Electric heaters – are most reliable, efficient and convenient to use in your greenhouse. Most of them are thermostatically controlled, so there will be no waist of energy. They don’t need regular refueling or special maintenance, do not produce fumes or water vapors. There are some types of electric heaters that can be used to heat your greenhouse: fan heaters, water-proof tubular heaters or convector heaters. Fan heaters are the most effective because they can be moved, promote good air circulation and also can be used in warm weather to cool the greenhouse. Tubular heaters need to be fitted to the sides of the greenhouse, just above floor level.
Gas heaters – connected to the mains supplies or from bottled gas. Their thermostats are not so efficient as to the electric heaters and if used with bottled gas they need to be regularly replaced. Propane gas releases fumes and water vapors as it burns, so a proper ventilation is required in this case. Always have two bottles connected by an automatic switch-over valve in case one bottle runs out. Keep the gas cylinders in a safe place and have them checked regularly.
Paraffin heaters – not as efficient as electric or gas heaters in their use of fuel because they are not thermostatically controlled. They may be expensive to run because some of energy is wasted if you need to maintain a high temperature in your greenhouse but they are inexpensive to buy and no installation cost is required. A proper ventilation is necessary in this case also because it produce plant-toxic fumes and water vapors. An other disadvantage is that you need to transport and store the fuel and check its level every day to make sure that it is burning cleanly.
Solid fuel hot water systems – rarely used now. Oil- and gas-fired circulated hot water systems are still sometimes used but only on a small scale. They need regular stoking and cleaning out of the fire.
Thermometers and frost alarms – a must if your heater is not thermostatically controlled. Use a min/max thermometer to check that the right temperatures are maintained overnight. In regions with extremely low temperatures you should use a frost alarm also. It will allow you time to protect plants if temperatures drop unexpectedly because of a power failure or heater breakdown. An alarm bell will sound remotely to announce you that temperature has drop to near freezing.