This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
It is not intended to speak of the ordinary type of stove, which is merely an iron casing lined with firebrick, as it possesses no special features worth describing. Description will therefore be confined to ventilating stoves, stoves which are made with air-warming passages within them, and are used for air-warming in some way. Stoves of this kind are largely used for hospital and institution work, and for the waiting-halls of railway stations, etc.
In Fig. 98 is illustrated a ventilating stove, the air-warming surfaces being extended, firstly, by the gills shown within them; and secondly, by the smoke flue being diverted down between the air passages, as will be seen. Provision is made, by means of a sliding damper, for the smoke to pass direct from fire-box to the flue nozzle, if required. It will be noticed that the warmed air is discharged through grated sides as well as at top. This is a stove in the strict sense of the word, the fire not being visible, and the fire-box receiving a charge of fuel sufficient to enable it to last for several hours without attention.
In Fig. 99 is shown a stove that could be described as an independent open grate, the vertical plate appearing to rest on the top front fire-bar is a blower for drawing up the fire, when required; at other times being removed to expose the fire to view. The air-heating surfaces are chiefly tubular in this case, passing to and fro across the heated flue-way.
In Fig. 100 is shown a stove similar to the last, but in double form, the two fires being back to back. The special feature in this illustration, which will bear describing, is that of the underground invisible flue-ways. In the majority of cases large stoves - certainly double ones - are placed centrally in the room or place to be warmed, and an ascending smoke pipe may not be possible or would be strongly objected to. In such cases the stoves are connected with a suitable brick chimney in the nearest wall, the connection being by underground flue. Needless to say, the floor must be of incombustible material, although the writer once saw an iron flue pipe carried through a wooden floor and run horizontally, suspended from the beams beneath, the whole being kept 9 inches from woodwork, as required by the London Building Act.
Underground flues should never be of less size than the flue nozzle of the stove, and provision must be made for sweeping them. To secure efficiency with an underground flue, the descending part should never exceed 5 feet, and the whole descending and horizontal portion should not exceed one-third the height of a perpendicular chimney to which connection is made. When an underground flue is of iron, or loses heat at all fast; or if it is unusually long, it will be found necessary to use a "pilot-stove" at the base of the vertical chimney to start the draught when the fire is lighted, or whenever the flue has been allowed to cool down. This stove can be of any close-fire kind, in small size, and connected so that when the draught is established it can be shut up air-tight and not prejudice the working of the other by being equivalent to a leak or a hole in the chimney. It is not always necessary to use a stove for the pilot fire. In many cases it is sufficient if a soundly fitted soot door is provided, this being opened to allow of a good handful of lighted shavings being thrust in to start the draught.
Whatever kind of warming stove is being fixed, but particularly those with underground flues or working under any unusual condition, the flue pipes, whether of iron or earthenware, or built of brickwork, must be absolutely air-tight at all points. There are extravagant ideas regarding chimneys - as to the draught being improved if a slanting hole is made in the chimney, and things of this kind - but as the efficacy of a chimney is in ratio with its heat, it is important that cold air be prevented from entering. The rule - and it has been called a golden rule in stove and range fixing - is to cause all the air which enters the chimney to first pass through the fire-box. The observance of this rule often causes the draught to be very strong, but this is distinctly better than a draught that is too weak. There are always legitimate means of controlling the draught in all fuel-burning appliances, therefore no excuse exists for leaky flues. Such leaks are an index to ignorant or careless work.