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         This is simple introduction to the methods used to calculate the heat
         requirements of a building. If you already understand this then there
         is no need for you to read it. It is intended for those who have
         acquired the program with no previous experience and who otherwise
         might not understand the terms used in the program instructions. I
         suggest that anyone in this position should also visit their local
         library and obtain a book on the subject.

         Remember all estimation of this type requires a certain amount of
         judgement backed by experience as well as mathematical calculation.

         I shall use only Imperial units as these are used in the simple
         version of the Central Heating Calculator program.


                 BASIC PRINCIPLES

         The concept of space heating is to calculate how much heat room will
         lose to it's surroundings. Knowing this we then know how much heat we
         need to supply to maintain the chosen temperature, whether from
         conventional radiators or other means.

         This primer does not go into the practical aspects of installation.
         It is not concerned with types of radiator, radiator placement,
         boiler types, etc. It is only intended to give you a very basic
         understanding of a rather complicated subject so that you will be
         better able to use the program.


                 HEAT LOSS THROUGH SURFACES

         The main heat loss from a building is by CONDUCTION through the
         various surfaces in contact with the colder outside, ie. floors,
         walls and roofs. Warm air in contact with the inside surface of a
         wall transfers some of its heat to the wall. The wall conducts heat
         through to its outside surface and transfers it to the outside air.

         Some materials such as metals conduct heat well, others such as
         asbestos are poor conductors of heat. Before we can work out how much
         heat will be passed through the walls of our building we need a
         method to quantify the conduction properties of the materials used by
         builders. We call this unit the 'U' value. The lower the 'U' value
         the better the insulation properties of the material. The higher the
         'U' value the greater the amount of heat lost.

         Modern building regulations lay down strict limits for the maximum
         permitted 'U' values of surfaces to ensure that buildings conserve
         heat. Lists of the 'U' values of common building materials can be
         found in most books on the subject.

         It is obvious that we must also take the area of the surface into
         account. All other things being equal the heat lost through a surface
         is proportional to its area.


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                 TEMPERATURE DIFFERENCE

         The second factor affecting the amount of heat lost is temperature.
         The actual temperature is not important, it is the DIFFERENCE in
         temperature between the 'hot' and 'cold' sides which matters.

         Remember that in a building we also have internal walls. If all the
         rooms are at the same temperature then there is no difference so no
         heat is conducted. If one room is warmer than another then heat will
         be lost from the warmer room. This heat will become a heat GAIN in
         the cooler room and this must also be taken into account. This
         effect is unlikely to become significant unless you have rooms of
         widely differing temperatures.

         In a terraced or semi-detached house the 'party' wall is treated as
         an internal wall. In theory your neighbours house is at the same
         temperature as your own but you should not rely on this. Consider
         what happens if they go for a skiing holiday and turn off the
         heating. The temperature on their side of the party wall could drop
         by 30 degrees.


                 VENTILATION

         There is another consideration. The air in the building also needs to
         be heated. It might seem that as the air has no contact with the
         outside once it has 'warmed up' there will be no heat loss. In
         practice no building is 100% airtight, if it were the occupants would
         suffocate. Even a fully draught-proofed building must have some
         ventilation to avoid it feeling 'stuffy', and opening a door for a
         few moments will introduce a lot of new cold air.

         The amount of ventilation is measured in air changes per hour. The
         minimum for comfort is about 1, rising to 2.5 or 3 for a room such as
         a kitchen. Most living rooms need 1 to 2 for comfort. As this new air
         comes originally from the outside it must be heated from the outside
         temperature to room temperature.


                 HEAT UNITS

         I must now introduce the concept of a 'unit' of heat. In the same way
         that we might measure distance in feet or speed in miles per hour we
         need to be able to quantify heat. The standard Imperial unit for this
         is the British Thermal Unit or B.T.U.

         This is actually defined as the amount of heat needed to increase the
         temperature of one pound of water one degree farenheight. To return
         to our 'U' values a 'U' value of 1 is defined as transferring one
         B.T.U. per square foot for every degree farenheight difference
         between its two surfaces.


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                 THE CALCULATIONS

         You should now have enough knowledge to understand the calculation
         for the heat required by a simple room. There are six surfaces to be
         taken into account, floor, ceiling and four walls and we must also
         include air changes into our calculations.

         The heat lost through each surface is equal to;

                 Area  x  'U' value  x  temperature difference

         We must calculate this for each surface and add them together. We
         must then add the heat lost through ventilation which is;

             Air changes x Volume of room x Temperature difference x 0.02

         The number 0.02 is the amount of heat required to increase the
         temperature of 1 cu.ft. of air by 1 degree.

         There is another factor we must take into account. We have seen that
         the amount of heat lost through a surface is defined by the 'U'
         value. If a wall has a window then the 'U' value of the glass will be
         different from that of the surrounding brickwork. The calculation for
         a wall with a window becomes;


             (Area of wall - Area of window) x Wall 'U' value

                        plus

              Area of window x Window 'U' value

                        multiplied by

              Outside temperature - Inside temperature


         If this is getting too complicated don't worry. This work is all done
         for you by the program. All you need to do is feed it the
         information.


                 CENTRAL HEATING CALCULATOR PRIMER Page 4



                 RADIATORS

         To maintain a constant temperature we need to work out the heat lost
         from a room and then arrange to feed just enough heat into it to
         balance the loss. Not enough and the room will not reach the proper
         temperature, too much and it will become too warm.

         When selecting a radiator it needs to have a slightly higher output
         than the room actually requires. There are two reasons for this.
         Firstly we need a margin to allow for unusually low temperatures (or
         if we have made a slight error in our estimate!). Secondly we must
         consider 'warm up' time. When the heating system is first turned on
         we not only have to make good the normal losses we must also heat up
         all the air in the building and the structure of the building
         itself. A margin of about 10% is normally sufficient for this. The
         bigger the margin the faster your house will reach the required
         temperature.

         The margin allowed in radiator sizing also affects the boiler size.
         When starting from cold all the radiators will be full on and the
         boiler must be big enough to cope with the total heat output of the
         radiators. In fact it needs to be even bigger. Radiator output
         assumes that the room is at normal temperature. The rule about heat
         loss being proportional to temperature difference also applies to
         radiators so that if the radiator reaches working temperature while
         the room is still cold its output will be increased. All boilers are
         at their most efficient when working near their maximum output.
         Boiler life is also shortened if the boiler is constantly switching
         on and off because it is too big. This means that if you allow a
         large radiator margin not only will the initial cost of your
         radiators and boiler be greater but the boiler efficiency and
         longevity will suffer.

         As always the actual size is a matter of judgement. Around 15%
         greater than the total B.T.U. required by the building will normally
         be adequate.

         So far I have not mentioned hot water supply as it is beyond the
         scope of this primer but it must be taken into account when
         considering the boiler size. It is usual to add 5,000 to 15,000
         B.T.U. to the boiler size if hot water is required. The exact amount
         will depend on how much hot water is used and how quickly it needs to
         be replenished.


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