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 177 terms in this directory beginning with the letter M.
monitoring and verification. See [[monitoring]] and [[verification]].

Mach number
ratio of the speed of a body or of a point on a body with respect to the surrounding air or other fluid or the ratio of the speed of a fluid to the speed of sound in the medium. Symbol M or NMa.

machinery room
a space that is designed to safely house compressors and pressure vessels.

magnetic ballast
a ballast constructed with magnetic core and a winding of insulated wire.

magnetic cooling
cryocooling by adiabatic demagnetization of certain paramagnetic substances.

magnetic flux
lines of force that exist about a magnetized body and collectively constitute a magnetic field.

magnetic flux density
degree of concentration of magnetic lines of force.

magnetic thermometer
one that indicates temperature from the measurement of magnetic properties (e.g., susceptibility) of any suitable substance; used for very low temperatures.

(1) pipe or duct for distributing flowing fluid to or collecting flowing fluid from various branches. (2) regulated compressed air piped to pneumatic controls.

main meter
the meter that measures the energy used for the whole facility. There is at least one meter for each energy source and possibly more than one per source for large facilities. Typically, utility meters are used, but dataloggers may also be used as long as they isolate the load for the facility being studied. When more than one meter per energy source exists for a facility, the main meter may be considered the accumulation of all the meters involved.

maintenance program
maintenance concept in terms of time and resource allocation. The maintenance program documents the objectives, establishes the criteria for evaluation, and commits the maintenance activities to basic areas of performance (e.g., prompt response to mechanical failure, repair, adjustment, and planned service functions that protect the capital investment and minimize downtime or failure response).

maintenance-free battery
A sealed battery to which water cannot be added to maintain electrolyte level.

majority carrier
Current carriers (either free electrons or holes) that are in excess in a specific layer of a semiconductor material (electrons in the n-layer, holes in the p-layer) of a cell.

makeup air
(1) dedicated replacement air. (2) air brought into a building from the outdoors to replace air that is exhausted. Makeup air may or may not be conditioned. (3) any combination of outdoor and transfer air intended to replace exhaust air and exfiltration. (4) in a cleanroom, air introduced to the secondary air system for ventilation, pressurization, and replacement of exhaust air. (5) in a laboratory or kitchen, outdoor air deliberately brought into the building from the outside and supplied to the vicinity of an exhaust hood to replace air, vapor, and contaminants being exhausted. Makeup air is generally filtered and fan forced, and it may be heated or cooled depending on the requirements of the application. Makeup air may be delivered through outlets integral to the exhaust hood or through outlets in the same room.

makeup air unit
air-handling unit that provides 100% outdoor air to offset air that is exhausted and exfiltrated, often providing conditioning or treatment of the outdoor air.

makeup water
water supplied to replenish the water of a system.

management information function
provision of information useful for management of a building environment and/or its energy efficiency and/or HVAC system maintenance.

portion of a main in which several branches are close together. Also, a single piece in which there are several fluid paths.

device for measuring pressure or head in a fluid.

requiring personal intervention for control. Nonautomatic does not necessarily imply a manual controller, only that personal intervention is necessary. See [[automatic]].

manual changeover
changeover from one operation to another by means of operator intervention.

manual damper
device that can be used to manually adjust the airflow rate by manual operation. Also see [[damper]]; [[valve]].

manual expansion valve
hand-operated, needle-type valve for controlling the flow of liquid refrigerant to an evaporator.

manual reset valve
automatic shutoff valve installed in a supply piping and set to shut off when unsafe conditions occur. The device remains closed until manually reopened.

manual switch
(1) device for interrupting or changing the path of electric current or mass flow by a physical means. (2) device used to manually turn on or interrupt an electric circuit.

manual valve
device that can be used to shut off the flow by manual operation. Also see [[damper]] and [[valve]].

manually adjusted air terminal device
see [[manual damper]].

the supplier who provides the rating of the combined appliance.

manway (manhole)
opening with removable cover in a vessel to permit periodic entry to the interior of the vessel or inspection of an area.

