Have you checked your vents for leaks? It is time to be sure that your greenhouse is tightly sealed from cold air. Just a few small air leaks can drop the temperature of your greenhouse immensely. Look into applying weather-stripping to all vents and doors. I prefer the more compressible (and cheaper) foam tape, because it allows for the vents and doors to close better and makes a better seal than the firmer rubbery weatherstrip. Also consider using a quality caulk to seal up other air leaks
Do you have a winter water source? Some people are lucky with a hose bib nearby. Others must depend on occasionally filling drums filled with water for their water source.
I have rigged up gutters on my greenhouse where I can capture natural moisture and then have it channeled into a rain barrel inside my greenhouse for winter watering.
The good news is greenhouses need much less water in winter then they did in summer so be careful about overwatering.
If you don’t want to run your heater a lot it is also helpful to focus on growing the more cold tolerant fruits, vegetables and ornamentals as explained in the book Greenhouse Gardener’s Companion.
While we are headed for colder weather be sure your vents still function as greenhouses can overheat even in the winter. Automatic vent openers can wear out over time and not have the pushing power that they once had. I have found that you get what you pay for with vent openers. The cheaper they are, the less they can push and the shorter life-span that they have.
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