Efficient Heating of Dwellings

New Zealand Journal of Agriculture, Volume 94, Issue 3, 15 March 1957, Page 293

Efficient Heating of Dwellings

By

H. W. T. EGGERS,

Engineer, Department of Agriculture, Wellington

LJUMAN beings can work and live with comfort and efficiency only within a restricted ■ ■ set of physical conditions, which include temperature and humidity. As new -facilities for space heating are now available, this article has been prepared to enable the most to be made from the application of heat as an aid to comfortable living.

/GENERALLY the temperature at which most human beings are most comfortable is between 60 degrees and 70 degrees F., but this depends on many other factors, such as humidity, clothing, air movement, and whether the individual is doing manual work or resting. Humidity has a very great effect on human comfort and health. Excessive humidity makes it very difficult for human beings to get rid of the heat produced in their tissues. This heat is normally largely dissipated by evaporation from the surface of the

skin. This occurs easily when the humidity is 50 per cent, and lower, but occurs . with increasing difficulty as the humidity rises to 100 per cent. Heat The unit of volume of heat .is the British Thermal Unit (commonly known as 8.T.U.). Simply expressed, this is the amount of heat required to raise the temperature of 11b. of water by 1 degree F. Every human being produces some 450 B.T.U. of heat every hour and unless this heat is dissipated, the body temperature rises.

The B.T.U. can be expressed in everyday things by the following example: If a kettle of water contains 2|lb. (2 pints) at 62 degrees F., to raise it to boiling point (212 degrees F.) will require 2J X 150 = 375 B.T.U. Therefore in round, figures 375 B.T.U. are needed to raise an ordinary kettle of water to boiling point. If the measurement of heat volume is understood, the merits of various space-heating appliances can be readily assessed. With oil-burning apparatus the heat output is usually expressed in 8.T.U., but with most of the solid-fuel equipment available in New Zealand, heating capacity is usually expressed in cubic feet of space effectively warmed. This is not a satisfactory method of indicating heating capacity, because the volume of air warmed and the temperature at which it is maintained will depend largely on insulation and air movement. However, where the heating capacity is expressed in

volume of air heated, the output in B.T.U. is roughly double the volume given for “background warmth” or four times the volume for “fullheating” rating. Though B.T.U. output calculated in this way will be very approximate, it is sufficiently accurate for comparing one appliance with another. Various fuels have stored chemical energy which, when converted into heat by combustion, will produce varying amounts of heat, depending on the amount and the composition of the combustible material they contain. The total number of B.T.U. which can be obtained from the combustion of any material is known as its calorific value; that is, the heating value of the fuel expressed in B.T.U. The B.T.U. is also used to express the heat-trans-mission value (U factor) of materials. This is the amount of heat (expressed in B.T.U. per square foot per degree difference of temperature per hour) which will pass through any material. The heating value of gas is measured by the “therm”, which is the ordinary commercial unit of heat and is equal to 100,000 B.T.U. Appreciation of what the British Thermal Unit is can be of great help in assessing the relative merits of materials or apparatus connected in any way with the production or movement of heat. Table 1 gives the calorific value of various . fuels and Table 2 the heattransmission value of various materials. The B.T.U. is also the basis for measuring the efficiency of heating apparatus for converting fuel into available heat. . This is known as “thermal efficiency” and is the ratio between the amount of heat made available by the apparatus and the potential heating (calorific) value of the fuel. An efficiency of 10 per cent, means that 101 b. of fuel will have to be burnt to obtain the calorific value of lib. as useful heat. For example the calorific value of good coal is 13,000 B.T.U. per pound (Table 1). To make this amount of heat available' from an open fire with a thermal efficiency of 10 per cent, requires the burning of 101 b. of coal. Heat Transfer There are three ways in which heat can be transferred from one body or substance to another: By radiation, convection, and conduction. Radiation of heat is the same as radiation of light. A luminous body radiates light in all directions and the intensity of light received per square foot varies inversely as the square of the distance of the object is from the source of light. This applies also in radiation of heat when a small surface only is the radiating source. Pure radiation can exist only in a vacuum, since whenever, it occurs in air it is always combined with some degree of convection. Convection is the transference of heat by particles of air or other fluid

