Simultaneous Heating and Cooling—The HVAC Blight

Despite the good intentions of the Energy Codes that restrict or prohibit it, simultaneous heating and cooling occurs too frequently in HVAC systems and represents a large opportunity for energy reduction. Like driving with the brakes on but with no smell, it is a silent consumer of energy; so simple that it is often overlooked. In air handlers, the waste of heat-cool overlap can occur within a few feet of each other. In systems with primary and terminal level temperature control the lack of coordination between control settings creates similar waste but in a less obvious way. Automatic control valves ‘told to close’ don’t always comply. Certain automatic control routines, even with DDC controls, can allow overlap on a transient or continuous basis. Ironically, the most simplistic HVAC systems are often the most immune. A few systems require overlap to accomplish a specific function, such as humidity control, but for many systems the expenditure of energy goes on with no benefit and is an obvious energy waste. Using experience from hundreds of working HVAC systems, the paper will point out common causes of unintentional heating and cooling overlap, how to spot them, how to correct them, and provide tips on quantifying the savings.