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.
