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Wahl Air Conditioning

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Glossary of Common HVAC Terms

Sorted alphabetically


2.5% Design
Outside summer temperatures and coincident air moisture content that will be exceeded only 2.5% of the hours from June to September. In other words, 2.5% design conditions are outdoor temperatures historically exceeded 73 out of the 2,928 hours in these summer months.

Absolute Pressure
Gauge pressure plus atmospheric pressure (14.7 lb.per sq.in.) equals absolute pressure.

Absolute Temperature
Temperature measured from absolute zero.

Absolute Zero Temperature
Temperature at which all molecular motion ceases(-460 F. and -273 C.

Substance with the ability to take up or absorb another substance.

Absorption Refrigerator
Refrigerator which creates low temperature by using the cooling effect formed when a refrigerant is absorbed by chemical substance.

A leading HVAC/R Association – http://www.acca.org/

Accessible Hermetic
Assembly of a motor and compressor inside a single bolted housing unit.

Storage tank which receives liquid refrigerant from evaporator and prevents it from flowing into suction line before vaporizing.

Air Changes Per Hour. The number of times that air in a house is completely replaced with outdoor air in one hour.

Acid Condition In System
Condition in which refrigerant or oil in system is mixed with fluids that are acid in nature.

ACR Tubing
Tubing used in air conditioning and refrigeration. Ends are sealed to keep tubing clean and dry.

Activated Alumina
Chemical which is a form of aluminum oxide. It is used as a drier or desiccant.

Activated Carbon
Specially processed carbon used as a filter drier ; commonly used to clean air.

That portion of a regulating valve which converts mechanical fluid, thermal energy or electrical energy into mechanical motion to open or close the valve seats.

Add On Heat Pump
Installing a heat pump in conjunction with an existing fossil fuel furnace. The result is a dual fuel system.

Adiabatic Compression
Compressing refrigerant gas without removing or adding heat.

Adjustable Grille
A grille with linear blades which can be adjusted to vary the direction of the discharged air. The linear blades are normally either vertical or horizontal, or both horizontal and vertical.

Substance with the property to hold molecules of fluids without causing a chemical or physical damage.

Act of combining substance with air.

AFLU (Annual Fuel Utilization Efficiency)
A rating that reflects the efficiency of a gas furnace in converting fuel to energy. A rating of 90 means that approximately 90% of the fuel is utilized to provide warmth to your home, while the remaining 10% escapes as exhaust.

AFUE (Annual Fuel Utilization Efficiency)
This number represents how efficiently a furnace converts fuel to energy. The ratio of annual output of useful energy or heat to the annual energy input to the furnace. The higher the AFUE, the more efficient the furnace — higher efficiency translates to more savings on fuel bills. This will range from 80% to 95%. Percentage of fuel used for heating.

Device used to cause motion in confined fluid.

AHU (Air Handler Unit)
The inside part of the A/C system that contains the blower, cooling (evaporator) coil, and heater.

Air Change
The amount of air required to completely replace the air in a room or building; not to be confused with recirculated air.

Air Cleaner
Device used for removal of airborne impurities.

Air Coil
Coil on some types of heat pumps used either as an evaporator or condenser.

Air Conditioner
Device used to control temperature, humidity, cleanliness and movement of air in a confined space.

Air Conditioning
Control of the temperature, humidity, air movement and cleaning of air in a confined space.

Air Cooler
Mechanism designed to lower temperature of air passing through it.

Air Core Solenoid
Solenoid that has a hollow core instead of a solid core.

Air Diffuser
Supply air terminal device, designed to direct airflow into desired patterns, usually placed in the ceiling, generally of circular, square or rectangular shape, and composed of divergent deflecting members.

Air Diffusion
Distribution of the air in a space, called the treated space, by means of devices, called air terminal devices, in a manner so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.

Air Distribution
The transportation of a specified air flow to or from the treated space or spaces, generally by means of ductwork.

Air Gap
The space between magnetic poles or between rotating and stationary assemblies in a motor or generator.

Air Handler
Fan-blower, filter, heat transfer coil, and housing parts of a system. Also known as the blower section and part of the split system, this unit is commonly in your home or attic and blows the air through your house. This has to be matched with the condenser properly to assure maximum efficiency. This will contain your heating and evaporator coil.

Air Infiltration
Leakage of air into rooms through cracks, windows doors and other openings.

