Greenhouse heating -- heated table top?

I'm pondering what I might do to more efficiently heat my small greenhouse to a minimal extent (40F). With the increase in the price of natural gas I wonder if electric would be about the same? I also want to consider assembling a table with a clear plastic cover to contain the heat to a smaller area. Perhaps I could create a heated table top? That would be useful next Spring. I suppose resistance wire in a thin bed of sand?
Dave
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Use horse poop to provide the heat. It's free, and the carbon dioxide produced will help the plants. Our combined goat and chicken house will be heated with goat and chicken poop.
Ray Drouillard
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I've seen that mentioned but how much manure depth would actually be required for decent results? Thanks. Dave
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price
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dioxide
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I haven't tried it yet, so I don't know offhand. You can probably find plans for hothouses, though. Try looking on google.
Ray
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On 2 Oct 2004 05:18:01 -0700, galt snipped-for-privacy@hotmail.com (Dave) wrote:

For a fairly small area (3'xg') devoted to germinating seeds, I used an aquarium heater in a tank of water with an aquarium pump to circulate the water via plastic pipe embedded in sand. Worked OK as long as I remembered to keep the reservoir topped up. That could be cured with a float valve, but I didn't bother. Likewise, metal pipe would transfer more heat.
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On 2 Oct 2004 05:18:01 -0700, galt snipped-for-privacy@hotmail.com (Dave) wrote:

There was a thread on...rec.gardens, I think...a while back on using a waterbed heater to warm a seed tray. There were several people who had tried it, I believe, but also a couple of warnings about fire hazards.
You could probably google under the rec.gardens hierarchy for "waterbed heaters" and pull up the information.
Penelope
--
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(Dave) wrote:

I think a waterbed heater might be a little scary. That resistance wire they use to prevent pipes from freezing might be safer. The idea of a circulating water scheme is attractive and you could use it with various sources of heat -- even solar -- but on the negative side it itself could become another freeze risk unless you used antifreeze in the water.
Dave
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