Margoulis number
see [[Stanton number]].

quantity of matter in a body measured in terms of resistance to acceleration by a force (i.e., its inertia). In SI, the standard unit of mass is the kilogram (kg); in I-P, the standard unit of mass is the slug.

mass diffusivity ratio
(also known as diffusion coefficient), ratio of the mass flow of a substance diffusing through a surface of unit area to the rate of variation in the concentration of this substance normal to this surface.

mass flow rate
mass of a substance flowing per unit time.

mass transfer
transfer of one component of a mixture relative to the motion of the mixture. It is the result of a concentration gradient. Compare to [[heat transfer]].

mass velocity
ratio of the mass flow rate of a fluid to the cross-sectional area of the flow.

mass-action law
rate of a chemical reaction for a uniform system at constant temperature is proportional to the concentrations of the substances reacting. Also known as Guldberg and Waage law.

master controller
(1) central device that develops corrective action, in response to the area control error, for execution at one or more generating units. (2) controller that contains the necessary circuits to operate slave controllers. (3) instrument whose variable output is used to change the setpoint of a submaster controller.

master meter
central utility meter for an aggregation of multiple units.

materials inherently of low hazard
all materials not hazardous by nature, existing at near ambient pressure and temperature, and not placing at risk people working on systems carrying such materials.

maximum allowable charging period
the period of time within which charging of the thermal storage device must be completed. This period is typically determined by the utility rate structure, the building operating schedule, and the design operating strategy.

maximum allowable pressure
(1) maximum gage pressure permitted on a completed system. (2) the setting of the pressure-relieving devices protecting the system.

maximum deviation
for a set of multiple measurements of a physical property, from which statistical methods have been used to remove spurious data points, the greatest of the deviations of the set of indicated values from the mean of the set.

maximum opening
the position of the sash at which the fume hood has the largest opening.

maximum outdoor air damper
a modulating damper or set of dampers used to control the outdoor airflow to the system in excess of minimum ventilation outdoor air for free cooling (airside economizer). Also called economizer outdoor air damper. May also serve to provide the minimum outside airflow control.

maximum permissible fill (MPF)
maximum amount of refrigerant to be packaged in a shipping container for safe transportation as defined by U.S. Department of Transportation.

maximum power point (MPP)
The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 volts.

maximum power point tracker (MPPT)
Means of a power conditioning unit that automatically operates the photovoltaic generator at its maximum power point under all conditions.

maximum power tracking
Operating a photovoltaic array at the peak power point of the array's I-V curve where maximum power is obtained. Also called peak power tracking.

maximum temperature glide
the difference between the saturated liquid temperature (bubble point) and the saturated vapor temperature (dew point) for the "as-formulated" blend composition at constant pressure. For a given pressure, the evaporator temperature glide in a direct-expansion system will typically be 70% to 80% of the maximum temperature glide (as the refrigerant blend entering the evaporator is a mixture of liquid and vapor) and not at the saturated liquid temperature of the “as-formulated” blend composition.

maximum usable cooling supply temperature
the maximum fluid supply temperature at which the cooling load can be met without adversely affecting latent space conditions.

maximum usable discharge temperature
the maximum fluid temperature at which usable cooling can be obtained from the thermal storage device.

maximum working pressure
see [[design pressure]].

used where a provision is permissible but not mandatory.

one thousand Btu per hour. An I-P unit of power. In SI, use watts or kilowatts.

