Expandable shelters having a heating, ventilation, and air conditioning (HVAC) system arranged to receive outside air away from a mechanical compartment are disclosed. The expandable shelters may comprise an air inlet vent disposed in a front of the expandable shelter interconnected to an in-floor air duct arranged to draw outside air below an interior floor and exchange heat with the coolest area of the expandable shelter. By virtue of having an air inlet vent receive outside air from a location substantially away from the mechanical compartment and exchange heat with the coolest area of the expandable shelter, an environmental control unit (ECU), housed in the mechanical compartment, receives relatively cool outside air, keeping air temperatures inside the mechanical compartment below the upper operating limit of the ECU.