Have you thought of trying to use solar to heat it. You can make a solar panel to gather heat from the sun. I used a garden hose that is wound across a sheet of plywood painted dark forest green or black. Wrap the plywood with 2x4 and cover with plexiglass. Insulate the bottom and cover with something. Use a submursible pump and pull water from a 55 gal drum. Pass the fluid through a radiator and let it drain back into the 55 gal drum. Let me know how it goes lol try it before you make it permanant. Use space heaters for heat at night if it gets too cold. Dont use antifreeze in the water or when it spills you have something that can kill everything you wanted to keep warm.

Mikee,

I'm trying to build on the idea you posted. Thoughts:

First, use an old water heater to retain the water. Use a timer to turn on a pump that will circulate water through hosing as you describe during the hours when the sun is shining bright. Another pump, also running on a timer, would circulate the water from the heater through small radiators - similar to the radiator heaters found in cars, during the night.

It would be an enhancement to have the night-time pump also controlled by a thermostat so if the water temperature being circulated drops below the determined temp, the pump shuts off.

Dreams!

Mike

Thanks for the ideas, Mikee, except that the greenhouse receives nearly no direct sunlight in the winter, and my budget is too small to purchase the parts you mention.

Certainly then your budget is too small to purchase a heating unit. The solar collector can be detached from the greenhouse, the hoses insulated, and the free solar heat can do better than free air.

Dan

You'd do better with a grow light in a heated indoor location then. Much cheaper to run light than to heat a greenhouse.

Quick question. I use christmas lights the few cooler months of the year here on my fruit trees. Could I do this in my Hoop house to keep things warm?

What are you growing? How many nights per season do you need to heat? I'm on the edge of zone 7a/7b and I heat about 30-40 nights over an average winter. Most things will withstand a cool greenhouse as long as it stays above freezing. I can't imagine you having many nights when you need heat in zone 9. How would space heaters be an option if the gas is turned off? Do you mean electric space heaters?

I have used a simple small electric heater bought at a garage sale for $7 to heat my 120sq. ft. greenhouse (8ft. x 15ft.). On our coldest night last winter, it got down to 26F outside and inside the greenhouse, it only got down to 40F. That night, I left the thermostat setting on medium. If I had put it on high, the temp probably would not have dropped below 46 or 47F. I forgot what the wattage is, but the whole heater is only about a foot long x 6 inches wide x about 8 inches tall. And that little one heated my whole greenhouse just fine last winter, with no insulation other than the two layers of 6mil contractors plastic covering the frame!

John

coconut_palm-
I use the same type of electric heater to warm my 150 sq. ft. GH. Most of those heaters are about 1500 watts. I got mine new for about $50 and it has a thermostat and a high (1500W) and low (750W) setting. I can maintain my GH at about 40F on just the low setting here in Zone 8. The only problem is that you have to have a decent circuit to plug into. A standard 15 amp circuit can only handle about 1800W before the breaker trips, so it's best not to have other things operating on the same circuit.

What are you trying to grow in this greenhouse without any direct sunlight?

The only plants I can think of that can do this would be leafy greens (spinach, lettuce, mache, etc.) However, these are also the plants that don't need a lot of heat, in fact, just the opposite! These plants would flourish without nighttime heat and would be quite healthy!

If you are looking for more heat-loving (fruiting) plants, they would require direct sunlight.

Thanks for the information. I have no idea how many nights I'll need to heat, but that depends on whether I close the vents (side and ridge) and how many cold fronts come through. My guess, though, is at least 30 nights through March of next year. The greenhouse was formerly used for biology labs at a university, so there is ample fluorescent lighting which I keep on for about eight hours each day, to simulate the daylight. (I wonder what the banana tree thinks about it.)
Responding to tsmith, yes I mean electric space heaters, except that with the variety out there I don't know what to pick - oil or ceramic heaters.
Eaglesgarden - I'm trying to grow mostly fruiting plants and seedlings.
So can anyone please refer me to a heater that is relatively water-resistant (usually mid-range humidity in the winter in the greenhouse) and costs between 40 and 60 dollars? Thanks.

Here is a link that might be useful: The greenhouse in question

A couple of questions? What is your greenhouse made of? Plastic? Glass? Do you need the entire area for your planting needs in the winter? If your greenhouse gets no direct sunlight in the winter does it get sunlight any other time? Is this because of trees? hills? location? Heaters - I would use the oil filled heaters which are more energy efficient along with a fan left on low. A neighbor of mine has a 20 X 10 greenhouse and uses two of these oil filled heaters. When the temp drops into the teens (we are 90 miles west of San antonio) she adds a propane heater that is fastened to the top of a 5 gallon propane tank. She also cover the entire north wall with tarps.

If you look at the picture I attached, you'll see that it's made of glass. The greenhouse gets a lot of sunlight during the summer - at least six hours (or more) of intense light in the summer, and about one hour on one section in the winter mornings. Mostly it's because of the tall university buildings all around it, and the one that form two of its walls. Thanks for the suggestion.

ykerzner-
I've had good luck with DeLonghi brand oil heaters, and they've been in this business for decades. Here is a link to one that is very similar to mine. The cost is in your price range and you can see it gets very good reviews.

Here is a link that might be useful: DeLonghi heater