body coming in contact with a hot surface, absorbing heat, and passing on and delivering this heat to some other body colder than itself. For example, when a kettle is put on a fire, the bottom is heated and cold water particles in contact with this surface are warmed and rise to the top, releasing some heat to the colder water particles in their passage. The moving particles are replaced by other water particles and this sets up a circulation which continues as long as a fire is maintained under the kettle. Conduction is the transfer of heat through material. For example, when the temperature on the inside of a brick wall is raised, heat travels through the wall to a lower temperature outside. Different materials have different rates of transferring heat. Metals and all very dense and heavy materials are usually good conductors; light and porous materials are usually bad conductors. When a material has a very low ability to conduct heat it becomes a good insulator. Example of Heat Transfer An everyday example of heat transfer occurs in the ordinary domestic refrigerator. In this machine expanded gas at low temperature is circulated through tubular coils inside an insulated cabinet. As the temperature of the gas is much lower than that inside the cabinet, heat will flow from the cabinet to the gas by conduction through the walls of the coils containing the gas. The circulating gas as it absorbs heat is carried away and replaced by more cold gas, which absorbs heat in its turn, maintaining an even cooling cycle. The warmed, expanded gas is drawn away from the coils in the insulated cabinet and compressed. This raises the temperature of the gas by concentrating the heat into a smaller volume of gas, the gas now being warmer than normal air temperature. This warmed gas is again passed through tubular coils outside the insulated cabinet and the heat flows from these coils into the lower temperature of the atmosphere. The compressed gas having given up its heat is returned and expanded to absorb more heat. The walls of the insulated cabinet are filled with a material which has a high resistance to the passage of heat by conduction and so prevents the flow of heat from the outside to the inside of the cabinet. Heating of Dwellings In temperate climates such as New Zealand’s the problem is how to attain comfortable warmth in dwellings during the cold periods of the year with the least cost in fuel or labour. This problem goes deeper than just supplying a source of heat, as the amount of heat required will depend on the degree of the insulation of the walls, roof, and floor of the dwelling. The efficiency with which fuel is used will

dictate the amount of heat which can be made available from any source. Under these circumstances good heating will depend on good insulation and ventilation, good heat distribution, and economic conversion of fuel to heat. Insulation Though New Zealand has a relatively moderate climate, insulation of dwellings is not given the consideration it should receive to produce more comfortable living. Insulation ensures warmth in winter and coolness in summer with the least expenditure of energy. With dwellings . general building practice relies : on a cavity wall to effect some degree of insulation, but building materials are not usually considered as much for their insulating qualities as for their ease of handling and suitability as structural materials. Excessive cost may preclude the use of some insulation materials, but reasonably good insulation can be obtained at very little extra cost if discrimination is used in the choice of materials and in design. When the outside temperature is low, the natural flow of heat is from the inside to the outside, or vice versa when the outside temperature is the higher. Walls designed to retard this, heat flow and constructed of good insulating materials considerably reduce the heat losses from the inside when the outside temperature is low. This means that less heat is required to maintain comfortable temperatures inside when the outside temperatures are low. Few people would tolerate a leaking water tank, yet many are content to construct dwellings which “leak” heat as a perforated tin leaks water. Materials are now available with good insulating qualities which, if incorporated in concrete or plaster, still retain their insulating qualities. Internal lining boards with very high insulating properties are available, as also are insulating materials which are suitable for filling cavities in hollow walls or placing between ceiling joists. Insulating material is available in the form of board, granular powder, slabs, or padding suitable for any structural application. Structural design can also contribute to better insulation. With cavity (hollow) walls, which is the usual construction for dwellings, the passage of heat from the inside to the outside is by conduction through the inner and outer skins and by convection inside the cavity. That is, heat passes through the inner skin by conduction, is carried to the outer skin by convection, with the moving air inside the cavity acting as a vehicle, and passes through the outer skin by conduction. Air is an excellent insulator if it can be restrained from moving. If air

