You might use one of the greenhouse heat loss calculators online like http://www.greenhousemegastore.com/greenhouse_btu_calculator to get an idea of what to expect. Using one, and your description above with a few assumptions, I came up with about 40000 btu/hr heat loss. I assumed short side walls, insulated ends and lows mid-30s. A 1000 watt light and ballast produces about 3500 btu/hr, so 10 such lights could provide about the required heat. A standard heater might be a more suitable choice.

Renais

Do you have room for some thermal mass items like containers of water, or bricks painted black, to collect heat during the day and release it at night?

Also, plants breathe in carbon dioxide so they might enjoy a propane heater.

Assuming a depth of 4ft, there will be around 200,000 lbs of thermal mass available in the greenhouse floor. To provide the same thermal capacity, you`d need about 5600 gallons of water, 755 cubic feet or a 14" depth of water covering the entire greenhouse floor.

Oh, sorry, i thought the op was trying to augment his heat retention. Evidently 200,000 lbs isn't doing the trick.

Thats because its just sitting there and not being put to use ;) Sink a tube down to 4ft and compare the soil temperature with the greenhouse air temperature to see what your missing.

The problem with any mass storage system is transferring the energy out of the mass at a high enough rate to do any good and then recharging it fast enough for the next demand cycle. Then the potential health risks come into play when a potential exists for mold and mildew growth in some of the subterranean systems I see advocated.

Mildew and mold isnt an issue unless you use non-perforated tubing. Worm burrows are one of natures solutions to increased soil aeration which encourages beneficial flora and fauna, not so much mildew and mold. Worm burrows dont experience the same airflow rates as the tubing which can be as high as 20ft per second (13.6mph).

I just have a HFGH 10 X 12 so I might not be much help but I use a 30 K BTU Kerosene heater (switching to Natural gas next year). I have 10 (50 gallon) steel barrels painted black filled with water. The sun and/or the heater heats the house and also the barrels. They retain the heat and then dissipate it back to the house during the night. There are formulas online to know how many gallons you need. No need for antifreeze. Just leave a couple of inches of space on the top and cover them. It doesn’t matter if the water freezes because water will still generate BTU’s when it changes from ice back to a liquid. I put plywood on top of the barrels so they serve as my work area bases as well.

I monitor the Greenhouse temp, barrel temp and outdoor temp from in my house so I know the barrels are working. It’s not the best solution but it’s free.

The cost of the Kerosene does add up but it is cheaper than electricity or propane. Natural gas will cost me less. I wish there was a way for me to avoid the night heater but it is tough here in MA