Answer: Many greenhouses are equipped with a very small recirculating fan which can be left on all the time. You might look into adding one of those because you certainly don't want to run extra heat. The air circulation need only be slight to help ward of fungal attacks; too strong a breeze will dry out the seedlings.
Different seeds have different optimum starting temperatures, but most will start fine at normal household temperaturesor slightly warmer and then grow on at cooler temperatures, say 60 degrees or so. For this reason many gardeners will germinate the seeds in a small container set in a warm spot, then transplant the seedlings to individual containers and set those in a location with optimum growing temperatures. It is also interesting that seedlings grown in warmer temperatures tend to be very tender when it comes time to set them outdoors, so warmer is not always better.
You might find the information in these two articles interesting.
Seedstarting Made Easy at http://www.gardeners.com/gardeners/bgbseedstart.html
Seedling Savvy at http://188.8.131.52/nga/articledetails.taf?id=514&kwd=seed&A...
Enjoy your greenhouse!
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