Air Source Equipment
Heat pumps or air conditioners that uses the outdoor air to transfer heat to and from the refrigerant in the unit.

Air Terminal Device
A device located in an opening provided at the boundaries of the treated space to ensure a predetermined motion of air in this space.

Air Terminal Device, Lighting Troffer
An air terminal device, usually in the form of a slot or combination of slots, designed for use with a troffer unit.

Air Terminal Device, Linear
Air terminal device (grille) with an aspect ratio of 10: 1 or more.

Air Terminal Device, Slot
A device with one or several slots with an aspect ratio of 10: 1 or more for each slot (the aspect ratio is the ratio of the length to the width of the closed rectangular opening). A slot may or may not have an adjustable member to vary the direction of the air jet(s) or air flow rate.

Air Terminal Device, Supply
An air terminal device through which air enters a treated space. It usually consists of one or several deflecting members which ensure reduction of the air velocity in the occupied zone as well as efficient mixing of the supply air with the air in the treated space. Moreover supply air terminal devices usually determine the direction of the air jet(s).

Air-Cooled Condenser
Heat of compression, plus the heat of absorption, is transferred from refrigerant within coil to surrounding air, either by convection or fan or blower.

The distribution or movement of air.

Ak value (of an air terminal device)
Quotient obtained by dividing a measured air flow rate by a measured air velocity according to a specific process and a specific instrument.

Normally in tablet form, placed in evaporator drain pan. Used to help slow the growth of bacteria.

ARI (Air-Conditioning and Refrigeration Institute)
Air-Conditioning and Refrigeration Institute is a nonprofit, voluntary organization comprised of heating, air conditioning and refrigeration manufacturers. ARI publishes standards for testing and rating heat pumps and air conditioners to provide you with a standardized measure of comparison. So, ARI ensures a level of performance within the industry.

A leading HVAC/R Association – American Society of Heating, Refrigerating and Air Conditioning Engineers – http://www.ashrae.org/

American Society for Testing and Materials.

Back Drafting
Reverse flow of combustion gases down the chimney of a vented combustion appliance, which is often caused by depressurization of the room where the appliance is located.

Balance Point
The lowest outdoor temperature at which the refrigeration cycle of a heat pump will supply the heating requirements. Usually a temperature between 30°F to 45°F — at which a heat pump’s output exactly equals the heating needs of the house. Below the balance point, supplementary electric resistance heat is needed to maintain indoor comfort.

The process of adjusting the flow of air in duct systems, or water flow in hot-water heating systems. Proper balancing is performed using accurate instrumentation to deliver the right amount of heating or cooling to each area or room of the home.

Blower door
A large powerful variable-speed fan mounted in a doorway that blows air into (pressurizes) or sucks air out of (depressurizes) a house. It’s used to test for air leakage in a house. The size and complexity of the fan varies, but all blower doors have adjustable frames around the fan so that they can fit snugly into most doorways.

Blower (Fan)
An air handling device for moving air in a distribution system.

A piece of duct used to connect ducts with registers.

BTU (British Thermal Unit)
The amount of heat that will raise or lower one pound of water 1 degree F. at 39.2 degrees F. One BTU is the equivalent of the heat given off by a single wooden kitchen match. The British Thermal Unit is a standard of measure for cooling and heating capacities. This is how the capacity of air conditioning is measured. For your home, it represents the measure of heat givens off when fuel is burned for heating or the measure of heat extracted from your home for cooling.

BTUH (British Thermal Unit Per Hour)
Establishes a time reference to btu input or output rates. A BTUH is how many BTUs are used per hour.

Buffer zone
An area within the home between the conditioned zones and the outside. Thus, it normally is not conditioned (for instance, attics, attached garages, crawlspaces, basements, and enclosed porches).

CAE (Combined Annual Efficiency)
A measure of the amount of heat produced for every dollar of fuel consumed for both home heating and water heating.

A device used to start a motor or compressor (or to keep it running after start up.)

Capacity (or System Capacity)
The output or producing ability of a piece of cooling or heating equipment. Cooling and heating capacity are normally referred to in BTUs. The capacity of an air conditioner is measured by the amount of cooling it can do when running continuously. The total capacity is the sum of the latent capacity (ability to remove moisture from the air) and sensible capacity (ability to reduce the dry-bulb temperature). Each of these capacities is rated in Btus per hour (Btu/h). The capacity depends on the outside and inside conditions. As it gets hotter outside (or cooler inside) the capacity drops. The capacity at a standard set of conditions is often referred to as “tons of cooling.”

