Warming. Warm-blooded animals require for health a certain temperature of the body, variable only within certain limits. If the external temperature become too high, the temperature of the body is regulated by perspiration and respiration; if it become low the bodily activity is increased, and the temperature thus kept up. Under ordinary circumstances, however, man needs to protect himself against excessive cooling, in order that he may lead the comparatively quiescent life of civilisation; he needs houses and clothing. If this precaution be neglected his vitality sinks; and as the external temperature falls the death-rate rises, which shows that there is a certain amount either of neglect or inability, or both, to take the measures necessary to avoid undue cooling of the body. This undue cooling may be avoided in two ways—by extra clothing or by artificial heating of apartments. In southern Europe and in China the former is the plan resorted to; in countries where fuel is not scarce the latter plan, which is preferable, is adopted.
The objects of artificial heating are the temperature most congenial to the human constitution, pure air to breathe, and air not too warm or dry. If the body be kept sufficiently warm the air which we breathe may be fairly cool; and an ideal system of warming would warm the body and leave the air cool and pure. The actual methods of warming are, to a greater or less extent, compromises, and they depend mainly either upon radiation of heat or upon convection of heat, or both. The former method is that of the open fire; the latter is, in the main, that of stoves, gas, steam, and hot water. In the former the walls and furniture of the room are warmed, while the air is not warmed directly by the fire, for the radiant heat streams through it without warming it; in the latter the air itself is warmed directly. In the former the walls are, so far as the fire is concerned, warmer than the air; in the latter the reverse is the case. In the former (the open fire) there is no tendency to deposition of moisture from the air upon the walls, unless the air becomes laden with excess of moisture from extraneous causes, such as the presence of a numerous assemblage, or unless the air becomes chilled and deposits its moisture even upon somewhat warmer walls; in the latter case, the walls being cooler than the air, there is a continuous tendency to the deposition of moisture on the walls, unless the air is at some distance from its saturation point. Heating by radiation is best exemplified by the open fire with a chimney or in the open air. The drawbacks of the open fire burning in a grate are the waste of fuel through imperfect combustion, the production of smoke, the comparatively small radiative power of flame (a blazing fire having less effect than a glowing one), the warming of surrounding objects on one side only, the want of an equally maintained temperature, and the waste of heat, which escapes with the smoke and chimney-gases. Its advantages are the satisfaction of the eye, cool air to breathe, and the ventilation produced. Smoke may be diminished and radiation increased by an extended use of coke; but coke does not do well if the fire burns low, and a blower should be used to make the fire draw quickly and become bright. A forced draught should always be used for a few minutes while lighting a fire; the amount of smoke produced is very materially diminished by this means. The forced draught is secured by narrowing the front aperture so that all the air which reaches the chimney must have passed through the fuel. If the aperture in front of the fire be large, air passes up to the chimney in front of the fire-gases, and the combustion is relatively slow. Dr Arnott (q.v.) developed the principle of limiting the access of air to the fuel by enclosing a store of fuel in an iron box beneath the grate, and bringing this gradually to the top by pushing up a false bottom; the air got access to the fire only at and near the top, and if the fire were left to itself it smouldered for many hours, ready to brighten up when it was pushed up so that air might gain freer access to it. In tending a fire it ought to be borne in mind that when the fire cannot radiate light it cannot radiate heat, and that it is therefore absurd to hide the fire under opaque masses of coal; and secondly, that the products of distillation of coal ought not to be allowed to escape as black smoke, but should pass up through a bright portion of the fire, and be perfectly burned. In special hearths it is possible, by means of false bottoms, to introduce fresh charges of coal underneath the existing fire so that the outer surface of the fire is always clear and bright. Even in ordinary grates it is possible to do a good deal towards minimising smoke and confining the active portion of the fire to the top and front; if, for example, a tile be fitted in the bottom of the grate; if a substantial amount of fuel be put in the grate and lit at the top; if this fuel contain some broken coke or cinder; if fresh fuel be added, not by throwing it on the top, but by raking the fire forward, throwing the fresh fuel into the hollow thus produced at the back of the fire, and then pushing the bright fire back upon it; if these things be done the fire is obviously brighter and more continuously cheerful and more nearly smokeless. The fire should always be bounded by fire-brick behind and on each side, for iron chills and blackens it. The fire-gases should not be allowed to escape at once into the chimney up a sloping iron back; but the back of the grate should be fire-brick all the way up, and should overhang the fire so that the ascending fire-gases impinge on it; by this means the 'throat' of the chimney is prevented from being excessively wide, the back-brick absorbs heat from the fire-gases and radiates it towards the floor, and the back-brick also reflects towards the room the heat radiated upwards from the top of the fire. For similar reasons the regulator or register should slope forwards. In every case, however, where there is flame there is great loss of heat, which escapes up the chimney, and even where the fire is smokeless this loss is considerable. The heat which is radiated into the apartment is never more than a fraction of the total heat of combustion of the fuel; and plans have been devised (see VENTILATION) for recovering some of this heat by causing it to warm the air which is supplied to the apartment.