is contained in a cell or pocket so small that it is prevented from circulating, it cannot transfer heat by convection. The best insulating materials are, therefore, those which contain the most air in the smallest pockets. The more restraint which can be put on air inside wall cavities the greater will be the degree of insulation obtained. On the other hand, with present conventional methods of construction, air circulation in cavities is essential to ensure dryness. These two conflicting aims can be reconciled by the employment of a double cavity, an outer one in which air is permitted to circulate to ensure dryness and an inner one in which the movement of air is restrained as an aid to better insulation. This is all a question of design and cost; if designed correctly, double cavity walls can be provided at a little extra cost, which is amply repaid by the added insulation obtained. In dwelling construction, unless expensive precautions are taken, some heat loss through windows and by air circulation under doors is inevitable. However, provided the total heat loss can be reduced to a minimum compatible with economy, the maintaining of a comfortable temperature inside the building is neither difficult nor uneconomic. Ventilation Humidity plays an important part in comfortable living. For example, if the air inside a dwelling is heated to a comfortable 60 degrees F. and maintained at that temperature - without any air change, the contained moisture would be lost by condensation and the air would become so dry that conditions would become uncomfortable. Equipment to maintain both temperature and humidity at comfortable levels by mechanical means is too costly to be considered for the average dwelling, so other methods must be used to maintain comfortable humidity. This can be done by ventilation, that is, by permitting frequent air change to occur with the least interference with temperature. . With the oldest method of dwelling heating, the open fire, this air change takes place automatically, as the heat of the fire causes a flow of air up the chimney which is replaced by air admitted under doors and round or through windows. If the heating system for a dwelling is designed in conjunction with the means of ventilation and with due regard to insulation, very satisfactory results can be obtained and the inside of the dwelling can be kept comfortable, both as regards temperature and humidity, as long as the outside temperature remains low. Heating Appliances Heating appliances may be divided into two groups: Those associated with

a central system and those where local heating in the various rooms of a building occurs. The former includes hot-water or steam-radiation systems, whether supplied with heat produced from electricity or any of the other fuels, and heated-air circulating systems, where the air is conveyed by duct and fan. The latter include the open fireplace, the various types of stoves, gas fires, electric radiators, or portable, liquid-fuel heaters. The first group is not generally applicable to domestic heating under New Zealand conditions, but as there is heating apparatus which combines

some features of both groups, central heating will be touched on in the following discussion of appliances. Open Fireplace Some people prefer an open fireplace, principally because of its cheerful appearance, but its disadvantages should be realised. The open coal fire, unfortunately, has a very low thermal efficiency of about 10 to 15 per cent., which means that from six to nine times as much fuel as is necessary is being burnt. Actually its efficiency is even worse than this, because an open fireplace as normally constructed does not

smoke out the room only when the chimney shaft is a good deal bigger than is theoretically necessary. A great deal of air has therefore to be discharged up the chimney, and this air has to be replaced with cold air which finds its way into the room through the porous walls and through cracks in doors and windows. If this replacement of air did not occur, the fire would go out. The heating of all this additional air from the outside temperature at which it enters to the temperature of discharge from the fireplace represents a large amount of additional heat which the fire has to provide and which is all wasted in excess ventilation. This additional air movement may be as much as 20,000 cub. ft. per hour. It may be argued that this excess ventilation will ensure a freshness of atmosphere and an adequate air change and so avoid stuffiness. This may be true, but who is not familiar with the situation when a door or window is on the side of the room remote from the fireplace: a person sitting in front of the fire has his face scorched by the fire while his

back is subjected to cold draughts. The position is usually only aggravated when more fuel is put on the fire to offset the draughts. Of course the draughts can be eliminated largely if the windows are really close fitting and if the air comes in from an opening under a door on to a corridor or lobby which is itself heated by radiators or Otherwise so that the incoming air is already warmed. Then the fire acts mainly as a centre of attraction focus for the social gathering and a source of radiant heat additional to the basic warming of the room. The only heat available from an open fire is that conveyed by radiation, unless steps are taken to transfer heat by installing a water jacket in the fireplace. Tire open fireplace has been discussed at some length, as the range of solid-fuel space-heating appliances covers various stages between the fully open fire and the totally enclosed heating stove. These will be discussed in the order of their stages. Solid-fuel Appliances The basic design of all solid-fuel space-heating appliances centres

round one or more of the following features: —• 1. The control of combustion by the control of air to the fire. 2. The utilisation of heat by convection. 3. The control of flue velocities. Designs based on these features have produced heating appliances with greatly increased thermal efficiency; that is, much more heat is made available from the same quantity of fuel or inversely less fuel is required to produce the same quantity of heat. Stage 1 Heaters For those who prefer the aesthetic appeal of an open fire to the totally enclosed heater but who also require better control of heat there are available types of fireplaces which control combustion by the control only of air to the fire. By fine regulation of the air available to the fire from under the ‘ grate the rate of combustion can be controlled fairly closely, as this air

supplies the oxygen which sustains combustion.

