Summarize the following:
Set up fans so that they are diagonal and create a airflow. They should run almost constantly during the winter months, to ensure the entire greenhouse is benefiting from the heater. They can also be located near the top of the supports.  Some ventilation of carbon dioxide is essential. The vents should be adjustable. You will need to open them wider in the summer months. Solar heat may only account for 25 percent of the heat in your greenhouse, so a backup heater is essential.  You can also use a wood or oil-based heater, but it must be vented to the outside to ensure good air quality, carbon dioxide poising is a real danger to be looked out for in such a close space. You should check with your town or council to see what heating options are available in your area. If you can afford to outfit your greenhouse with its own temperature control system, then you can set it up to grow almost anything.  Employ an electrician and a contractor to install your system. It may need regular maintenance to ensure it can handle ventilation and heating in the winter. You should install several thermometers in case 1 breaks.  Place them at different levels of the greenhouse so that you can observe the temperature in your greenhouse at all times. You can buy a thermometer that measures the temperature inside your house and inside your greenhouse, so that you can watch it closely during the winter months.

Summary:
Place fans in the corners of the greenhouse. Install vents in the ceiling of your greenhouse. Consider installing an electric heater. Install a forced-air system, if you are using a glass-frame greenhouse. Install thermometers or thermostats.