ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

unit thermal conductance or overall heat transfer coefficient, W/(m2·K) (Btu/(h·ft2·°F).


Uniform Building Code

ultimate strength

the stress at which rupture occurs.

ultraviolet (UV) radiation

range of electromagnetic radiation wavelengths from about 400 nm, just beyond the violet in the visible spectrum, to about 4 nm, on the border of the x-ray region. UV photons have enough energy to initiate some chemical reactions, to degrade most plastics, and to kill microorganisms.

ultraviolet C (UVc), air cleaner

process uses UVc light energy to reduce or deactivate bioaerosol. UVc light has a nominal wavelength of 253 nm.

unconditioned space

space within a building that is not conditioned space.

underfloor air-distribution system (UFAD)

an air-distribution system that uses an underfloor plenum (open space between the structural concrete slab and the underside of a raised-floor system) to deliver conditioned air into the space, typically through floor diffusers. Air is returned at a level above the occupied zone (typically at the ceiling level). Under cooling operation, UFAD systems produce a stratified environment, similar in principle to that of displacement systems. The primary difference between these systems is that UFAD outlets deliver air at higher velocity, producing greater mixing in the area near the outlet discharge.


condition where voltage is lower than rated.

unglazed solar collector

a solar collector in which the absorbing surface is directly exposed to the atmosphere.

uniflow compressor

compressor in which the suction gas passes through the piston.

uniform velocity

a smooth, straight line motion of a gas or a liquid across the area of flow.

uninterruptable power supply (UPS)

a system intended to deliver continuous, stable power to the critical load. The majority of modern UPS systems are of two fundamental types: (a) “static,” in which incoming alternating current (AC) power is rectified to direct current (DC) and then inverted back to AC, with batteries in the DC portion that assume the load when incoming power fails or anomalies occur, and (b) “rotary,” in which incoming AC power drives a propulsion unit that turns a generating device, with a heavy flywheel storing kinetic energy that continues to turn the generating portion when incoming power fails or anomalies occur. Either type can be made up of one or more modules running in parallel to add capacity or redundancy or both. DC UPS systems, which eliminate the inverter and deliver DC power to the ITE, are also used.

unit control panel (UCP)

location where one or more accessory panels are grouped to serve a single remote system. Contains both accessory devices and local loop controls.

unit cooler

(1) an assembly of cooling coils with drain pan, fan, and enclosure. (2) direct cooling, factory made encased assembly including an air-cooling coil, refrigerating compressor and condenser, fan and motor (usually), and directional outlet, including the necessary automatic controls.

unit heater

heater consisting of a fan for circulating air over a heat exchange surface, all enclosed in a common casing.

unit ventilator

fan coil unit package devised for applications in which the use of outdoor and return air mixing is intended to satisfy tempering requirements and ventilation needs.

unitary system

one or more factory-made assemblies that normally include an evaporator or cooling coil and a compressor and condenser combination.

unmet load hour

an hour in which one or more zones is outside of the thermostat set point plus or minus one half of the temperature control throttling range. Any hour with one or more zones with an unmet cooling load or unmet heating load is defined as an unmet load hour.

unprotected tubing

tubing that is unenclosed and therefore exposed to crushing, abrasion, puncture, or similar damage after installation.

unshrouded impeller

open impeller with the blades left uncovered. Caution should be used in placing access for servicing requirements.

unsteady state

state of a system when the various physical quantities at any location vary randomly.

upfeed system

piping arrangement for a heating, air-conditioning, or refrigerating system in which heat transfer fluid is circulated through supply mains that are below the levels of heating or cooling units they serve.

usable storage capacity

total amount of cooling discharged from a thermal storage device, at or below the maximum usable discharge temperature, for a particular storage cycle.

use factor

percent capacity realized over a period of time that a system is operated.

useful refrigerating effect

rate at which heat is removed by the primary refrigerant, or by the secondary refrigerant, between two specified points, taking into account the conditions of use.

utility meter

the meter used to calculate a monthly energy and/or demand charge at a specific utility/customer connection, more than one may be installed per customer and per site due to different supply voltages, capacity requirements, physical separation distances, installation periods, or for specific customer requirements or utility programs.

utilization factor

see use factor.