If air circulation through the fire is not permitted, the fire will go out, so under-grate air control can produce any rate of combustion up to the completely free combustion of the open fire. A fireplace with under-grate control may not be any more efficient in supply of heat from fuel used than an uncontrolled open fire, as heat is still conveyed by radiation only. However, as combustion is slowed down the amount of cold air drawn through the room and passed up the chimney may also be decreased. A stage 1 heater is illustrated on page 293.

Stage 2 Heaters

There is also available a fireplace combining control of combustion, utilisation of heat by convection, and control of flue velocities, but at the same time having the aesthetic appeal of an open fire. A fireplace of this type is shown on page 297.

In this type the firebox is brought forward a few inches and an outer surround provided to permit induced circulation of air up the sides of the firebox and over a portion of the top of the throat plate. . Combustion is controlled by under-grate air control as for stage 1 and heat additional to that given off by radiation is made available by convection with air flow round the protruding firebox.

The throat of. this fireplace is provided with an adjustable damper which can restrict the smoke outlet to an area of about 4 sq. in. This limits very severely the amount of cold air which will be drawn through the room and passed up the chimney. This not only increases the heating efficiency, but eliminates most of the cold draught through the room, which is one of the most unsatisfactory features of the conventional open fire.

The efficiency of these fireplaces is approximately 35 per cent, under normal burning conditions, . but, whereas the open fire requires as much as 20,000 cub. ft. of air per hour, this fire requires only 2000 to 2500 cub. ft. per hour. .

Stage 3 Heaters

There are a number of various makes of space-heating stoves which can be classed as stage 3 heaters, as all are provided with doors to the firebox, which can be opened to give some of the comfort of the open fire. Types of these stoves are illustrated on page 299.. These all have control of combustion by the control of air to the fire and utilisation of heat by convection. Control of flue velocities is provided by total enclosure rather than by the use of dampers or a restricted throat.

These stoves consist roughly of two boxes, one inside the other, the space

between the inside firebox and the ventilated outer surround being used for the transfer of heat by convection. All these stoves protrude for their full depth in order to utilise all the sidewall heat for transfer by convection. Combustion is controlled by undergrate air control, and the entrance of cold air directly into the flue is not possible when the doors are closed. With the doors closed the amount of heat available by radiation is relatively small in comparison with the fireplaces of stages 1 and 2, but a very much greater amount of heat is transferred by convection. This transfer of heat by convection is excellent for space heating of dwellings, in which ceilings are not usually very high, as the heated air circulates upward from the stove toward the ceiling and returns at lower levels to the stove. The stove does not induce cold draughts, but keeps one room pleasantly and evenly warmed. If doors to the room are left open to allow warm air to circulate, the warm air will take the chill from the whole house, though its efficiency in this respect will depend on the design of the house and position of the heater. Models of most of these stoves can be obtained with hot-water boilers incorporated for connecting to hotwater storage cylinders. This, though reducing the heat output available for space heating, does reduce electric water-heating costs, and when the stove water-heating system is coupled with electric water-heating ensures a constant supply of hot water within the capacity of the storage provided. The , efficiency of these stoves, examples of which are shown on page 299, is about 50 to 60 per cent., and air consumption is about 200 cub. ft. per hour. Stage 4 Heaters Stage 4 space heaters (see illustration on page 301) are totally enclosed. Doors are provided for admission of fuel only and the stoves will not operate effectively if the doors are left open. The fronts or doors of these stoves are usually provided with transparent windows, as are stage 3 stoves, so that the fire is visible, but the amount of heat available by radiation is very small, nearly all heat .transfer being by convection. Instead of circulating and warming internal air, some models can be arranged to draw in air from outside. This is heated and passed into the room. If the necessary controls are provided, circulation of outside air can be arranged as an alternative to internal circulation. This arrangement is an advantage, because, comfortable humidity can be obtained without the need for admission of unheated outside air through normal ventilation channels. . Stage 4 heaters embody combustion control by control of air to the fire

