The other form is that known as convexed heat - heat conveyed to our bodies by the surrounding air, which has received it previously from some heated surface, such as the iron casing of a stove or the coil 0f a hot-water system. Such heat will pass wherever the air which carries it can find access.
The two forms in which heat reaches us. Radiant heat is the more pleasant and healthful, but also the more extravagant
Radiant heat warms us without producing those symptoms of stuffiness and discomfort which are associated with convexed or airborne heat. •
The open grate diffuses radiant heat in a greater degree than any other warming appliance, and hence the good favour in which it is held. But it is not economical, and in its more primitive forms is stated to waste five-sixths of the heat it produces. This waste heat, of course, escapes up the chimney with the products of combustion.
The most modern and improved forms of grate probably waste one-half the heat they produce; and it is questionable whether any appreciable further economy could be effected in the open grate without destroying its efficiency as a dispenser of radiant heat.
So long as coal does not show any tendency to increase permanently in price, we shall probably endure the wastefulness. The dust we have already learnt to tolerate; and as for the attention, we most of us regard that duty as a privilege, some even as a fine art.
Taken together, these shortcomings of the open grate are more than counterbalanced by its advantages.
The closed stove is not yet common in our living-rooms, though it has long been used in churches, schoolrooms, and other places where a large space has to be warmed economically.
Its principal drawback is the uncomfortable dryness it imparts to the air, which is not entirely removed by the common practice of placing a pan of water upon the stove top.
Improved modes of construction have been devised, with the object of counteracting some of these disadvantages, the most successful of which aim at reducing the external temperature of the stove casing ; but unless this reduction of temperature be accompanied by an increased amount of surface, the heat thus conserved will pass into the flue - and be wasted.
Increased surface is sometimes introduced in the form of "gills," or radiating plates, and at other times by increasing the dimensions of the outer casing of the stove.
All systems of heating are, or should be, co-related with ventilation. It has already been mentioned that the open grate acts as a means of ventilation. This it does by means of the draught induced by the ascending current of heated gases from the burning coal. The closed stove, on the other hand, unless arranged in connection with a special system of ventilation designed to utilise its heat, carries away no more air than it needs for the combustion of its fuel.
A closed stove heats the room more uniformly than an open grate, producing a continually ascending current of warm air, which displaces the cooler air around it. This latter in time reaches the stove, and is in turn warmed, and thus a circulation of air is produced which carries the heat into every corner of the room.
Closed stoves are designed to burn coke or anthracite, both of which are smokeless fuels, and a hopper is usually provided for storing a considerable supply of fuel in the stove itself, so that it need only be replenished at long intervals.
There is one application of the closed stove which might receive more favour than it has done at present. A great defect in our ordinary system of domestic heating by means of open grates is that our living-rooms are well warmed, but the passages, stairways, and bedrooms are often at something near freezing temperature. If a closed stove be fixed in the hall, and kept burning continuously through the winter months, it will diffuse a pleasant warmth through those neglected parts of the house, and will do much to eliminate those draughts in our rooms caused by the action of the grate in drawing air into them from the colder parts of the house.
To sum up the advantages of the closed stove, they are : Economy, more uniform heating, little attention needed, practically no dust.
The disadvantages are: Stuffiness and discomfort, unpleasant smell, need for supplementing ventilation, possible danger from noxious fumes.
The question of appearance will probably influence some people; and it cannot be said that much has been done to redeem the closed stove from ugliness. But recent patterns show a tendency to improvement in that respect, and if not altogether things of beauty, at least are inoffensive.
Of other systems of heating, those which employ gas, oil, or electricity, may be classed as supplementary.
Heating by hot water or by steam falls into a separate category, and will now be noticed.
In the ordinary open grate about five-sixths of the heat produced escapes up the chimney, but a useful service is rendered by the ventilating of the room incidental to an open fire
. For large houses and rooms these systems prove more generally efficient than any other mode of heating ; for small houses they are less suitable, owing to the difficulty of preventing overheating.
Whenever the open grate is abolished in favour of hot water or steam piping, it becomes essential to instal an efficient separate ventilating system, with properly arranged inlets and outlets ; otherwise, as the valuable influence of the fireplace in changing the air of the room is lost, the result of the pipe system would be to create a circulation of warm foul-air currents round the room - a result which would be most injurious and unpleasant to the inmates.
The usual plan is to introduce fresh air by an inlet placed so that the air passes through, or around, the coil or radiator, and thus becomes warmed before it enters the room. Proper outlets to remove the foul air have also to be arranged in the upper part of the room.
Two systems of hot-water apparatus are in use, known respectively as the high and low pressure. In both systems the arrangement consists of a complete circuit through which the water circulates in obedience to the law that when the water in one half of the circuit is heated it ascends, and is replaced by the cooler water in the other half.
In the low-pressure system the temperature of the pipes is about 200 degrees Fahr. They are of large diameter, and, the system being open to the air, is at atmospheric pressure. In the high-pressure system the water circulates under pressure in a closed system of pipes, and the temperature of the pipes may be 300 degrees to 400 degrees Fahr., the pipes being of small diameter. The latter system is open to the objection that the air is liable to be overheated, but it is more economical to instal, as the piping is smaller, and no boiler is needed.
The steam system of heating is unsuited for domestic use in this country, though extensively employed in America, where houses are often built in large blocks subdivided into tenements. It is liable to overheat the air, and, besides other objections, is subject to emitting at intervals unpleasant noises known as "hammering."
The advantages of these systems of pipe heating may be stated as ; Uniform distribution of heat, economy of fuel, absence of dust, smell, and discomfort.
Their disadvantages are : High first cost, need for a separate ventilating system, if no fires are used with them, inapplicability to small rooms.