Carbon Monoxide
A colorless, odorless, highly poisonous gas produced when carbon burns without sufficient air nearby.

Carboxyhemoglobin Saturation
Carbon monoxide poisoning.

Central Forced-Air Heating System
A piece of equipment that produces heat in a centralized area, then distributes it throughout the home through a duct system.

CFC (Chlorofluorocarbon)
A class of refrigerants. Generally, refers to the Chlorofluorocarbon family of refrigerants. Sometimes called Freon

CFM (Cubic Feet per Minute)
A standard measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced through the system. A typical system produces 400 CFM per ton of air conditioning.

Amount of refrigerant placed in a refrigerating unit.

Comfort Zone
The range of temperatures, humidity’s and air velocities at which the greatest percentage of people feel comfortable.

The heart of an air conditioning or heat pump system. The large (usually black) part in the condenser (outdoor unit) that pumps refrigerant. The pump of a refrigerating mechanism which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities to meet the cooling requirements of the system.

This is the unit that will sit outside and is part of a split system, it contains the compressor which is the heart of your air conditioner or Heat Pump, it pumps the refrigerant through your system. Some people call the condenser the compressor, but the compressor is a component of the condenser along with the fan motor and condenser coil. Coil or outdoor coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid.

Condenser Coil
[Also see Outdoor Coil] The outdoor portion of a heating or cooling system that either releases or collects heat from the outside air, depending on the time of year. The Condenser Coil is connected directly to the home’s Air Handler and is also known as the Outdoor Coil.

Condensing Unit
Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser and returns it to the refrigerant control. The outdoor portion of a split system air conditioner contains the compressor and outdoor coil ignoring the reverse cycle operation, also the outdoor in a heat pump system.

The transfer of heat through a solid material.

This describes the direction in which a furnace outputs heat. A furnace may have an upflow, downflow or crossflow (horizontal) configuration.

In the condenser, the main switch that turns the condenser on.

The movement of heat by air flow.

COP (Coefficient of Performance)
COP compares the heating capacity of a heat pump to the amount of electricity required to operate the heat pump in the heating mode. COPs vary with the outside temperature: as the temperature falls, the COP falls also, since the heat pump is less efficient at lower temperatures. ARI standards compare equipment at two temperatures, 47 F and 17 F, to give you an idea of the COP in both mild and colder temperatures. Geothermal equipment is compared at 32 F enter water temperature. COP & HSPF cannot be compared equally. Air Source Equipment is rated by HSPF or COP and Geothermal equipment is rated by COP.

A device that is located in ductwork to adjust air flow. This movable plate opens and closes to control airflow. Dampers are used effectively in zoning to regulate airflow to certain rooms. There are basically two types of dampers: Manual and motorized. A manual damper generally consists of a sheet metal (or similar material) flap, shaped to fit the inside of a round or rectangular duct. By rotating a handle located outside of the duct a technician can adjust (see Balancing) air flow to match the needs of a particular area or room. A motorized damper is generally used in a zoned system (see Zoning) to automatically deliver conditioned air to specific rooms or zones. In particular, the following types, can be distinguished:
Multiple leaf dampers, comprising of a number of blades (or shutters) of opposed or parallel leaf type.
Single leaf dampers (the flap being mounted at one end), commonly called splitter dampers.
Hit-and-miss dampers, having two or more slotted slide mechanisms.
Butterfly dampers, with two flaps in “V” arrangement.

Dry Bulb

db (Decibel)
A decibel describes the relative loudness of a sound. Some common sounds are fairly close to a typical air conditioner or heat pump’s sound level: human voice, 7.0 decibels; blender, 8.8 decibels.

DDC (Direct Digital Control)
Direct Digital Control

Defrost Cycle
The process of removing ice or frost buildup from the outdoor coil during the heating season.

The reduction of water vapor in air by cooling the air below the dew point; removal of water vapor from air by chemical means, refrigeration, etc.

Design Conditions
Cooling loads vary with inside and outside conditions. A set of conditions specific to the local climate are necessary to calculate the expected cooling load for a home. Inside conditions of 75 degrees Fahrenheit and 50% relative humidity are usually recommended as a guideline. Outside conditions are selected for the 2.5% design point.