In the more primitive plans in which there is no ventilation—an open fire in a cave, a tent, a wigwam, or a cabin, or a charcoal brazier in a room with a chimney, or a gas or petroleum stove isolated in a room—the whole heat of combustion of the fuel may be utilised in warming the walls and air of the room; but the products of combustion vitiate the air. Warming of this kind is effected by gas-burners and petroleum-lamps, which raise the temperature very considerably in non-ventilated rooms, but seriously vitiate the air at the same time.
By the use of close stoves the actual vitiation of the air by products of combustion is in great part avoided. The Dutch stove, for example, is a hollow cylinder or other form of iron, standing on a stone slab on the floor, close at the top, and having bars at the bottom on which the fire rests. The door by which the coals are put in being kept shut, the air for combustion enters below the grate; and a pipe issuing from near the top carries the smoke into a flue in the wall. If this pipe be made long enough the fire-gases traversing it may be very materially cooled down before they enter the chimney, and thus the bulk of the heat of combustion remains in the room. As far as mere temperature is concerned, this is a most effective and economical warming arrangement; but it has serious faults. The iron often becomes red-hot or even hotter; any carbonic oxide existing inside as the result of an inadequate draught passes through hot iron and acts as a slow poison, causing anaemia; the dust in the air is charred when it approaches the hot metal, and gives rise to offensive and unwholesome odours; the air is rendered very 'dry' by being strongly heated. These faults are more or less obviated by increasing the mass and the cooling surface of the stove so that it cannot become too hot externally when a moderate fire is kept up within; by regulating the fire; by adjusting the fire-capacity of the stove itself; by allowing the access of sufficient air to ensure complete combustion; by surrounding the fire with brick instead of iron, or building the whole stove of brick or earthenware; and by placing a vessel of water upon the stove, the water evaporated from which may supply the moisture necessary to bring the air to a congenial degree of saturation, appropriate to its new temperature. If this vessel of water be placed upon the stove the air takes up moisture from the evaporating pan, and does not then parch the skin and lungs; but when the room cools down again the air may readily prove supersaturated, and deposit moisture on the walls, a condition favourable to mould.
In most continental stoves the fire is surrounded by a mass of brick, lined externally with porcelain. The smoke goes along a winding passage in the structure and issues nearly cold. The brickwork becomes warmed, and keeps up a moderate heat for a long time after the fuel has burned out. Open-fire stoves have also been devised; and an open fire-grate might be built out into a room at a distance from the wall, and the flue might go at once into the chimney, or go up an ornamental column through the room above. By such means the waste heat of the chimney would be utilised in warming the air of the apartments.
Gas-stoves have come into considerable vogue of late years. They depend either upon radiation from luminous flames or from asbestos heated by Bunsen Burners (q.v.), or upon heating of a metal casing by Bunsen burners with or without direct contact between flame and casing. Bunsen flames are in themselves of no use for pure radiation, and bare flames without ventilation simply heat the air by pouring hot water-vapour and carbonic acid into it, as any other fuel would do. In gas-stoves as much as in open fires the products of combustion, though they are invisible, must be taken out of the room, and the means of access of these products to the chimney must be ample. The air of apartments is also frequently warmed by steam or hot-water pipes: these are iron pipes containing hot steam or hot water, and warm externally by reason of the heat-conductivity of the metal. Air coming in contact with them is warmed and ascends, its place being taken by cooler and heavier air, which in its turn ascends. The whole air thus becomes warmed. Radiation from the pipes is also favoured by a coat of paint, not by a smooth metallic surface. These systems lend themselves readily to distribution of heat throughout a building from one central fireplace. The pipes can be so arranged that the steam or water can be shut off from any part at will; and the tubes may, by being connected with or attached to plates or wings of metal, have their heating surface and their heating efficiency increased. Steam heating is useful where there is waste steam available, as in factories and railway trains. As long as steam goes on condensing it remains at . (.) until it is wholly converted into water at ; overheating is thus not possible unless the steam is itself superheated at a high pressure. The pipes must be so laid, in the case of a building, that all condensed water may flow back into the boiler, and allowance must be made for expansion of the pipes by heat. When hot water is employed it may be made to circulate either at low pressure or at high pressure.