Mechanical Contractors Association of America

the sum of measurement values divided by the number of measurements. It is considered the best approximation of the true value.

mean monthly outdoor air temperature
temperature is based on the arithmetic average of the mean daily minimum and mean daily maximum outdoor (dry-bulb) temperatures for the month in question.

mean radiant temperature
theoretical uniform surface temperature of an enclosure in which an occupant would exchange the same amount of radiant heat as in the actual nonuniform enclosure. Compare to [[operative temperature]].

mean tank temperature
arithmetic mean of tank-water temperatures. Determination of number and type of measurements can be as specified by standards or guidelines.

mean temperature
can be calculated as the average of temperature readings over a period of time or the average of the high and low temperatures over a given time.

mean velocity
an average of the instantaneous air velocity over an interval of time.

means of egress
a continuous and unobstructed path of travel from any point in a building or structure to a public way.

measurement and characterization
A field of research that involves assessing the characteristics of photovoltaic materials and devices.

measurement station
element inserted in the ductwork that facilitates the determination of air temperature, air humidity, airflow rate, and/or pressure.

measurement system verification
that full range of checks and tests carried out to determine if all installed measurement system components, subsystems, systems, and interfaces between systems function in accordance with the measurement plan. Also used to ascertain the as-installed uncertainty of the measurement system.

mechanical anemometer
mechanical instrument that directly measures air velocity; a device, that is sensitive to air movement, with a calibrated airspeed measuring indicator.

mechanical atomizing burner
burner supplied with fuel oil under pressure, causing self-atomization.

mechanical chiller
refrigerating machine using mechanical energy input to generate chilled water.

mechanical cooling
reducing the temperature of a fluid by using vapor compression, absorption, desiccant dehumidification combined with evaporative cooling, or other energy-driven thermodynamic means. Indirect or direct evaporative cooling alone is not considered mechanical.

mechanical efficiency
ratio of the compression energy or work of a compressor to the energy or work input.

mechanical energy
capacity for doing work, usually expressed in work units (foot-pounds or newton-meters); sometimes in heat units (Btu or joule). Energy may be inherent in the speed of a body (kinetic energy) or in its position relative to another body (potential energy).

mechanical heating
raising the temperature or change of phase of a solid or fluid by use of fossil-fuel burners, electric resistance heaters, heat pumps, or other systems that require energy to operate.

mechanical joint
joints obtained by joining parts through a mechanical construction (such as flanged joint, screwed joint, flared joint). Also see [[compression joint]].

mechanical properties
properties of a material that reveal the elastic and inelastic reaction when force is applied or that involve the relationship between stress and strain; for example, the modulus of elasticity, tensile strength, and fatigue limit.

mechanical refrigerating system
refrigerating system using mechanical compression to move the refrigerant from the low-pressure side and to deliver it to the high-pressure side of the system.

mechanical seal
seal with small enough clearance between moving parts to provide pressure tightness and minimize leakage between mechanical parts.

mechanical shaft work
energy delivered or absorbed by a mechanism, such as a turbine, air compressor, or internal combustion engine.

mechanical ventilation
(1) the active process of supplying or removing air to or from an indoor space by powered equipment such as motor-driven fans and blowers but not by devices such as wind-driven turbine ventilators and mechanically operated windows. (2) ventilation provided by mechanically powered equipment, such as motor-driven fans and blowers, but not by devices such as wind-driven turbine ventilators and mechanically operated windows.

mechanical, draft-water cooling tower
tower through which air movement is effected by one or more fans. There are two main types: forced draft with fans located at the air inlet and induced draft with fans located at the air exhaust. See also [[cooling tower]].

medium pressure refrigerant system
system whose gage pressure at room temperature (74°F [23.3°C]) is greater than atmospheric pressure but typically less than 100 psig (689 kPa). Common medium pressure refrigerants include R-12, R-500, and R 134a.

medium temperature refrigerated storage
the temperature range for maintaining food product above freezing in refrigeration applications. Typically 35°F to 40°F (3°C to 5°C).

megawatt (MW)
1,000 kilowatts, or 1 million watts; standard measure of electric power plant generating capacity.

1,000 kilowatt-hours or 1 million watt-hours.