Direct Gas-Fired Heater
The burner fires directly in the air stream being heated, rather than through a heat exchanger. 100% of available BTUs are delivered to the heated space because no flue or heat exchanger is required. This results in no wasted energy.

DOE (Department of Energy)
The Department of Energy is a federal agency in charge of setting industry efficiency standards and monitoring the consumption of energy sources.

Downflow Furnace
A furnace that pulls in cool return air from the top and blows/expels warm air at the bottom – common where your furnace must be located in a second-floor closet or utility area.

Sometimes called filter/drier, it removes moisture and keeps the refrigerant clean.

Dry-bulb Temperature
The temperature measured by a standard thermometer.

Dual Fuel System
A dual heating system, for example a heat pump and a fossil fuel furnace.

A pipe or closed conduit made of sheet metal, fiberglass board, or other suitable material used for conducting air to and from an air handling unit.

Duct tape
This (initially) sticky tape is unfortunately the most common material used to seal duct connections. Care must be taken when it’s applied. For effective sealing, the surface it is applied to must be clean–free of dust, dirt, oil, or other substances. Duct tape has a tendency to lose adhesion with age, especially when used on ducts in unconditioned spaces.

Hollow pipes or channels that carry/transfer air from the Air Handler to the air vents throughout your home. The delivery system through which warm air from the furnace is brought to where it’s needed. Ductwork is made of sheet metal, fiberglass, or flexible plastic, and can be round or rectangular in shape. Ductwork is one of the most important components of a home heating and cooling system.

EER – (Energy Efficiency Ratio)
A ratio calculated by dividing the cooling capacity in Btu’s per hour (Btuh) by the power input in watts at any given set of rating conditions, expressed in Btuh per watt (Btuh/watt). EER & SEER cannot be compared equally. Air source equipment is rated by SEER and geothermal equipment is rated by EER. EER changes with the inside and outside conditions, falling as the temperature difference between inside and outside gets larger.

Effective area (of an air terminal device)
The smallest net area of an air terminal device used by the air stream in passing through the air terminal device.

A rating on comfort equipment is similar to the miles per gallon rating on your car. The higher the rating number, the more efficient the system and the lower your fuel consumption will be. You can save a lot of money with a high efficiency unit. Depending on your local climate, lifestyle and electricity rates, savings will vary. For furnaces. it is the rate at which a furnace maximizes fuel use. This rate is numerically described as a ratio called AFUE (see AFUE). As of January, 1991, no furnaces can be manufactured with efficiencies lower than 78% afue. High efficiency furnaces will be rated 85 to 95% afue.

Emergency Heat (Supplementary Electric Heat)
The backup electric heat built into a heat pump system. The same as an auxiliary heater, except it is used exclusively as the heat source when the heat pump needs repair.

The geometrical surface of the points of an air jet, corresponding to a determined value of the measured air velocity. This velocity is generally called «terminal velocity». Also, the air barrier that separates the conditioned space from the outside and from unconditioned spaces like attics and garages.

EPA (Environmental Protection Agency)
Environmental Protection Agency – http://www.epa.gov/

ERV (Energy Recovery Ventilator)
This device preheats incoming outside air during the winter and pre-cools incoming air during the summer to reduce the impact of heating and or cooling the indoor air. This means that smaller capacity heating and cooling systems can be used in homes, which results in lower installation costs, lower peak demand for energy, and lower operating costs.

Evaporator Coil
[Also see Indoor Coil] The evaporator coil is located inside your house in a split system in the air handler, or above the gas furnace. This will produce cooling in the air conditioning mode and heating in a Heat Pump mode. This coil section in the evaporator is where refrigerant evaporates and absorbs heat from air passed over the coil. This is also very important in removing humidity from your home.

Uncontrolled air leakage out of a building.

The air flow leaving the treated space. Exhaust may be accomplished by one or more of the following methods:
a. Extraction: exhaust in such a manner that the air is discharged into the atmosphere.
b. Relief: exhaust in such a manner that the air can escape from the treated space if the pressure in that space rises above a specified level.
c. Recirculation: exhaust in which the air is returned to the air treatment system.
d. Transfer: exhaust in which air passes from the treated space to another treated space.

