Internal gains are the heat produced by people, lighting, and appliances inside the home. In winter, internal gains reduce the heat the HVAC system must supply; in summer, they add to the cooling load. How important are these internal gains? How do they factor into the design of mechanical systems? And how can understanding them help us keep our homes comfortable and efficient year-round?

Energy conversions

Energy exists in many forms: the chemical energy in food, the kinetic energy of a running toddler, the electrical energy that turns a motor, the light emitted from a TV screen. Each time energy changes form, a portion of that energy is converted to heat. When these processes occur inside a home, they produce internal gains.

Before digging into the different types of internal gains, we need to establish a common energy currency. In the U.S., heating and cooling energy is typically measured in British thermal units (Btu). A Btu is the amount of heat required to raise the temperature of a pound of water one degree Fahrenheit; it’s also about the amount of heat released by burning one kitchen match. A small, energy-efficient home might require less than 20,000 Btu/hr to keep it comfortable in very cold weather; a larger or less well-insulated home might need 50,000 Btu/hr.

Air-conditioner and heat-pump capacity is also described in “tons.” A system that delivers one ton of cooling removes 12,000 Btu/hr from a space. The term is a legacy of the days when refrigeration was provided by delivered ice; a ton of ice melting over 24 hours absorbs 12,000 Btu/hr from the space. (Because AC and heat-pump capacity depends on indoor and outdoor conditions, real-world HVAC systems will deliver more or fewer Btu than their nominal tonnage would suggest.)

Human metabolic energy is measured in…