Buffcoat,
Mad-at-arms and Blakewilson (although I do not agree with the window comment – plenty of herp rooms do not have one) have got it right.
What the diagram is showing is the flow of air as a result of convection. The placement of the vents will, in simple terms, affect how much air in an enclosure is exchanged in a given period of time. The diagram shows how to get the maximum amount of air exchanged. Minimum would be both vents on the hot side.
Say you wanted to have a section with high humidity, you wouldn’t want that air to be constantly exchanged. By putting both vents at the hot end, most of the air in the cool end stays put and you can build up the humidity there.
Say you have a carpet enclosure and you want warm air in the top section because that’s where it likes to hang out a lot. Set up the vents as per the diagram, except the top vent would be positioned only 2/3 to 3/4 of the full height. This would result in a layer of warm air being trapped in the top section of the enclosure. As it cools off it is constantly being replaced by the rising warm air.
The thermodynamics of enclosures is not simple to dissect and explain. It involves the size, shape, construction material, surrounds and positioning of an enclosure. It also involves the type of heating, the wattage thereof, the position and how long it runs and if controlled by a thermostat. It is also influenced by the position, size and nature of vents plus any incidental openings, such as sliding door gaps, which allow airflow in or out.
Thank goodness, so long as we have a few basics in place, the temperatures can be fine tuned by playing with the wattage of heating elements and using a thermostat.
Blue