Exhaust Air Flow Rate
Volume of air leaving an exhaust air terminal device within a time unit.

Exhaust Air Terminal Device
Air terminal device through which air leaves the treated space.

Fan Coil
An indoor component of a heat pump system used in place of a furnace, to provide additional heating on cold days when the heat pump does not provide adequate heating.

FAQ (Frequently Asked Questions)
Used to mean a list of frequently asked questions.

A device for removing dust particles from air or unwanted elements from liquids.

Fire Valves (or Fire Dampers)
Components which are installed in an air distribution system between two fire separating compartments and are designed to prevent propagation of fire and/or smoke. Generally, are kept open by mechanical restraint, whose effect is canceled under specific conditions. The valve is then closed automatically.

Flame roll-out
A dangerous situation that occurs when flame is pushed out of the bottom of a combustion appliance. It can lead to fires.

Flex duct
Usually installed in a single, continuous piece between the register and plenum box, a flexible duct usually has an inner lining and an insulated coating on the outside.

Flow hood
A diagnostic tool used to measure air flow through ducts, supply registers, and return grilles.

Forced Air
This describes a type of heating system that uses a blower motor to move air through the furnace and into the ductwork.

That part of an environmental system which converts gas, oil, electricity or other fuel into heat for distribution within a structure.

Furnace, Horizontal
A furnace that lies on its side, pulling in return air from one side and expelling warm air from the other.

Geothermal Equipment
Heat pumps that uses the ground to transfer heat to and from the refrigerant in the unit. The unit circulates water through a heat exchanger in the to a closed loop buried in the ground or by pumping water from a well through the unit.

An air terminal device with multiple passages for the air.

Coverings for the ducts where they open to the conditioned space. (Same as Registers)

HCFC (Hydrochlorofluorocarbon)
A class of refrigerants. Generally, refers to Halogenated Chlorofluorocarbon family of refrigerants.

Heat Exchanger
This is a device that enables furnaces to transfer heat from combustion safely into breathable air. The primary heat exchanger transfers heat from combustion gases to the air blowing through the ductwork. It’s vital that none of the combustion gas itself gets into the airstream. The primary heat exchanger handles the hottest gases. This device transfers heat from outgoing stale air to incoming cold air. In warm climates, this process can be reversed. In high efficiency furnaces, secondary heat exchangers recover heat that used to be vented up the chimney with the exhaust gases. By recovering this heat, the furnace becomes more efficient. Part of the heat recovered here causes the water and acid to condense out of the exhaust gas. Because this liquid is corrosive, secondary heat exchangers must be designed to prevent deterioration. Usually this means they are made of stainless steel or some derivative of it.

Heat Gain
The amount of heat gained, measured in BTU’s, from a space to be conditioned, at the local summer outdoor design temperature and a specified indoor design condition.

Heat Loss
The amount of heat lost, measured in BTU’s from a space to be conditioned, at the local winter outdoor design temperature and a specified indoor design condition.

Heat Pump
A heating and air conditioning unit that heats or cools by moving heat. A Heat Pump is a reverse cycle air conditioner. The Heat Pump uses a compression cycle system to supply heat or remove heat remove a temperature controlled space. When you run your air conditioner, your outdoor unit will be blowing hot air, (in other words, removing the heat from your home and sending it outside). When you run your heat pump, you reverse the flow of refrigerant and remove the heat from the atmosphere outside and blow it inside. When the temperature dips below 40 degrees outside, the Heat Pump labors in producing heat so they install a backup or auxiliary electric heat strip to supplement the Heat Pump. Electric Heat strips are very expensive to operate. In southern climates where it rarely dips below 40 degrees the heat pump is very efficient. A 3 to 1 savings in heating compared to electric heat strips.

Heat Pump Cooling Mode
In the cooling cycle of a Heat Pump, you are removing hot air from inside the house and sending it outside. You can feel the hot air outside, over the condenser fan motor.

Heat Pump Heating Mode
In the heating mode of a Heat Pump the refrigerant cycle is reversed and you are now removing the heat from the outside and sending it inside the house. You can feel the cool air outside, over the condenser fan motor. When temperatures go below 40 degrees Heat Pumps labor in producing heat and must use back up electric heat strips. Heat strips cost 3 times as much to operate as a Heat Pump when producing heat.

