journal

18 THE CACTUS AND SUCCULENT January, 1951
ELECTRICITY IN THE GREENHOUSE
By A. J. EDWARDS
(A lecture at the monthly meeting, 10th October, 1950)
In November, 1947 I spoke of the advantage of electric heating over other methods, such as boilers and pipes,
oil heaters, etc. Since then I have had many enquiries about types of appliances, thermostats and soil heating.
Firstly, i will deal with greenhouse heating and various types of appliances and their adaptation to the greenhouse,
thermostatically controlled, to maintain a steady temperature. To attain this we must have sufficient appliances
and a thermostat properly positioned for everything depends on their efficiency.
Contrary to the common belief, the amount of current consumed does not depend on the size or number of
heaters, for, when the apparatus is used in conjunction with a thermostat, they will switch on and off in accordance
with the thermostat setting. Thermostats are switches operated by a bi-metal strip. The surrounding temperature
causes the bi-metal strip to open or close. I have been asked why, with my thermostat set at a given figure, the
temperature of the house falls three or four degrees below that of the setting. This is because of insufficient
heaters to overcome the time lag with a quick failing temperature outside. You may calculate that you have the
minimum amount of apparatus to maintain a certain temperature, but there is always time lag while the heat builds
up, but once achieved it is maintained.
Therefore, always have that extra amount of apparatus to overcome the time lag ; running cost will be no
more. The apparatus will not be on for all the time as the thermostat cuts out the moment the air temperature
is raised to the setting.
The position of the thermostat is very important ; to give the best results, fix in a position not affected by
draughts or hard against the glass where cold air radiating off the glass influences the thermostat instead of the
general air of the house.
The general effect of heating is that the tubes or panels, whichever are used, are mounted on the wall under
the benches, and the air becoming warm, rises and forces the cold air down.
With the thermostat in my greenhouse, I have fixed a one-inch thick wood panel over the glass and at a height
of about eighteen inches to two feet above the level of the plants. This works satisfactorily and, with the thermostat
set at forty degrees, the temperature has never fallen below that level.
During observations of the last six winters, I have recorded, on a few occasions, a temperature outside of ten
and twelve degrees of frost on a maximum and minimum thermometer fixed on a wooden post at a height of five
feet, six inches from the ground level.
Type of heaters : (I) round tubular heaters with a current consumption of sixty watts per foot ; (2) oval
tubular heaters with a current consumption of eighty watts per foot ; (3) panel heaters with a current consumption
of five hundred watts each ; (4) convector heaters with a current consumption of two to three thousand watts
each ; (5) fan heaters with a current consumption of three thousand watts each. All are recommended to give
satisfactory results, provided adequate apparatus is installed.
Tubular and panel heaters are fixed on the wall just above floor level, on brackets, so as to give the maximum
area of the tube or panel for the'air to pass round, hot air rises and cold air falls to the bottom and this process
warms the air as it passes the heaters. Convectors stand on the floor or fixed against the wall and the same thing
happens, but air passes through the convector. Fan heaters are heaters with a small fan which forces the air
through the heater much quicker than by the method of convection and they reduce the time lag.
Soil heating. This is becoming very popular in greenhouses and frames. If used in borders and built up beds
on the staging, air temperature can be kept at a lower level as the equipment will keep up a steady soil temperature.
Methods before soil heating raised air temperatures and the heat was conducted to the soil, resulting in a period
of time before the soil temperature was raised to encourage root action, therefore, soil heating equipment gives
a steady soil temperature, not too high an air temperature, the plants a more balanced atmosphere to live in. The