Meissner state
thermodynamic state shown by all superconductors in a sufficiently weak magnetic field, characterized by the exclusion or expulsion of magnetic flux from everywhere within the superconductor except a thin surface layer.

change of state from solid to liquid, caused by absorption of heat.

melting point
for a given pressure, the temperature at which the solid and liquid phases of the substance are in equilibrium.

membrane manometer (diaphragm manometer)
pressure gage with a flexible membrane whose deformations are used to indicate pressure.

memory stop
an adjustable mechanical device that allows a valve to be closed (for service) and limits the valve to a predetermined position when reopened.

(1) free surface of a liquid which, near the walls of a vessel, is curved because of surface tension. (2) in a manometer, the datum point measured at the center of the free surface.

mercury switch
glass tube partially filled with mercury. Electrical contact is established when the tube is tilted so that the mercury bridges the gap between contacts located at the same end. Tilting in the opposite direction opens the circuit.

mercury thermometer
bulb and attached glass capillary tube containing mercury which expands or contracts with changes in temperature; so marked that the end of the mercury column indicates the ambient temperature.

see [[minimum efficiency reporting values (MERV)]].

MERV rating
see [[minimum efficiency reporting values (MERV)]].

metabolic heat
heat produced by oxidation of food elements (i.e., metabolism) in humans or animals. The met represents the average heat produced by a sedentary man, approximately 90 kcal/h or 100 W (340 Btu/h).

metabolic rate
(1) rate of energy production of the body. The rate varies with the type of activity. (2) the rate of transformation of chemical energy into heat and mechanical work by metabolic activities within an organism, usually expressed in terms of unit area of the total body surface. Metabolic rate is expressed in met units.

chemical changes in living cells by which energy is provided for vital processes.

metered data
energy end use data collected over time using a measuring device or group of measuring devices.

metered demand
the average time rate of energy flow over a period of time.

collection of energy data using a measuring device or group of measuring devices.

methanol (methyl alcohol)
CH3OH, colorless, toxic, flammable liquid with a boiling at 148.9°F (65.0°C) and having a flash point (open cup) of 54°F (12.2°C).

The science of measurement.

conditions such as temperature, humidity, and motion of air within an enclosure or limited outdoor area.

A small groove scribed into the surface of a solar cell, which is filled with metal for contacts.

micrometer (micron)
One millionth of a meter.

a microscopic organism, especially a bacterium, fungus, or protozoan.

mineral-fiber thermal insulation
insulation composed principally of fibers manufactured from rock, slag, or glass, with or without binders.

minimum efficiency reporting values (MERV)
scaled rating of the effectiveness of air filters. The scale is designed to represent the worst-case performance of a filter when dealing with particles in the range of 0.3 to 10 micrometers. The MERV rating is from 1 to 16. Higher MERV ratings correspond to a greater percentage of particles captured on each pass, with a MERV rating of 16 filter capturing more than 95% of particles over the full range.

minimum motor efficiency
the minimum efficiency occurring in a population of motors of the same manufacturer and rating.

minimum operating pressure
the minimum pressure where effective operation is achieved

minimum outdoor air damper
a damper in parallel with the maximum outdoor air damper to provide the minimum outdoor air required for ventilation.

minisplit system
an encased, factory-made assembly or assemblies designed to be used as permanently installed equipment to provide conditioned air to an enclosed space(s). It normally includes multiple evaporators, compressor(s), and condenser(s). Such equipment may be provided in more than one assembly, the separated assemblies of which are intended to be used together.

minority carrier
A current carrier, either an electron or a hole, that is in the minority in a specific layer of a semiconductor material; the diffusion of minority carriers under the action of the cell junction voltage is the current in a photovoltaic device.

minority carrier lifetime
The average time a minority carrier exists before recombination.

ability of a liquid or gas to dissolve uniformly in another liquid or gas.

specific activities, technologies, or equipment designed or deployed to capture or control substances upon loss of containment to minimize exposure of the public or the environment. Passive mitigation means equipment, devices, or technologies that function without human, mechanical, or other energy input. Active mitigation means equipment, devices, or technologies that need human, mechanical, or other energy input to function.

mixed air
(1) air that contains two or more streams of air. (2) combined outdoor air and recirculated air.

