If you decide to have heat in the greenhouse itself, and not just in propagating frames inside, you have several choices available. Natural gas, domestic fuel oil, and solid fuel heating are really only suitable for larger structures and work with water pipes in a similar way to central heating in the home.
Paraffin stoves are perhaps the easiest and cheapest forms of free-standing heaters and can vary from the very simple to the highly sophisticated but they have a common problem in that they must be well maintained or they will emit fumes harmful to plants and because of this, and also because they give off water vapour as they burn, ventilation must be watched carefully.
Natural gas and propane gas can also be used in flueless heaters but again there is a problem of fumes so adequate ventilation must be provided.
Electricity is possibly the most expensive but the most efficient form of heating (though if you are on the Economy 7 tariff your night costs are considerably reduced). It can be used to power fan, convector, panel or tubular heaters, soil-warming cables or prefabricated electric blankets. It gives off no fumes and can be accurately regulated by thermostat. In addition, an electric fan heater can be used as an extra means of ventilation and air circulation in summer by blowing cold air around the greenhouse.
If you are away a lot, you may like to consider some aids to help the watering situation. Capillary matting can be placed under the pots on the staging and kept constantly damp by dipping one end into a reservoir. The plants absorb what they need through the compost behind the holes in the base of the pots (or through a wick up the central hole of a clay pot).
Microbore irrigation is a more controllable method, in which each plant or group of plants is watered by drip-ends fed by thin plastic pipes. Some drip-ends are capable of being regulated for rate of flow, and the whole system can be connected to an electric timer for complete automation.
Mist units are devices for keeping the air humidity high and surrounding conditions very damp and are especially useful for striking cuttings under glass. A fine spray of water is turned on and off automatically as necessary, usually by an ‘electronic leaf’ of 2 electrodes which turn on the water when the film of water between them dries out. They should be used with care in greenhouses also containing established plants as they can cause leaf rotting and disease.