Reference

Glossary

26 terms that appear on boxes, spec sheets and product listings across this site. Where a term is commonly used to mean more than it can support, the entry says so.

MERV: Minimum Efficiency Reporting Value, a filter rating produced by an ASHRAE laboratory test method. Residential filters generally run from about 1 to 16, with higher numbers describing media that captures a wider range of particle sizes. The rating says nothing about how much resistance the media adds to airflow, which is the number your equipment actually reacts to.MERV, and What a Higher Number Costs You Furnace & HVAC Filters
MPR: Microparticle Performance Rating, a scale published by one filter manufacturer for its own products and weighted toward the finest particles it measures. It produces numbers in the hundreds and thousands where MERV produces numbers under twenty. There is no arithmetic that converts one into the other.MPR and MERV Are Not the Same Scale
FPR: Filter Performance Rating, a scale published by one large retailer for the filters it sells. Like MPR it is a real measurement defined by the party publishing it, and like MPR it does not convert into MERV. Three scales exist because each party would rather you compared within its own.MPR and MERV Are Not the Same Scale
HEPA: A defined filter medium grade, described by how much of a challenge aerosol the medium captures at the particle size where it performs worst, under specified test conditions. It is a property of the medium rather than of the appliance the medium sits in. A grade met in a laboratory does not describe what a room contains after a machine has run in it.HEPA, True HEPA, and Words That Look Like Them Air Purifier Filters
True HEPA: A marketing phrase, not a separate grade. Sellers adopted it to distinguish their products from things described as HEPA-type or HEPA-style, which signal resemblance without stating the grade is met. This site quotes all three as listing wording and treats none of them as an additional standard.HEPA, True HEPA, and Words That Look Like Them
Activated carbon: A carbon material processed to have a very large internal surface area, used in filter cartridges to adsorb gases and odors rather than to trap particles. It works on a different mechanism from the particle-capture layer and reaches the end of its useful life on its own schedule. Several purifier cartridges in this catalog state it as part of the assembly.Air Purifier Filters Core 300-RF Replacement Filters
Nominal size: The size printed on a filter frame and used in its product name, rounded to whole inches. It names the slot the filter is meant for and is not a measurement of the filter. A filter marked 20x20x1 does not measure twenty inches on any face.Nominal Size Is a Name, Actual Size Is a Measurement Size Finder
Actual size: What a tape measure gets when applied to the frame. Filters are built under their nominal figure so they slide into a slot, and how far under is a decision each manufacturer makes for itself. In this catalog the gap on a nominal 20-inch face runs from 0.12 in to a full 0.50 in.Nominal Size Is a Name, Actual Size Is a Measurement 20x20x1 Furnace Filters
Static pressure: The pressure a duct system works against, measured in inches of water column. Every component in the air path contributes to it, including the filter. Equipment manufacturers publish the external static pressure their blowers are designed for, and that figure is what a filter choice has to respect.MERV, and What a Higher Number Costs You Furnace & HVAC Filters
Airflow restriction: The practical consequence of finer filter media or a loaded filter: less air moves through the system than it was designed to move. It is invisible from the room and shows up at the equipment. A clean filter at a moderate rating restricts less than a loaded filter at a fine rating.MERV, and What a Higher Number Costs You The Replacement Calendar
CFM: Cubic feet per minute, a measure of air volume moved per unit of time. It is the honest way to compare how much air a fan moves, and exactly one fan listing in this catalog states a figure for it. Throw distance in feet, speed counts, and watts are all different quantities and none of them convert into CFM.Pedestal & Standing Fans
ASHRAE BTU: The cooling capacity figure produced by an ASHRAE test procedure, and the larger of the two numbers a portable air conditioner carries. It is the one that appears in the product name because it is bigger. It does not account for the heat a single-hose unit brings back into the room.The Two BTU Numbers on One Portable AC Box BTU Check
SACC: Seasonally Adjusted Cooling Capacity, the figure produced by the Department of Energy test procedure for portable air conditioners. It accounts for infiltration and duct heat that the older method does not, and it is the smaller of the two numbers on the box. It is the figure to size a room against.The Two BTU Numbers on One Portable AC Box BTU Check
