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Hi JD530, Before you spend a lot of time and money on building your own downdraft booth, consider some numbers that would be required to meet standard design principles. The ventilation design standard for a cross draft spray booth is 100 feet per minute air velocity across the section - in your case the section is 224 sf and the cfm required becomes about 22,000. A downdraft booth will be somewhat less in velocity because of the greater floor area used for a "section", but the air changes per minute should be about the same. The 22,000 cfm is the equivalent of about 4 air changes per MINUTE for a 23 foot long booth. A downdraft booth should be similar. To heat that amount of air from 0F to 70F will require about 1.7 million btu per hour of heat energy. This agrees favorably with the 1.5 million btuh that glennster indicates for his commercial booth. That's enough heat energy to heat between 5 and 10 conventional houses! Of course you can make do with less heat input by limiting your painting to milder outside temperatures - or taking the makeup air from a much larger, already heated space, assuming temperature drops are acceptable in that space and that the existing heating system can recover in a reasonable time. You should remember that the ventilation standard for spray booths is to prevent a buildup of fume concentration exceeding one-fourth the lower explosive limit of typical paint solvents evaporated during the spraying process. That's the primary purpose. It's not for personal respiratory protection. While a properly designed downdraft booth does well in minimizing fumes and overspray in the painters breathing zone, it is not a substitute for proper respiratory protection. The standard also requires all electrical equipment in the booth, including lighting, to be explosion proof, plus explosion proof exhaust fan(s). The numbers (and dollars) get very big, very fast. You can minimize them by lowering your ceiling height below 16 ft if possible, but they are always going to be big. I suspect the 12 in fan you are contemplating will provide only a small percentage of what really should be used. Do you absolutely have to meet such standards? Probably not, if only you are involved, depending on what you consider to be acceptable risks - but you likely can forget about any kind of fire or liability insurance for the installation - and for any larger building that it's in or a part of. Both the cost and the complexity of a booth that meets standards are well beyond the usual DIY category. It's why I only do my painting outside in warm weather. If you are only going to be painting a couple of weeks in the year, I'd suggest you do something similar, save yourself time and money and schedule your work to avoid painting during cold weather months. Or make friends with the owner of a local autobody shop that has the necessary facilities. Just some food for thought.  Rod
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