I remember walking around my university campus that is quite large and seeing that it had a centralized HVAC system with huge cooling towers that has water running over huge radiators. I imagine that black mesa uses the same kind of huge HVAC system to supply the whole complex and keep it at that pleasant 67 deg Maybe if there is room on one of the outside levels a model can be added in the distance or close by to reflect this massive HVAC system.
I always find it fun to muse about the possible inner workings of such an amazing complex. Imagine all the work it takes to upkeep that facility and to keep track of all the ongoings. I imagine it would be fun.