ASTM. American Society for Testing and Materials.

ASHRAE. American Society of Heating, Refrigerating and Air Conditioning Engineers.

Air handler. The cabinet for a central furnace, air conditioner, or heat pump, which contains a fan that propels air through the duct system (see "Air Handler Fan: A Driving Force for Air Infiltration," HE Nov/Dec '89, p.11).

Backdrafting. Reverse flow of combustion gases down the chimney of a vented combustion appliance, which is often caused by depressurization of the room where the appliance is located (see "Backdrafting Causes and Cures," HE May/June '91, p.30).

Blower door. A large powerful variable-speed fan mounted in a doorway that blows air into (pressurizes) or sucks air out of (depressurizes) a house. It's used to test for air leakage in a house (see "Leak Detectors: Experts Explain the Techniques," p.26). The size and complexity of the fan varies, but all blower doors have adjustable frames around the fan so that they can fit snugly into most doorways.

Buffer zone. An area within the home between the conditioned zones and the outside. Thus it normally is not conditioned (for instance, attics, attached garages, crawlspaces, basements, and enclosed porches).

Boot. A piece of duct used to connect ducts with registers.

Conduction. The transfer of heat through a solid.

Convection. The movement of heat by air flow.

cfm. Cubic feet per minute, a measure of air flow.

Duct tape. This (initially) sticky tape is unfortunately the most common material used to seal duct connections. Care must be taken when it's applied. For effective sealing, the surface it is applied to must be clean--free of dust, dirt, oil, or other substances. Duct tape has a tendency to lose adhesion with age, especially when used on ducts in unconditioned spaces (see "Guidelines for Designing and Installing Tight Duct Systems," p.55).

Ductwork. Round or rectangular passageways for conditioned air.

Envelope. The air barrier that separates the conditioned space from the outside and from unconditioned spaces like attics and garages.

Exfiltration. Uncontrolled air leakage out of a building.

Flame roll-out. A dangerous situation that occurs when flame is pushed out of the bottom of a combustion appliance. It can lead to fires.

Flex duct. Usually installed in a single, continuous piece between the register and plenum box, a flexible duct usually has an inner lining and an insulated coating on the outside.

Flow hood. A diagnostic tool used to measure air flow through ducts, supply registers, and return grilles (see "Leak Detectors: Experts Explain the Techniques," p.26).

Heat exchanger. This device transfers heat from outgoing stale air to incoming cold air. In warm climates, this process can be reversed.

HVAC systems. Heating, ventilating, and air conditioning systems.

Infiltration. Unintentional movement of outdoor air into a house. It results from the forces of wind, temperature difference, and HVAC operation.

Mastic. An adhesive paste used in the fabrication and sealing of thermal insulation on piping fittings, equipment, and duct work. It spreads easily and dries permanently. Its applications include new ducts and old, sheet metal seams, and ductboard. Not all mastics are created equal. Some are toxic, some are water-based, and some take longer to cure, making them more suitable for new construction than retrofit.

Manometer. An instrument that measures air pressure differences between locations. Tubes are usually attached to a manometer and run to the spaces where pressures are measured (see "Leak Detectors: Experts Explain the Techniques," p.26).

Pascals (Pa). A small unit of air pressure. One pound per square inch equals 6,895 pascals.

Supply. The ductwork that carries air from the air handler to the rooms in the house.

Radiation. The transfer of heat directly from one surface to another (without heating the intermediate air acting as a transfer mechanism).

Return. The ductwork that carries air from the house to the air handler.

Plenum. Air flow passage made of duct board, metal, drywall, or wood. Joins supply and return ducts with HVAC equipment.

Pressure balancing. The process of neutralizing pressure differences within a home (see "In Search of the Missing Leak," HE Nov/Dec '92, p.27).

Registers and grilles. Coverings for the ducts where they open to the conditioned space.

Smoke stick. A diagnostic tool used to observe air flow. Usually it consists of a chemical in a squeezable container. When squeezed it emits smoke which visibly follows air flow currents.

Zone. 1) Conditioned space in a house under the control of a thermostat. 2) A space within a house with a distinct pressure compared to other pressure zones. Also see Buffer zone.

Figure 1. Schematic of a duct system.

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