Class of man-made chemicals, including chlorofluorocarbons (CFCs), hydrocholorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), whose heat-trapping properties are among the most damaging of the greenhouse gases. This, coupled with their tendency to remain in the atmosphere for hundreds of years, has resulted in limits on their use. Halocarbons are most commonly used in refrigeration, air conditioning, and electrical systems, and as blowing agents in some foam insulation products.

Device that transfers heat from one material or medium to another. An air-to-air heat exchanger, or heat-recovery ventilator, transfers heat from one airstream to another. A copper-pipe heat exchanger in a solar water-heater tank transfers heat from the heat-transfer fluid circulating through a solar collector to the potable water in the storage tank.

Increase in the amount of heat in a space, including heat transferred from outside (in the form of solar radiation) and heat generated within by people, lights, mechanical systems, and other sources. See heat loss.

Heating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump.

Where heat is dumped by an air conditioner or by a heat pump used in cooling mode; usually the outdoor air or ground. See air-source heat pump and ground-source heat pump.

An appliance that uses an air-source heat pump to heat domestic hot water. Most heat-pump water heaters include an insulated tank equipped with an electric resistance element to provide backup heat whenever hot water demand exceeds the capacity of the heat pump. Since heat-pump water heaters extract heat from the air, they lower the temperature and humidity of the room in which they are installed.

(HRV). Balanced ventilation system in which most of the heat from outgoing exhaust air is transferred to incoming fresh air via an air-to-air heat exchanger; a similar device, an energy-recovery ventilator, also transfers water vapor. HRVs recover 50% to 80% of the heat in exhausted air. In hot climates, the function is reversed so that the cooler inside air reduces the temperature of the incoming hot air.

The difference between the 24-hour average (daily) temperature and the base temperature for one year for each day that the average is below the base temperature. For heating degree days, the base is usually 65 degrees Fahrenheit. For example, if the average temperature for December 1, 2001 was 30 degrees Fahrenheit, then the number of heating degrees for that day was 35.

Rate at which heat must be added to a space to maintain a desired temperature. See cooling load.

Metallic elements with high atomic weights, including mercury, lead, cadmium, arsenic, and chromium. Released as industrial pollutants, some heavy metals are toxic and may accumulate to hazardous levels in the food chain.

Different from Heavy Metal, which is a type of music frequently heard on job sites.

Index or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0.

The older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5.

(HET). Toilet that provides at least 20% water savings over the federal standard of 1.6 gpf and still meets the most rigorous standards for flush performance. HETs include pressure-assist toilets that use as little as 1.0 gpf, gravity-flush toilets that consume 1.28 gpf, and dual-flush toilets that offer two different flush volumes.

Energy audit that also includes inspections and tests to assess moisture flow, combustion safety, thermal comfort, indoor air quality, and durability.

A residential weatherization program jointly administered by the U.S. Department of Energy and the U.S. Environmental Protection Agency. The Home Performance With Energy Star program connects homeowners interested in improving the energy performance of their homes with contractors trained to assess home performance and perform energy retrofit work.

Water-distribution piping system in which individual plumbing lines extend from a central manifold to each plumbing fixture or water-using appliance; piping is typically cross-linked polyethylene (PEX). Depending on patterns of use, hot water may be delivered more quickly in such a system because the diameter of the tubing can be matched to the flow of the fixture or appliance.

(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building.

(HCFC).
Compound commonly used as a refrigerant in compression-cycle mechanical equipment (refrigerators, air conditioners, and heat pumps) or as a blowing agent to produce foam insulation. HCFCs are damaging to the Earth’s protective ozone layer, but less so than chlorofluorocarbons (CFCs).

A device that measures relative humidity of air. Mechanical hygrometers that rely on a coil of thin metal are not terribly accurate; electronic hygrometers available at most electronic or hardware stores are usually accurate to about plus or minus 2 - 3%.

A term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out.