factor 

°C

°F


50

90

0.87

53

95

0.94

56

100

1.00

58

105

1.07

61

110

1.13

64

115

1.20

67

120

1.27

Where the outputs quoted on the manufacturers data sheet are based on temperature differences other than 56 °C (100 °F), the figure used should also be quoted on the data sheet as should the appropriate corrective factors. Always check these figures and adjust the quoted output figures before selecting a radiator.
Sizing heat loss from a building
The basic principles
The idea behind sizing is twofold, firstly to establish the heat requirement of each room and then (by adding all the room requirements together) to establish the total heat requirement for the property – this page is written around a domestic resistance but the principles can be adopted for any sort of building.
Thermal energy moves from a temperature to a lower one, the bigger the difference, the faster the transfer. As well as the temperature difference, the material through which the energy moves also affects the speed of thermal transfer. Each type of material has its own particular rate of thermal conductivity, this is usually referred to as it’s ‘kvalue‘ and is quoted in Watts per square metre/degree Celsius difference for a one metre thickness of material (or in imperial as BTU per square foot/degree Fahrenheit for 1 inch thickness).
For different building materials/methods, the thermal conductivity is quoted as U values, these are calculated from the kvalue of the materials used taking into account the thickness of each material and material combinations. U values are quoted as in metric or for imperial. This makes life easy as a single figure is available for calculating thermal loss through common construction methods.
From the above, it can be seen that to calculate thermal loss from a room, it is necessary to establish:
 The temperature difference across each wall, ceiling and floor (this is done by defining the required internal temperature and the anticipated outside temperature).
 The type of construction of the walls, ceiling and floor, from this the appropriate U values can be established.
 The area of each material.
There is one more factor to take account of, namely Air Changes. No matter how well a room is draught proofed, there will be a certain amount of natural air change – if there were not, people would tend not to live long as all the oxygen would be taken out of the air! Air changes are quoted as so many changes per hour (i.e. the complete volume of air in the room changes so many times).
The figures:
Room temperatures

°C  °F 
Lounge  21 
70

Dining Room  21 
70

Bedsitting Room  21 
70

Bedroom  18 
65

Hall and Landing

16 
60

Bathroom  22 
72

Kitchen  18 
65

WC  18 
65

Temperature difference
The temperature difference should not be thought of as just between the inside of a property and the outside temperature; the differences between adjacent rooms should also be considered. In the , the recognise temperatures for domestic rooms are given in the table to the right.
The temperature outside of the property is normally taken as 30° F (1°C) which is supposed to represent the normal lowest winter temperature.
U/R values
Typical U/R values for some of the more common types of construction are given in the included on this site. Please see below
Area of material
The area of each type of material used, is simply measured. Where a window or door is included on a wall, just measure area of the whole wall, and take away the area of the window/door. For metric calculations, measure the areas in square metres whilst for imperial use square feet.
air changes per hour


Lounge 
1

Dining Room 
2

Bedsitting Room 
1

Bedroom 
0.5

Hall and Landing 
1.5

Bathroom 
2

Kitchen 
2

WC 
1.5

Air changes
Again in the there are recognised Air Changes for each room (see right). Where the rooms have not been draught proofed, the number of air changes should be increased.
With these air changes, it is necessary to calculate the energy required to heat the volume of air by the temperature difference. To heat one cubic metre of air by one degree Celsius required 0.36 watt (or for imperial 0.02 BTU to heat one cubic foot one degree Fahrenheit).
So the energy loss is simply:
the appropriate watt or BTU figure x volume of the room x number of air changes x temperature difference
It is normal to take the temperature difference as being the design temperature of the room and the outside temperature 30° F (1°C).
To be on the safe side, a universal figure of 3 air changes per hour can be used with confidence.
Aspects not included
The above calculations do no take into account thermal energy introduced within the property. The two main contributors are the people occupying the property and the heat generated by cooking and use of hot water for washing etc. In general these contributions can be ignored.
Estimating for the domestic hot water supply.
The size of hot water tank will vary between houses, larger houses having larger tanks as it is assumed that the need for hot water is greater. For this exercise, a 120 litre tank is assumed.
First a design reheat time needs to be established, this normal assumes that all the hot water has been used and the tank is filled with cold mains water. Three hours is probably reasonable in most cases although the ‘life style’ of those living in the house may cause a shorter or longer time to be chosen.
In the , mains cold water is generally assumed to be 4°C whilst ‘hot water’ for a domestic system is usually around 60°C (this will avoid scalding). So the cold mains water needs to be raised by 56°C within the hot watertank.
As 1 litre of water needs about 1.16 watts to raise it through 1°C in an hour, a 120 litre tank of cold mains water needs a total energy input of about 7800 wattshours (120*1.16*56) to raise its temperature to the required level. With a 3 hour design reheat time, this means a power input of about 2600 watts.
The calculations are simply adjusted to take account of different tank sizes and/or the design reheat heat time but don’t mixed up the imperial and metric units (i.e. gallons, degrees F with litres, degrees C). .
The final hourly energy requirement figure is then simply added to the figure calculated separately for the central heating energy requirement, the sum of these two figures, gives the required output for the boiler.
Sizing the pipes
Once the positions of the radiators and pipe runs have been determined, the size of the pipes must be calculated. For this, the pipe runs must be broken down into individual runs between radiators etc. starting furthest from the boiler and working back towards it.
U values of typical building methods
U values (overall coefficient of heat transmission) indicate the heat flow through materials – the higher the figure, the higher the heat loss. While the values do vary for each particular material and method of construction, the following table gives general figures for some common modes of construction.
is the imperial unit for quoting U values while is the metric (SI) unit. When using these figures, make sure that the appropriate units for area and temperature difference are used.
(If you have any U values for other types of construction, email us and we’ll add them)
Wall (outer)  BThU per sq ft, degree F  watt m2, degree C 
9″ solid brick  0.38  2.2 
11″ brickblock cavity – unfilled  0.18  1.0 
11″ brickblock cavity – insulated  0.10  0.6 
Wall (internal)  
plaster, 4.5 inch brick, plaster  0.39  2.2 
plaster, 4 inch heavyweight block, plaster  0.44  2.5 
plaster, 4 inch lightweight block, plaster  0.22  1.2 
plasterboard, 4 inch studding, plasterboard  0.32  1.8 
Floor (Ground)  
solid concrete  0.14  0.8 
suspended – timber  0.12  0.7 
Floors (Intermediate)  
Plasterboard/ 8 inch joist space/ T&g boards – heat flow up  0.29  1.7 
Plasterboard/ 8 inch joist space/ T&g boards – heat flow down  0.25  1.4 
Roof  
pitched with felt, 50mm insulation  0.10  0.6 
pitched with felt, 100mm insulation  0.05  0.3 
flat, 25mm insulation  0.16  0.9 
flat, 50mm insulation  0.12  0.7 
Window  
wooden/uvpc frame, single glazed  0.88  5.0 
wooden/uvpc frame, double glazed  0.51  2.9 
(the post April 2002 standard)  
wooden/uvpc frame, double glazed – 20mm gap, LowE  0.29  1.7 
metal frame, single glazed  1.02  5.8 
Door  
external solid timber  0.42  2.4 
external glazed  treat as glazing  
internal  assume the same as the wall 
Note: for conversions
 multiply BThU, ft 2, degree F by 5.675 to convert to watt m2, degree C or
 divide watt m2, degree C by 5.675 to convert to BThU, ft 2, degree F
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