Heat Source
A body of air or liquid from which heat is collected. In an air source heat pump, the air outside the house is used as the heat source during the heating cycle.

HFC (Hydrofluorocarbon)
A class of refrigerants. Generally, refers to Hydrofluorocarbon family of refrigerants

HRV (Heat Recovery Ventilator)
This device brings fresh, outside air into a home while simultaneously exhausting stale indoor air outside. In the process of doing this, an HRV removes heat from the exhaust air and transfer it to the incoming air, pre-heating it. This allows for the reclamation of much of the energy that otherwise would simply be vented outside. The end result: home comfort systems operate more efficiently.

HSPF (Heating Seasonal Performance Factor)
Indicates how efficiently a Heat Pump is working. A higher number means the unit works more efficiently. Heating Seasonal Performance Factor is similar to SEER, but it measures the efficiency of the heating portion of your heat pump. Like SEER, industry minimums have been raised recently, and the minimum is now 6.80 HSPF. Most new units have ratings from 7.0 to 9.4. The total heating output of a heat pump during its normal annual usage period for heating divided by the total electric power input in watt-hours during the same period. COP & HSPF cannot be compared equally. Air Source equipment is rated by HSPF or COP and Geothermal equipment is rated by COP. ARI standards compare air source equipment at two temperatures, 47 F and 17 F. Geothermal equipment is compared at 32 F enter water temperature.

A device that adds moisture to warm air being circulated or directed into a space. This adds necessary moisture to protect your furnishings and reduce static electricity.

A device designed to regulate humidity input by reacting to changes in the moisture content of the air. Much like a thermostat but turns the system on & off by sensing the humidity level.

The amount of moisture in the air. Air conditioners remove moisture for added comfort.

HVAC (Heating, Ventilating and Air Conditioning)
Heating, Ventilating and Air Conditioning

HVAC/R (Heating, Ventilating, Air Conditioning, & Refrigeration)
Heating, Ventilating, Air Conditioning, & Refrigeration

IAQ (Indoor Air Quality)
Indoor Air Quality

Indoor Coils
[Also see Evaporator Coil] Refrigerant containing portion of a fan coil unit similar to a car radiator, typically made of several rows of copper tubing with aluminum fins. A homes comfort system consists of two components: the outdoor unit (air conditioner or heat pump) and the indoor unit (coil or blower coil). Combinations of various units will result in vastly different efficiency ratings. Unreasonably high efficiency ratings can be created by using unrealistic indoor and outdoor equipment combinations. The term “most popular coil” indicates the actual tested combinations; other ratings may be simulated and unrealistic. Be sure that the efficiency ratings you are comparing are for “most popular coil.” You’ll know the ratings are attainable and close to reality.

Indoor Unit
This is usually located inside the house and contains the indoor coil, fan, motor, and filtering device, sometimes called the air handler.

Process by which the primary air sets into motion an air volume, called secondary air, in the room.

Induction ratio (i)
Ratio of the total air flow rate to the primary air flow rate.

Unintentional movement of outdoor air into a house. Air flow inward into a space through walls, leaks around doors and windows or through the building materials used in the structure. It results from the forces of wind, temperature difference, and HVAC operation.

ISO 9000
A family of international standards for quality management and assurance by the ISO (International Standards Organization).

IWC (Inches of water column)
Commonly used in the USA

kW (kilowatt)
A kilowatt equals 1,000 watts.

kWh (kilowatt hour)
A kilowatt hour (kWh) is the amount of kilowatts of electricity used in one hour of operation of any equipment.

Latent Cooling Load
The net amount of moisture added to the inside air by plants, people, cooking, infiltration, and any other moisture source. The amount of moisture in the air can be calculated from a combination of dry-bulb and wet-bulb temperature measurements.

Latent Heat
Heat, that when added or removed, causes a change in state – but no change in temperature.

Load Estimate
A series of studies performed to determine the heating or cooling requirements of your home. An energy load analysis uses information such as the square footage of your home, window and door areas, insulation quality and local climate to determine the heating and cooling capacity needed by your furnace, heat pump or air conditioner. When referring to heating, this is often known as a Heat Loss Analysis, since a home’s heating requirements are determined by the amount of heat lost through the roof, entry ways and walls.

An instrument that measures air pressure differences between locations. Tubes are usually attached to a manometer and run to the spaces where pressures are measured.