DOE rating: A capacity figure produced under a Department of Energy test procedure. On portable air conditioner listings the phrases DOE and SACC are used for the same smaller number, and one listing in this catalog states 8,000 BTU DOE behind a 12,000 BTU ASHRAE headline.The Two BTU Numbers on One Portable AC Box Portable Air Conditioners
Single-hose and dual-hose: Two arrangements for getting exhaust out of a portable air conditioner. A single-hose unit draws air from inside the room, passes it over the condenser, and pushes it out of the window, which lowers pressure in the room and pulls outside air in through gaps. A dual-hose unit draws its condenser air from outside instead. The SACC test procedure accounts for the difference.The Two BTU Numbers on One Portable AC Box Portable Air Conditioners
Evaporative cooling: Cooling produced by water changing to vapor, which takes heat from the air passing over it. A machine using this principle adds humidity to the space and does not remove heat from it. It is not an air conditioner and publishes no BTU figure, which is why the one in this catalog sits in its own group.Portable Air Conditioners BTU Check
C-wire: A conductor some thermostats state as a requirement. Whether one is present behind a particular wall plate is not something this site, or any product listing, can tell you. This site publishes no wiring guidance of any kind, and a thermostat listing stating that a C-wire is required is describing the product rather than your wall.Thermostats
Line voltage: Heating systems that switch mains voltage directly — electric baseboard heaters and wall convectors are the common residential examples. The thermostats in this catalog are not built for line voltage, and connecting one to such a system can damage equipment. Identify the system type before shopping at all.Thermostats An Apartment With No Central Air
Millivolt system: A control arrangement powered by a thermopile rather than a transformer, found on some standing-pilot heating appliances. It is another system type that common smart and programmable thermostats are not built for. The manufacturer compatibility checker for a specific model is where this gets settled.Thermostats
Heat pump: Equipment that moves heat in either direction, providing both heating and cooling from one machine. Some configurations, particularly those with auxiliary or emergency heat stages, need thermostat support that not every model provides. Compatibility is a question for the manufacturer checker and, where unclear, a licensed HVAC professional.Thermostats The Whole House and the Furnace Itself
Oscillation: The side-to-side sweep of a fan head, stated as an angle. A 90 degree sweep keeps air over a narrower area; 120 degrees spreads it across more of a room and spends less time on any one spot. One fan in this catalog states oscillation on two axes rather than one.Pedestal & Standing Fans A Home Office With Equipment In It
Ultrasonic humidifier: A humidifier that breaks water into fine droplets using a high-frequency vibrating plate, with no heating involved. Because the water is not boiled, minerals dissolved in it can be carried into the room and settle as fine dust on nearby surfaces. It is one of the cool mist arrangements and carries no burn hazard.Humidifiers A Bedroom That Runs Warm at Night
Warm steam vaporizer: A humidifier that boils water to produce steam. Both the steam and the water inside are hot enough to scald, which makes placement a safety decision rather than a convenience one. Keep any boiling unit out of reach, on a stable level surface, never on the floor and never on or in a bed.Humidifiers A Nursery or a Kids Room
Relative humidity: The amount of water vapor in air expressed as a percentage of the maximum that air could hold at its current temperature. Because the maximum changes with temperature, the same absolute moisture reads as a different percentage in a warm room and a cool one. This site publishes no target figure as a recommendation; the EPA publishes general guidance on indoor moisture.Humidifiers A Basement or Another Damp Room
AHAM Verifide: A voluntary verification program run by an industry association, under which participating manufacturers have performance figures for air cleaners independently confirmed. Several purifier filter listings in this catalog state the status. It is reported here where a listing states it, with no additional meaning attached.Air Purifier Filters HEPA, True HEPA, and Words That Look Like Them
CADR: Clean Air Delivery Rate, a figure describing how much filtered air an air cleaner delivers, reported separately for different particle categories. It is a property of the whole machine rather than of the cartridge in it, which is why replacement filter listings generally do not state one. It is the figure that relates a purifier to the size of the room it sits in.Air Purifier Filters HEPA, True HEPA, and Words That Look Like Them