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How to Build Heated Walls

Charles McIntosh, The Practical Gardener, 1828


In the eighteenth and nineteenth centuries many large kitchen gardens had heated walls on which to grow fruit. The heat helped protect against frost and to ripen the shoots and fruit. The walls could be very large - in the Botanical Gardens in Tasmania there is a heated wall some 200 metres long, in which you can still see the flues and hearths needed to heat it. The following comments come from Charles McIntosh, writing in 1828.

Hollow walls are often recommended as possessing the same strength [as a straight, solid brick wall] ... and such walls are capable of being heated by artificial means, as the occasion may require, for the purpose of ripening late fruit, but more especially for ripening the young shoots, which is still more important and is, in fact, the principal use of hollow or flued walls and, when fuel is moderate in expense, is found to be extremely useful. But the success in this case, as in many others, depends upon the judgment and assiduity of the gardener.

The cellular wall is a recent invention. The central part of the wall is built hollow, or at least with connecting vacuities, equally distributed form the surface of the ground to the coping. If the height do not exceed 10 or 12 feet, these walls may be formed of bricks set on edge, each course or layer consisting of an alternate series of bricks set edgeways, and one set across, forming a thickness of nine inches, and a series of cells, nine inches in the length of the wall, by three inches broad. The second course is laid in the same way, but the bricks alternating or breaking joint with the first. This wall is not expensive to build, saves much material, and is simple and efficient to heat, but the bricks and mortar must be of the best quality.

This wall has been tried in several places near Chichester, and at Twickenham, by F G Carmichael, and found to succeed perfectly as a hot-wall, and at 10 feet high to be sufficiently strong as a common garden wall, with a saving of one brick in three.

As a whole, indeed, it is stronger than a solid nine-inch wall, on the same principle that a hollow tube is less flexible than a solid one.

It is evident, that the same general plan might be adopted in forming cellular walls of greater height by increasing their width. A very high wall might have two systems of cells divided vertically, one or both of which might be heated at pleasure. Piers might be formed either on both sides of the wall (a) or on one side by bricks (b) so as to bond in with the rest of the work.

A great advantage may be derived from walls built so as to be heated as the occasion may require; these are denominated hot walls, and have hitherto been constructed by introducing a system of common smoke-flues (as fig. 1) distributed through the walls at certain distances. These flues are objectionable, as the need to be frequently swept, which is not readily effected. Independently of which they are, like all flues heated by hot air or smoke, liable to become cool soon after the fire ceases to burn.

An improvement has been designed by W. Atkinson of Grove End, and for its utility and simplicity deserves to be in more general use. It consists in building the walls hollow, which will be found more economical and equally strong, and introducing, within a few inches of the bottom of such cavity, hot-water pipes, supplied from boilers, which may be built in the wall, and the fire fed and managed from behind, such boilers being placed at a distance of from fifty to one hundred feet apart - or one boiler, placed in the middle, will heat one hundred feet of wall sufficiently by having the pipe branching both from the right and the left.

These pipes require no cleaning or repair, if properly placed, and can be built at a very moderate cost. They possess a decided advantage over hot air or smoke flues by continuing to give out heat to the wall long after the fire has ceased to burn, and this property will increase according to the size of pipes inbuilt. For the side walls, which have an eastern and western aspect, the pipes may be placed in the centre of the walls, so that both sides may derive an equal degree of heat. For walls having only a southerly aspect, the walls being thicker, the pipes may be arranged as to have only one brick of thickness in front so that the remainder off the thickness is on the side where heat is not required.

The water being heated by the boiler will flow along the pipe to its extreme point, say one hundred feet, and there make a turn by an elbow joint, and return to the boiler by a pipe immediately below it, which will enter the boiler near its bottom. The water in this pipe will travel with more rapidity by forming an inclined plane from the extremity to the boiler; the top pipe may be perfectly level.

Thus the water will continue to circulate in the pipes long after the boiler is extinguished or so long as any heat remains in the pipes or the wall which surrounds it. it has been ascertained that the water heated by this method travels at a rate of forty feet per minute with an ordinary fire, but this rate may be much increased. The distribution of heat by this method is so equal, that the pipes will be found as warm fifty or sixty feet from the boiler as they are where they are connected to it. This is ever the case with smoke flues, from which arise the many complaints that hot walls are burnt up in one part, and little affected by heat in others.

Please also visit Old London Maps on the web as many of the maps
and views available there have plans and depictions of gardens from
the medieval period through to the late nineteenth century.

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