Manual D
Manual D is the ACCA method for designing duct systems. Contractors often find it a laborious process and most duct systems are just installed, not designed. The amount of time necessary to design a duct system is certainly warranted in tract construction where the design is used repeatedly and for custom homes where the total cost of the home warrants a proper design. In short, designing a duct system is essential for proper equipment performance and customer comfort.

Manual J
Manual J is a widely accepted method of calculating the sensible and latent cooling (and heating) loads under design conditions. It was jointly developed by the Air Conditioning Contractors of America (ACCA) and the Air-Conditioning and Refrigeration Institute (ARI).

Manual S
Manual S is the ACCA method of selecting air conditioning equipment to meet the design loads. It ensures that both the sensible capacity and the latent capacity of the selected equipment will be adequate to meet the cooling load.

An adhesive paste used in the fabrication and sealing of thermal insulation on piping fittings, equipment, and duct work. It spreads easily and dries permanently. Its applications include new ducts and old, sheet metal seams, and duct board. Not all mastics are created equal. Some are toxic, some are water-based, and some take longer to cure, making them more suitable for new construction than retrofit.

Matched System
A heating and cooling system comprised of products that have been certified to perform at promised comfort and efficiency levels when used together, and used according to design and engineering specifications.

MHPU (Mobile Home Package Unit)
The Mobile Home Package Unit is an air conditioner with electric or gas heat or can be a Heat Pump. This unit has the condenser and air handler all-in-one package. Commonly used in mobile homes and sits outside. (Self-contained unit)

An air terminal device designed to generate a low energy loss and thus produce a maximum throw by minimum entertainment.

Outdoor Coil
[Also see Condenser Coil] Refrigerant containing portion of a fan coil unit similar to a car radiator, typically made of several rows of copper tubing with aluminum fins.

Package Unit or Package System
A self-contained unit or system that has the Air Handler & Condenser in same unit. Normally placed outside the home and connected to a duct system by a penetration through the homes foundation. Except for geothermal which is a self-contained indoor unit that is place in a closet, attached garage, basement, or mechanical room.

Pascals (Pa)
A small unit of air pressure. One pound per square inch equals 6,895 pascals.

Air flow passage made of duct board, metal, drywall, or wood. Joins supply and return ducts with HVAC equipment.

Pressure balancing
The process of neutralizing pressure differences within a home.

Primary Air Flow Rate
Volume of air entering a supply air terminal device within a time unit.

The transfer of heat directly from one surface to another (without heating the intermediate air acting as a transfer mechanism).

Processing or returning used refrigerant to the manufacturer or processor for disposal or reuse.

Substance used in refrigerating mechanism. A substance that produces a refrigerating effect while expanding or vaporizing. Air conditioning systems use Refrigerant in the Evaporator Coil to cool air as it passes by. Refrigerants absorb heat in evaporator by change of state from a liquid to a gas, and releases its heat in a condenser as the substance returns from the gaseous state back to a liquid state.

Refrigerant Lines
Two copper lines that connect the Condenser (Outdoor) Coil to the Evaporator (Indoor) Coil.

Combination grille and damper assembly covering an air opening or end of an air duct.

Coverings for the ducts where they open to the conditioned space. (Same as Grilles)

The ductwork that carries air from the house to the air handler.

Return Air
Air drawn into a heating unit after having been circulated from the heater’s output supply to a room.

Relative Humidity

A leading HVAC/R Association – Refrigeration Service Engineers Society – http://www.rses.org/

Saturation Temperature
Also referred to as the boiling point or the condensing temperature. This is the temperature at which a refrigerant will change state from a liquid to a vapor or vice versa.

SEER (Seasonal Energy Efficiency Ratio)
An efficiency measurement that is similar to Miles Per Gallon for cars. The higher this number, the more energy efficient they system is. The amount of cooling your equipment delivers per every dollar spent on electricity. The higher the number the lower the operating cost (not more cooling.) SEER applies to air conditioners and heat pumps. In the past, a unit with a SEER of 8.00 was considered standard efficiency, and a unit with a 10.00 SEER was considered high efficiency. After January 1, 1992, the minimum SEER required by the DOE is 10.00 and 15.00+ SEER is considered high efficiency. EER & SEER cannot be compared equally. Air source equipment is rated by SEER and geothermal equipment is rated by EER. New units have SEER ratings from 10 to 17 BTUs per watt. The total cooling of a central unitary air conditioner or unitary heat pump in Btu’s during its normal annual usage period for cooling divided by the total electric energy input in watt-hours during the same period. The Seasonal Energy Efficiency Ratio is a standard method of rating air conditioners based on three tests. All three tests are run at 80 degrees Fahrenheit inside and 82 degrees Fahrenheit outside. The first test is run with humid indoor conditions, the second with dry indoor conditions, and the third with dry conditions cycling the air conditioner on for 6 minutes and off for 24 minutes. The published SEER may not represent the actual seasonal energy efficiency of an air conditioner in your climate.