and by control of flue velocities, the latter being obtained either by an uptake damper or a restricted throat. They rely almost entirely on convection for heat transfer and accordingly are designed with as large a heatreleasing surface as possible in the heat-transfer chamber. Efficiency is in the vicinity of 50 to 60 per cent, and air consumption is over 200 cub. ft. per hour. Oil Heating All the space-heating equipment so far mentioned is designed to burn solid fuel. Other equipment suitable for domestic installation is available which uses oil fuel. Oil and electricity may be considered the ideal sources of heat, as both can be subjected to automatic control. Once switched on and given a constant supply of power or fuel, equipment using them will maintain internal air temperature at constant levels, irrespective of outside variations unless temperatures inside and outside vary so greatly that heat loss through the fabric of the building exceeds the capacity of the heating plant. Oil-burning space heaters can be arranged to circulate internal air, external air, or both, and burners only can be obtained for converting existing convection heaters or water heaters to oil firing. The principle of heat transfer is the same as that employed in stage 3 and stage 4 solid-fuel stoves in that the walls of the firebox or combustion chamber. transmit the heat to the circulating air passages. Flue velocity plays an important part in oil firing and special attention is given to ensure that this is suited to the particular installation. Though manually controlled oil burners can be obtained for incorporation in space heaters, all burners are arranged for electric control so that firing can be started or stopped by moving a switch. Thermostats placed at strategic points and pre-set to the temperature required regulate the flow of oil in accordance with the demand for heat. Like all apparatus oilburning space heaters require periodical maintenance to keep them functioning at highest efficiency. A great advantage of oil-fired heaters is that they can be placed below floor level with a grating to provide air circulation. If the heater is below floor level on a wall line, hot air circulation. to two rooms simultaneously can be provided. As oil is burnt (after it has vaporised) as- a gas with a mixture of air, the thermal efficiency of oilburning apparatus is generally higher than that of solid-fuel apparatus. The efficiency ,of well-designed oil-burning equipment may be as high at 80 per cent.; that is, 80 per cent, of the calorific value of the fuel can be made available as useful heat.

Summary of Appliances The great advantage of heating appliances of stages 2,3, and 4 is that they can be left burning at a reduced rate for long periods provided there is sufficient fuel in the firebox. It is possible to arrange all-night burning for stage 1 fireplaces, but not nearly so readily or as safely as can be done with stage 2 fireplaces or stage 3 and 4 stoves. In any case there is little advantage in maintaining a fire in a stage 1 fireplace unless people are near the fire, as little heat transfer by convection occurs. Heat transfer by convection ensures that heat is carried to wherever the warmed air can circulate, so that some .warmth can be maintained throughout a house during a cold night if a heating appliance which distributes its heat by convection is kept burning. By bringing the source of heat under more control than is available with the open fire and by arranging heat transfer by convection rather than radiation, much more heat is available from the fuel consumed and the available heat is used more effectively for the purpose for which it is required— an aid to comfortable living. Choice of Heating Appliance Though there is not a great variation in the thermal efficiencies of the various makes of space , heaters in the different stages or classes, each possesses features which, though not influencing efficiency, affect convenience of operation and appearance. When a decision has been made on the general type of heater required choice of make or model will depend more on heating capacity and appearance in relation to cost than on efficiency. As efficiency varies with the type more than with make, the claims, both for efficiency and capacity, for .some makes may be extravagant. The efficiencies given for the various classes can be used as a reasonable check for any heating appliance within .any class. fuels Wood The choice of fuel for space heating will depend on availability and cost. Many farms have an abundant supply ■of firewood, which for consumption in a space heater of any type requires only to be cut small enough to fit into the firebox. As the calorific value of. wood is : about half that of good coal, about twice the weight of wood will be required for equivalent heating. However, by combining a fuel of low calorific value, such as wood, with a fuel of higher calorific value, such as coke or briquettes, a heating value ■ can be obtained which is an average ■of those, of the two fuels in the proportions in which they are used. This .is a good practice with fuels which

are difficult to ignite, such as coke, as the more readily combustible wood induces thorough ignition of the coke. Coal The heating qualities of coal can vary considerablyfrom 9250 to 14,650 B.T.U. (see Table' 1) —because coal varies greatly in composition. Anthracite coal is rich in pure carbon, whereas bituminous coals contain a high percentage of hydrocarbon. Fuels containing relatively large amounts of water have a lower calorific value than fuels containing less water, as is shown in Table 1 by the calorific values for wood and peat with 20 per cent, of moisture and the same materials dry. This difference is due to the water vapour escaping during combustion and taking its latent heat with it. Anthracite is more difficult to ignite than bituminous coal, as, it has to be raised to a fairly high temperature before it will support combustion, whereas bituminous coals at a much lower temperature give off gases, such as methane, which are readily ignited. Bituminous coals are therefore more suitable for fires in open grates and anthracite for stoves and . boilers. Bituminous coals if used in stoves or boilers produce much more smoke than does anthracite. The ash content of coal varies considerably, being as little as 2 per cent, in a. good anthracite, 4 per cent, for Westport steam coal, 5 per cent, to 8 per cent, in a good bituminous coal, and as high as 20 per cent, in slack.

