I was thinking there must be an easier way to sterilize the potting mix by other means than using an oven. All it needs is to be heated. I snooped around this morning sniffing out other methods. Came across other methods and I like this one as I can do a larger amount at once. Just buy a large metal garbage can, do what the instructions say and be done with it. I could set the can on concrete blocks and start a fire underneath using charcoal as a heat source. Charcoal would not create a lot of smoke either.  All you need to do is get the water boiling and maintain boiling temperature the required length of time. I like this metal barrel steaming method.

http://homeguides.sfgate.com/homemade-soil-sterilizers-37986.html

Metal Barrel Steaming
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Sure would be easier than building an oven. getting an adequate heat source, dealing with temperature control. I was even thinking of buying a used electric stove with an oven. Seen one last night for $75.00. Where would I store it? Look like he!! sitting covered in the back yard. No room in the garage.

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See comment for another heat source.