The luxury of walking into a cool room on a sweltering summer day may perhaps be one of the most under-appreciated advances of the 20th century. Heating, ventilation, and air conditioning, or an HVAC system, provides the sort of comfort that is most noticeable when it is absent. Meaning, it is the thought of a moment to turn your indoor air conditioning or heating on, but becomes something you cannot stop thinking about as soon as it stops working. HVAC companies recommend regular maintenance, such as changing the HVAC filters, for their heating and air conditioning systems. It is estimated…