In low-pressure systems the arrangement may be illustrated by the figure, in which a is a boiler; b is a tube which circulates through the building; c is a small tank at the top of the circuit and open to the air, by which the tubes and boiler are kept full; d is the return tube. When the boiler is heated the heaviest portion of the water within the system, the cool water in d, tends to sink by gravity to the lowest level, and thus circulation is immediately set up, and kept up as long as the boiler maintains a difference of temperature.
The portion of the pipe which contains the coldest water should be vertical, and the comparative coolness of the water in the return pipes is maintained by the loss of heat experienced by the water on its way round the building. In high-pressure systems the pipe is narrower and very strong (wrought-iron of special make and thickness), and it forms a closed endless coil throughout the building. It is completely filled with water, except at the top, where there is a strong closed cylinder (the 'expansion-pipe') containing air to provide for the expansion of the water by heat. The pipe is led in a 'boiler coil' round a fire at the basement. The water circulates for the same reason as in the low-pressure system, but it travels very rapidly, since the water can be heated in the boiler coil to temperatures far exceeding . This is because the whole apparatus is equivalent to a closed vessel, capable of standing great pressures, and in such a vessel water may be highly heated without attaining a boiling-point. The apparatus is tested for pressures of 2000 to 3000 lb. per square inch, and at 750 lb. pressure it would be possible to heat the water to . The usual heat employed is from to , which corresponds to a pressure of from five to nine atmospheres.
The advantages of the high-pressure system are the use of smaller pipes, which are more convenient and more seemly; the possibility of making them dip without risk of the bends being blocked by air (the removal of which must, on the low-pressure system, be provided for and attended to); the ease of application of radiating surfaces to the smaller tubes; the yielding of the system by alternate compression and release of the air in the expansion-tube, which acts as an elastic cushion and tends to prevent fracture; the small quantity of water used, the rapidity of circulation and the consequent promptness of action; the freedom from any access of dirt to clog the tubes; and the advantageous form of the boiler-coil as a rapid heater. The disadvantages are the quick cooling down when the fire goes down and the want of uniformity of temperature when the fire fluctuates, the uncomfortable heat of the pipes when touched, the fact that the pipes must be kept at a greater distance from plants, the slight charring of dust in the air, the slight charring of some kinds of wood laid too near the pipes, and the greater chance of freezing if the fire goes out. For the last reason the pipes should be charged not with water but with a non-freezing solution. In a modification of the system specially applicable to cases in which portions of the system are to be shut off from time to time, there are outlet and inlet safety valves to let hot water out or cold water in when the pressures are greater or less than certain limiting values. In that case the expansion-pipe is often dispensed with. In these cases the air which is in the room is heated. The heating of air to be brought into a room will be found under VENTILATION.
Small spaces may sometimes be warmed by the introduction of hot water, as railway carriages by hot-water tins. Better than hot water is a tin case filled with crystallised acetate of soda; this is exposed to heat until it becomes warm; the heat absorbed is partly expended in melting the acetate, which then dissolves in its own water of crystallisation; the mass therefore absorbs much heat; and as it cools down it keeps on liberating its latent heat for a protracted period.
As to conserving the warmth of a room by preventing heat from escaping, the leading methods are to make the walls, doors, &c. bad conductors and air-tight. Air-tightness is incompatible with ventilation, but bad conduction is desirable both in winter and summer. The best material for a badly-conducting wall is one of a porous or spongy texture, such as porous stone or brick, which contains air in its interstices; but the best structural form is that which contains a film or jacket of air. Even iron houses may be made warm in winter by this means, if plaster-lined. Windows, again, if made double—double panes or, better, double sashes—allow very much less heat to escape than single ones, and even window-blinds and curtains have to a smaller extent the same action. The intervening air-film or layer is prevented from flowing away, and it is a very bad conductor.
See Edwards on Ventilation and Heat (Longmans, 1881), and Dye's edition of C. Hood's Warming Buildings (1891), and literature there cited. See also FUEL.