HVAC GLOSSARY - D

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Damper: A device that is located in ductwork to adjust air flow. This movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms. There are basically two types of dampers: Manual and motorized. A manual damper generally consists of a sheet metal (or similar material) flap, shaped to fit the inside of a round or rectangular duct. By rotating a handle located outside of the duct a technician can adjust (see Balancing) air flow to match the needs of a particular area or room. A motorized damper is generally used in a zoned system (see Zoning) to automatically deliver conditioned air to specific rooms or zones. In particular, the following types can be distinguished: Multiple leaf dampers, comprising of several blades (or shutters) of opposed or parallel leaf type. Single leaf dampers (the flap being mounted at one end), commonly called splitter dampers. Hit-and-miss dampers, having two or more slotted slide mechanisms. Butterfly dampers, with two flaps in "V" arrangement.

Dampers: Controls that vary airflow through an air outlet, inlet, or duct. A damper position may be immovable, manually adjustable or part of an automated control system.

db: (Decibel) A decibel describes the relative loudness of a sound. Some common sounds are fairly close to a typical air conditioner or heat pump's sound level: human voice, 7.0 decibels; blender, 8.8 decibels.

DDC: (Direct Digital Control) Direct Digital Control.

Defrost Cycle: The process of removing ice or frost buildup from the outdoor coil during the heating season.

Dehumidification: The reduction of water vapor in air by cooling the air below the dew point; removal of water vapor from air by chemical means, refrigeration, etc.

Delta: (or Delta T) A difference in temperature. Often used in the context of the difference between the design indoor temperature and the outdoor temperature.

Demand Billing: The electric capacity requirement for which a large user pays. It may be based on the customer's peak demand during the contract year, on a previous maximum or on an agreed minimum. Measured in kilowatts.

Demand Charge: The sum to be paid by a large electricity consumer for its peak usage level.

Demand: (Utility) The rate at which electricity or natural gas is delivered to or by a system, part of a system, or piece of equipment, e.g., to end users, at a given instant or averaged over any designated period of time. Electricity demand is typically expressed in kilowatts.

Design Conditions: Cooling loads vary with inside and outside conditions. A set of conditions specific to the local climate are necessary to calculate the expected cooling load for a home. Inside conditions of 75 degrees Fahrenheit and 50% relative humidity are usually recommended as a guideline. Outside conditions are selected for the 2.5% design point.

Dewpoint: Is the temperature at which air becomes saturated with water and begins to condense - forming a dew. Therefore at 100 % RH the ambient or process temperature equals the dewpoint temperature. The more negative the dewpoint temperature is from the ambient temperature the less the risk of condensation and the drier the gas or air stream.

Diffusers and Grilles: Components of the ventilation system that distribute and return air to promote air circulation in the occupied space. Generally speaking, supply air enters a space through a diffuser or vent and return air leaves a space through a grille.

Dilution: A mitigation strategy that lowers the concentration of airborne contaminants by increasing the fraction of outdoor air in the supply airstream.

Direct Current: (DC) Electricity that flows continuously in the same direction.

Direct Expansion:(Refrigeration) Any system that, in operation between an environment where heat is absorbed (heat source), and an environment into which unwanted heat is directed (heat sink) at two different temperatures, is able to absorb heat from the heat source at the lower temperature and reject heat to the heat sink at the higher temperature. The cooling effect is obtained directly from a fluid called a refrigerant that absorbs heat at a low temperature and pressure, and transfers heat at a higher temperature and higher pressure.

Direct Gas-Fired Heater: The burner fires directly in the air stream being heated, rather than through a heat exchanger. 100% of available BTUs are delivered to the heated space because no flue or heat exchanger is required. This results in no wasted energy.

DOE: (Department of Energy) The Department of Energy is a federal agency in charge of setting industry efficiency standards and monitoring the consumption of energy sources.

Double Glazing: Windows having two sheets of glass with an airspace between.

Downflow: A type of furnace that takes cool air from the top and blows warm air to the bottom - common where your furnace must be located in a second-floor closet or utility area.

Downflow Furnace: A furnace that pulls in cool return air from the top and blows/expels warm air at the bottom - common where your furnace must be located in a second-floor closet or utility area.

Drain Trap: A dip in the drainage pipe of sinks, toilets, floor drains, etc., which is designed to stay filled with water, thereby preventing sewer gases from escaping into the room.

Drier: Sometimes called filter/drier, it removes moisture and keeps the refrigerant clean.

Dry Bulb Temperature: (DB) The temperature measured by a standard thermometer. A measure of the sensible temperature of air.

Dual Fuel System: A dual heating system, for example a heat pump and a fossil fuel furnace.

Dual-Duct System: A central plant heating, ventilation, and air conditioning (HVAC) system that produces conditioned air at two temperatures and humidity levels. The air is then supplied through two independent duct systems to the points of usage where mixing occurs.

Dual-Paned: (Double-glazed) Two panes of glass or other transparent material, separated by a space.

Duct: A pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit. A passageway made of sheet metal or other suitable material used for conveying air or other gas at relatively low pressures.

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 tends to lose adhesion with age, especially when used on ducts in unconditioned spaces.

Ductwork: A pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit. Hollow pipes or channels that carry/transfer air from the Air Handler to the air vents throughout your home. The delivery system through which warm air from the furnace is brought to where it's needed. Ductwork is made of sheet metal, fiberglass, or flexible plastic, and can be round or rectangular in shape. Ductwork is one of the most important components of a home heating and cooling system.

Dust: Dust is comprised of particles in the air that settle on surfaces. Large particles settle quickly and can be trapped by the body's defense mechanisms. Small particles are more likely to be airborne and can pass through the body's defenses and enter the lungs.