14 ANSI/ASHRAE/IESNA Standard 90.1-2007 dry-type transformer: a transformer in which the core and coils are in a gaseous or dry compound. liquid-immersed transformer: a transformer in which the core and coils are immersed in an insulating liquid. U-factor (thermal transmittance): heat transmission in unit time through unit area of a material or construction and the boundary air films, induced by unit temperature difference between the environments on each side. Units of U are Btu/h?ft2??F. unmet load hour: an hour in which one or more zones is outside of the thermostat setpoint range. unconditioned space: see space. unenclosed space: a space that is not an enclosed space. unitary cooling equipment: one or more factory-made assemblies that normally include an evaporator or cooling coil and a compressor and condenser combination. Units that perform a heating function are also included. unitary heat pump: one or more factory-made assemblies that normally include an indoor conditioning coil, compressor(s), and an outdoor refrigerant-to-air coil or refrigerant-to-water heat exchanger. These units provide both heating and cooling functions. variable-air-volume (VAV) system: HVAC system that controls the dry-bulb temperature within a space by varying the volumetric flow of heated or cooled supply air to the space. vent damper: a device intended for installation in the venting system of an individual, automatically operated, fossil-fuelfired appliance in the outlet or downstream of the appliance draft control device, which is designed to automatically open the venting system when the appliance is in operation and to automatically close off the venting system when the appliance is in a standby or shutdown condition. ventilation: the process of supplying or removing air by natural or mechanical means to or from any space. Such air is not required to have been conditioned. vertical fenestration: see fenestration. voltage drop: a decrease in voltage caused by losses in the lines connecting the power source to the load. wall: that portion of the building envelope, including opaque area and fenestration, that is vertical or tilted at an angle of 60 degrees from horizontal or greater. This includes above- and below-grade walls, between floor spandrels, peripheral edges of floors, and foundation walls. For the purposes of determining building envelope requirements, the classifications are defined as follows: above-grade wall: a wall that is not a below-grade wall. below-grade wall: that portion of a wall in the building envelope that is entirely below the finish grade and in contact with the ground. mass wall: a wall with an HC exceeding (1) 7 Btu/ft2??F or (2) 5 Btu/ft2??F, provided that the wall has a material unit weight not greater than 120 lb/ft3. metal building wall: a wall whose structure consists of metal spanning members supported by steel structural members (i.e., does not include spandrel glass or metal panels in curtain wall systems). steel-framed wall: a wall with a cavity (insulated or otherwise) whose exterior surfaces are separated by steel framing members (i.e., typical steel stud walls and curtain wall systems). wood-framed and other walls: all other wall types, including wood stud walls. wall area, gross: the area of the wall measured on the exterior face from the top of the floor to the bottom of the roof. warm-up: increase in space temperature to occupied setpoint after a period of shutdown or setback. water heater: vessel in which water is heated and is withdrawn for use external to the system. wood-framed and other walls: see wall. wood-framed and other floors: see floor. zone, HVAC: a space or group of spaces within a building with heating and cooling requirements that are sufficiently similar so that desired conditions (e.g., temperature) can be maintained throughout using a single sensor (e.g., thermostat or temperature sensor). 3.3 Abbreviations and Acronyms ac alternating current ach air changes per hour AFUE annual fuel utilization efficiency AHAM Association of Home Appliance Manufacturers ANSI American National Standards Institute ARI Air-Conditioning and Refrigeration Institute ASHRAE American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ASTM American Society for Testing and Materials BSR Board of Standards Review Btu British thermal unit Btu/h British thermal unit per hour Btu/ft2??F British thermal unit per square foot per degree Fahrenheit Btu/h?ft2 British thermal unit per hour per square foot Btu/h?ft??F British thermal unit per hour per linear foot per degree Fahrenheit Btu/h?ft2??F British thermal unit per hour per square foot per degree Fahrenheit CDD cooling degree-day CDD50 cooling degree-days base 50?F cfm cubic feet per minute c.i. continuous insulation