A comprehensive online glossary of terms and definitions related to built environment, heating, ventilating, and air conditioning (HVAC), as well as refrigeration, building envelope, electrical, lighting, water and energy use, solar power, concentrating solar power (CSP), and measurement terms.

HVAC and Solar energy engineering Terminology

There are currently 30 terms in this directory beginning with the letter U.
unit thermal conductance or overall heat transfer coefficient, W/(m2·K) (Btu/(h·ft2·°F).

Underwriters' Laboratories, Inc.

ultimate strength
the stress at which rupture occurs.

Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

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.

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.

underground feeder (UF)
May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures.

underground service entrance (USE)
May be used within battery enclosures and for interconnecting balance-of-systems.

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.

uninterruptible power supply (UPS)
The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

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.

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.

utility-interactive inverter
An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system's output is fully synchronized with the utility power.

utilization factor
see [[use factor]].