In article .com,
chuckie wrote:
My problem is that my greenhouse does not hold heat very well.

How big is the greenhouse and what is it made of? For at delta T of 40
degrees (70 inside, 30 outside), a 10,000 BTU/hour heater should be
adequate for a structure with 200 - 300 sq feet of surface area
depending on the material it is made from. That's like a 6x12 quonset.
Bigger if it is insulated like yours.

Also, what is the variation in temperature inside the greenhouse? If
you are getting a lot of solar gain during the day, but loosing it at
night, then you need to add thermal mass (rocks, bricks, barrels of
water, etc.)

--
Steve