Coke The coke generally available in this country is gasworks coke, which is produced when a bituminous coal is heated at the gasworks in closed retorts. The volatile gases pass out and, after purification, are collected in the gasholders for distribution. The non-volatile residue, of the coal, which is dumped together with the ash, is cleaned and broken down into various sizes for domestic and industrial use. As coke burns readily with a smokeless flame when once ignited, it is excellent for space heating and can also be used in an open grate after the fire has been thoroughly set going with coal or wood. An advantage of coke is that as usually it contains very little dust, the air passing through the fire bed will pass with. less resistance and therefore a lower draught than when coals containing dust are used. The ash content of coke is usually about 5 per cent, to 8 per cent, unless it is a low-grade or breeze coke, when the ash content may be as high as 20 per cent. Briquettes Briquettes are manufactured fuel in which coal which is carbonised to char is compressed with a, binding agent such as'pitch into oval pieces. . The shape and freedom from dust result in an ideal fuel with high calorific value and low ash content. The calorific value is a combination, of that of the coal char and binding agent. This fuel is very suitable for burning in any apparatus.

Oil Distillate oil, such as diesel fuel, when used in burners designed to ensure an abundant air supply and efficient vaporisation burns with a relatively smokeless flame. . With highly efficient heat-transfer arrangements a large proportion of the heat generated can be made available and, as oil has a high calorific value, much more useful heat is produced than is possible from a similar weight of any solid fuel. The great advantage of oil fuel is that heat is generated at a constant rate in proportion to the fuel supply and no firing is. required, as fuel supply is automatic. Gas There is a great deal of sulphur in most bituminous coals and most of this passes out during the distillation of gas. Gas containing any sulphur if burnt in a room under conditions when the products of combustion are not entirely removed, has a deleterious effect { on human health and on plants, fabrics, leather upholstery, the binding of books, and other materials. However, the danger of sulphur in gas has been considerably reduced owing to improved techniques at gasworks, but it is of some importance with flueless gas heaters, from which the products of combustion are discharged into the- room. The great advantage of gas heaters is that they are almost 100 per cent, efficient. Where there is a good air change their use should not be detrimental. Gas heating is very convenient and in any comparison between it and heating with solid fuels due weight should be given the labour-saving advantages of gas. Cost of Fuels The criterion of domestic space heating is to obtain as much heat as possible with least cost in fuel and greatest convenience. Though initial cost, aesthetic considerations, and other factors may influence the choice of a heating appliance, the cost and availability of fuel are important aspects. The average home of, say, 1100 sq. ft. floor area with a stud height of Bft. 6in. contains about 9300 cub. ft. of air. If insulation is reasonably efficient, approximately 37,200 B.T.U. per hour will be required to maintain a comfortable temperature throughout the house. This amount of heat, though within the capacity of space-heating oil-burning appliances, owing to their high efficiency, is generally beyond the range of the usual domestic space heaters which burn solid fuel. For this reason the home owner usually

compromises by heating part of the house comfortably and probably taking the chill off the remainder by warm-air circulation. Such a compromise can be made in the average New Zealand home by providing a stage 3 heater which, if required, can be used as an open fire and the capacity. of which is given as 5000 cub. . ft. of space effectively warmed. The effective heat available from a space heater with a heating rating of 5000 cub. ft. is approximately twice its volume rating in 8.T.U., which in this case is 10,000 B.T.U. per hour. On this basis, and allowing a thermal efficiency of 60 per cent, for a stage 3 heater, a comparison of fuel costs can be made.

Assuming that a small fire of about 71b. is started and combustion controlled to give the required heat output of 10,000 B.T.U. per hour, the following would be the quantities of various fuels required to produce this heat output when due allowance is made for thermal, efficiency and calorific value: — '

lb. Anthracite coal .. .. .. ■ . . 1.13 Bituminous coal .. . . . . 1.28 Coke .. 1.38 Briquettes .. .. .. . . 1.22 Wood (20 per cent, moisture) .. 2.97

The consumption of oil fuel to produce the same heat with an oil-fired appliance having an efficiency of 80 per cent, would be 0.63 pint.

