In
the eighteenth and nineteenth centuries many large 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.