mixed flow fan
fan whose characteristics combine both centrifugal and axial airflow.

mixing air diffusion
air diffusion where the mixing of supply air and room air is intended. Also see [[air diffusion]].

mixing box (blending box; mixing unit)
compartment into which two air supplies are mixed together before being discharged.

mixing section
a section for the mixing of two air streams at differing temperatures or humidities or both.

mixing system
a type of air-distribution system in which conditioned air is delivered to the space at a velocity sufficient to promote complete mixing of supply air with room air, thereby maintaining the entire volume of air in the space at a relatively uniform temperature, humidity, and air quality condition. A conventional overhead air distribution, which supplies and returns air at ceiling level, is an example of a mixing system.

mixing valve
three-way valve to mix two fluids. Compare to [[diverting valve]].

mode of vibration
in a system undergoing vibration, a mode of vibration designates the characteristic pattern of displacement assumed by the system vibrating at one of its resonance frequencies.

mode shape
the shape of the structure when vibrating at a natural frequency. Note: mode shapes should be normalized (usually by referring all values to a fraction of the motion at some reference point).

a mathematical representation or calculation procedure that is used to predict the energy use and demand in a building or facility. Models may be based on equations that specifically represent the physical processes (refer to [[simulation model]]) or may be the result of statistical analysis of energy use data.

modified sine wave
A waveform that has at least three states (i.e., positive, off, and negative). Has less harmonic content than a square wave.

modular air-conditioning system
on-site assembly of prefabricated components, each with a functional role in an air-conditioning (air-circulation, air-filtration, cooling, heating, humidification, etc.) system.

modular boiler
boiler designed and constructed to include increments of capacity.

The use of multiple inverters connected in parallel to service different loads.

(1) to adjust by small increments and decrements. (2) to vary a voltage or other variable with a signal.

modulating control
(1) a step-modulating control that is capable of controlling flow rate between the maximum and the minimum adjustable input rate in response to varying heating or cooling load. (2) method of control in which the output of the controller may vary infinitely over its range.

modulating diffuser terminal unit
diffuser with features of an air terminal unit and with an integral airflow control device.

modulating-control valve
valve capable of increasing or decreasing by increments the fluid flow according to deviation from the set control value.

adjustment of controller action by increments and decrements.

See photovoltaic (PV) module.

module derate factor
A factor that lowers the photovoltaic module current to account for field operating conditions such as dirt accumulation on the module.

modulus of elasticity
ratio of stress (nominal) to corresponding strain below the proportional (elastic) limit of a material, expressed in force per unit area based on the average initial cross-sectional area. Also known as Young’s modulus.

moisture carryover
retention and transport of water droplets in a gas stream (usually air) (e.g., water droplets formed by bridging fins of a coil and transported by the airstream).

moisture content
the amount of moisture per unit volume of porous material, w, in lbm/ft3 (kgm/m3).

moisture permeability
moisture flux through a unit thickness of a homogeneous material in a direction perpendicular to the isosuction planes, induced by a unit difference in suction. Units are lbm/ft·s·in. Hg [kgm/(m·s·Pa)].

moisture proofing
process for making things impervious to liquid. Note: if not completely impervious, use moisture retarder.

moisture ratio
the amount of moisture per unit weight of dry porous material or the volume of moisture per unit volume of dry material, in percent.

moisture removal capacity
the mass of water vapor removed from the process air per unit of time and expressed in kg/h (lb/h).

moisture removal rate
the mass of water vapor removed from the desiccant per unit of time via the regeneration process (desorption) and expressed in kg/h (lb/h).

mol fraction water vapor
in humid air, the ratio of the number of water vapor mols to the total number of mols in the mixture. Compare to [[humidity]].

biological growth or fungi growing on moist organic matter.

molecular flow
gaseous flow where the average free path of the gas molecules is greater than the largest dimension of the cross section of the duct. Parallel flow.

molecular sieve
adsorbent composed of porous aluminosilicates with pores of uniform molecular dimensions that selectively adsorb molecules of the substance to be adsorbed.

