The type of system you choose will depend on how much you want to spend.
The five types are:
Natural or L.P. Gas, No electricity required - These units are designed to be used singly or in groups to meet individual greenhouse requirements. These thermostatically controlled heaters circulate warm air over the greenhouse somewhat like the heat rays from sunshine, from the floor up. The temperature at the top of the greenhouse can be kept between 5 to 7 degrees of the floor temperature.
Space heaters - For low-cost heating for small greenhouses, use one or more ordinary space heaters.
Forced-air heater - The best system for heating a small greenhouse is a forced-air furnace with a duct or plastic tube system to distribute heat. You can use a thermostat to control the temperature in the greenhouse.
Hot-water or steam heater - A hot-water system with circulator or a steam system linked with automatic ventilation will give adequate temperature control. In some areas, coal or natural gas is readily available at low cost. This fuel is ideal for hot-water or a central steam system. Steam has an advantage in that it can be used to sterilize growing beds and potting soils.
Electric heaters - Overhead infrared heating equipment combined with soil cable heat provides a localized plant environment, which allows plants to thrive even though the surrounding air is at a lower than normal temperature. Electric resistance-type heaters are used as space heaters or in a forced air system.
Ceramic Electric heaters - Keep your plants
warm and over wintered plants safe from frost with our durable, efficient
electric heater. Features auto safety Shut-off, overheat protection, and caution
light for extra safety. A heavy-duty, enclosed fan circulates heat evenly.
Maximum 5000 BTUs with a 1000 and 1500 watt setting. (not intended for use in
misting or open sprinkler environments)