Answer: Fortunately, most greenhouse heaters that use propane and natural gas are vented, so harmful gasses don't build up inside the greenhouse. Gardener's Supply Co. (www.gardeners.com; ph# 800/863-1700) has a selection of greehouse accessories, including a good selection of heaters for various situations (they used to carry a style, called the Southern Burner, that didn't require venting!). I didn't find the greenhouse heaters listed on their website, but you can request their Greenhouse Accessories Catalog via the web. Hope you find what you need!
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