The cost of fuel to produce 10,000 B.T.U. per hour with a stage 3 heater to warm 5000 cub. ft. of space effectively would be as follows:

Cost of fuel for 10,000 *Cost B.T.U. per Fuel per lb. hour d. d. Anthracite coal .. 2.05 2.316 Bituminous coal .. 0.88 1.126 Gas coke .. ..1.0 1.380 Briquettes .. .. 1.66 2.025 ■' Wood .. .. 1.1 3.267 Oil (per pint) .. 2.31 1.455

Comparative costs with gas or electricity as the source of heat would be: —

Gas (per cub. ft.) .. 0.12 2.664 Electricity (per unit) .. 0.7 2.044

Approximate calorific Value B.T.U. per lb. Coal, anthracite . . .. 14,650 Coal, bituminous .. 13,000 Coal, poor quality . . .. 9,250 Coke, gas .. . . .. 12,000 Coke, breeze .. . . .. 10,000 Coke, furnace .. . . .. 13,500 Briquettes ... . . .. 13,500 Wood, 20 per cent, moisture 5,600 Wood, kiln dried .. .. 8,000 Peat, 20 per cent, moisture .. 6,500 Peat, kiln dried .. 10,000 Charcoal from wood, dry .. 13,000 Charcoal from peat, dry .. 11,600 Liquid Kerosene .. . . ... 20,050 Diesel fuel . . . . . . 19,760 , Light fuel oil .. . . .. 19,260 Heavy fuel oil . . ..' .. 18,900 Gas B.T.U. per cub. ft. Coal gas ~. ' . . . . .. 450 Liquid petroleum gas .. ' . . 3,333

TABLE I—CALORIFIC VALUE OF FUELS Solid

Though in a city area such as Wellington, coal and coke are the cheapest fuels, oil is little dearer. In country districts wood probably would be the least expensive. Gas and electricity are both expensive for space heating, but against the cost must be weighed the labour saving and convenience resulting from their use.

Heat Transmission The heat-transmission value for glass or flat sheets of thin asbestos or iron is generally taken as 1 and for other materials commonly used may be taken at the following figures: —

TABLE 2—HEAT-TRANSMISSION VALUE OF MATERIALS Walls

4Jin. brickwork . . .. .. 0.59 9in. brickwork . . . . .. 0.39 13Jin. brickwork .. .. . . 0.29 18in. brickwork .. .. .. 0.23 (When walls are plastered these figures are 10 per cent. less). Ilin, cavity walls . . .. . . 0.27 4in. concrete walls .. .. 0.75 6in. concrete walls .. '.. .. 0.61 Sin. concrete walls . . . . . . 0.52 lOin. concrete walls .. . . . . 0.45 Corrugated iron .. .. .. 1.5 Corrugated asbestos . . . . .. 1.4 Walls with Some Degree of Insulation Brick with lin. cork and plaster: 4Jin. brick .. . . . . 0.19 22jin. brick .... .. .. 0.12 Concrete with lin. cork and plaster: 4in. concrete .. .. .. 0.21 12in. concrete ... .. .. 0.17 Corrugated iron or asbestos lined with tin. wood .. .. .. 0.41 Lined with plaster board .. .. 0.53 Lined with gin. fibre board .. . . 0.31 Doors and Windows Wooden doors . . .... . . 0.5 . Glazing . . . . . . . . . . 1.0 Double glazing . . .. .. .. 0.5 Floors Wooden floor on joists with plaster ceiling .. .. .. .. 0.27 6in. concrete floor with screed .. 0.48 6in. concrete floor with wood floor finish . . . . . . . . 0.33 Roofs Corrugated iron . . . . .. 1.5 Corrugated asbestos .. .. .. 1.4 Treated metal sheets .. .. .. 0.9 Tiles or slates unlined .. ... 1.5 Tiles or slates on sacking with felt or paper . . .. .. .. 0.56 Tiles or slates unlined with plaster ceiling .. .. .. 0.56 Tiles or slates over sarking and felt with plaster ceiling .. .. 0.32 Floors Resting on Ground 6in. concrete with hard aggregate (such as granite chips) finish . . 0.5 6in. concrete with wood blocks or boards on fillets .. . . . . 0.35

The above figures help calculation of the amount of heat required in. B.T.U. to balance the heat loss from a building to the outside when theweather is cold. Reference ‘‘Heating and Ventilating”, by Oscar Faber (E. and F. N. Spon, London, 1948).

♦Costs based on those in ' Wellington.