Mollier diagram (Mollier chart)
graph of enthalpy versus entropy of a vapor on which isobars, isothermals, and lines of equal dryness are plotted.

observing, supervising, controlling, or verifying the operations of a system.

monitoring and verification (M&V) plan
(1) a plan for gathering of relevant measurement data over time to evaluate equipment or system performance. The plan defines specific M&V methods to be used, including baseline determination, performance period measurements, savings verification calculations, and acceptance criteria. The M&V methods chosen are consistent with the current facility requirements (CFR). During the implementation phase, a list is developed of specific instrumentation and data-gathering equipment that must be maintained at the site. During the hand-off phase, the type, frequency, and distribution of M&V reports to be submitted for approval is confirmed. (2) equipment to measure and record the parameters of the HVAC&R systems (i.e., temperature, humidity, pressure, electric current, kW, and volts). (3) gathering of relevant measurement data over time to evaluate equipment or system performance (e.g., chiller electric demand, inlet evaporator temperature and flow, outlet evaporator temperature, condenser inlet temperature, and ambient dry-bulb temperature and relative humidity or wet-bulb temperature, for use in developing a chiller performance map (e.g., kW/ton versus cooling load and versus condenser inlet temperature).

Fabricated as a single structure.

Moseley’s law
the square root of the frequency of an x-ray spectral line belonging to a particular series is proportional to the difference between the atomic number and a constant that depends only on the series.

motor overload protection
devices that automatically disconnect a motor from its energy supply when predetermined unsafe temperatures or overcurrent conditions exist. See [[line-break motor protection]].

motor power, motor engine
machine in which power is applied to do work by the conversion of various forms of energy into mechanical force and motion.

motorized valve (motor operated valve)
valve operated by an electric motor or actuator.

Short for metal oxide varistor. Used to protect electronic circuits from surge currents such as those produced by lightning.

[[mean radiant temperature]]

Manufacturers' Standardization Society of the Valve and Fittings Industry

Mu (μ)
dynamic air viscosity, kg/m·s (lbm/ft·s)

(1) device intended to reduce noise; more particularly, an expansion chamber in the exhaust line of an internal combustion engine. (2) device to reduce sound level.

multi-stage controller
A charging controller unit that allows different charging currents as the battery nears full state_of_charge.

multiblade damper
damper having two or more blades or leaves.

multicellular metal-foil thermal insulation
thermal insulation consisting of waffled or corrugated sheets of metal foil to form a cellular structure.

multichannel heat exchanger
apparatus in which fluids exchange heat by flowing in an assembly of separated channels.

A semiconductor (photovoltaic) material composed of variously oriented, small, individual crystals. Sometimes referred to as polycrystalline or semicrystalline.

multijunction device
A high-efficiency photovoltaic device containing two or more cell junctions, each of which is optimized for a particular part of the solar spectrum.

multilayer thermal insulation
many thin layers of materials combined to obtain a very high thermal resistance (superinsulation).

multiple-leaf damper
a number of shutters in opposed blade or parallel arrangement.

multishell condenser
condenser consisting of a number of closed shell-and-tube units.

multistage compression
compression in two or more stages; usually the low-stage compressor discharges to the suction of a higher-stage compressor.

multistage compressor
compressor in which compression is accomplished in more than two stages in separate cylinders or impellers.

multistage expansion
allowing refrigerant to pass through two or more expansion valves in series with each other.

multistage refrigerating system
system in which compression of refrigerant is carried out in two or more steps.

multivane rotary compressor
sliding vane compressor with several vanes, sliding-in slots in the rotor, and that maintains continuous contact with the inner periphery of the fixed casing.

(1) air-conditioning unit capable of handling variable loads from different sections of a building simultaneously. (2) spatial divisions of a building having different air-conditioning loads.

mushroom valve
poppet valve whose head is mushroom shaped.