Believe it or not, I have seen and have worked in some heated barns. Some heat with overhead force hot air systems and others with radiant heat conducted through the concrete floor. Fans and water seem to be the cooling mechanisms of choice. Some veterinary hospitals have both central heating and cooling.
If a barn is heated, proper ventilation should still be the priority to maintain the health of the horses.
The gallery has images of these systems in barns I have seen.
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