
                CENTRAL HEATING CALCULATOR PRIMER
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
          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 that all estimation of this type requires a certain amount
       of judgement 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 a
       building 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.


                TEMPERATURE DIFFERENCE
                ~~~~~~~~~~~~~~~~~~~~~~
          The second factor which affects 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 1.5 to 2 for comfort. As this new
       air comes originally from the outside it must be heated 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.


                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 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.


                RADIATORS
                ~~~~~~~~~
          To maintain a constant temperature we need to work out the heat
       loss of 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 it's 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.
