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The older pellet stoves used room air. The combustion fan was right at the bottom of the stove, pulling air just above floor level. There is a reason insurance companies wouldnt cover garages and outbuildings that had them installed... because they had a high rate of burning the buildings down. Gas fumes can become a powerful beast.
Like rd 2N in mo mentioned, code usually says 18 inches above the floor and sometimes as high as 48 inches above the floor but insurance companies many times just cancel because the risk is too great. Corn stoves and pellet stoves are a nitch market (low percentage of houses) that are many times homeowner installed. Thats too big of a risk due to uneducated installers and homeowners saying "ya, that will work".
A quick google search shows that the old style, room air feed systems are still being sold, at least the website for them is still active. http://www.bucknercornstoves.com/
Many other companies copy that design, its a very simple stove so its impossible to fully patent.