Sensible Cooling Load
The heat gain of the home due to conduction, solar radiation, infiltration, appliances, people, and pets. Burning a light bulb, for example, adds only sensible load to the house. This sensible load raises the dry-bulb temperature.

Sensible Heat
Heat, that when added or removed, causes a change in temperature but not in state.

The temperature to which a thermostat is set to result in a desired heated space temperature.

Single Package
A year ’round heating and air conditioning system that has all the components completely encased in one unit outside the home.

Refers to the procedure a heating contractor goes through to determine how large a furnace (measured in btuh) is needed to heat a house efficiently. Too small a furnace won’t deliver enough heating; too large a furnace increases energy costs and can have an adverse effect on comfort. Sizing depends on the square-footage of the home, the amount of ceiling and wall insulation, the window area, use of storm doors, storm windows, and more.

Smoke stick
A diagnostic tool used to observe air flow. Usually it consists of a chemical in a squeezable container. When squeezed it emits smoke which visibly follows air flow currents.

Sound Attenuators
Components which are inserted into the air distribution system and designed to reduce airborne noise which is propagated along the ducts.

Split System
Refrigeration or air conditioning installation, which places condensing unit outside or away from evaporator. These unit are connected by a supply and return refrigerant lines. Also, applicable to heat pump installations. A combination heat pump or air conditioner with indoor components such as a furnace or blower coil. To maximize effectiveness, Split Systems should be matched.

Spread (LS) (for a supply air terminal device)
Maximum distance between two vertical planes tangent to a specified envelope and perpendicular to a plane through the core center. The spread is generally referred to the envelope corresponding to 0.25 m/s for zero supply temperature differential (i.e., under isothermal conditions).

SRN (Sound Rating Number)
Sound is measured in bels (a bel equals 10 decibels). The SRN of a unit is based on ARI test, performed at ARI standard rating conditions. Average sound rating ranges from 7.0 to 8.0 decibels. The lower the SRN rating, the quieter the unit.

Straight Cool
This is an air conditioner that uses different forms of heating such as Natural Gas, LP Gas, Electric Resistance heat and oil.

Subcooled Liquid
Liquid refrigerant which is cooled below its saturation temperature.

Superheated Vapor
Refrigerant vapor which is heated above its saturation temperature. If a refrigerant is superheated, there is no liquid present.

Supplementary Heat (Emergency Heat)
The auxiliary or emergency heat provided at temperatures below a heat pump’s balance point. It is usually electrical resistance heat.

The ductwork that carries air from the air handler to the rooms in the house.

Supply Air
The air flow entering the treated space.

Switchover Valve
A device in a heat pump that reverses the flow of refrigerant as the system is switched from cooling to heating. Also called a reversing valve or four-way valve.

Another measurement of heat. One therm equals 100,000 BTUH.

A temperature sensitive switch for controlling the operation of a heater or furnace. Typically found on a wall inside the home, that consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system. A device that allows you to control the temperature inside your home by telling the heating or cooling system how much air to produce.

Thermostat, Air-Sensing
Thermostat unit in which sensing element is located in refrigerated space.

Thermostat, Setback (Programmable Thermostat)
A state-of-the-art electronic thermostat with a built-in memory that can be programmed for different temperature settings at different times of the day.

Throw (Lt)
The maximum distance between the center of the core and a plane which is tangent to a specified envelope and perpendicular to the intended direction of flow The throw is generally referred to as the envelope corresponding to 0.25 m/s for zero supply temperature differential (i.e., under isothermal conditions).

Time Delay
Usually refers to a device that will not allow the condenser to restart for an average of 5 minutes.

A cooling unit of measure. Each ton equals 12,000 Btuh. Heat pumps and air conditioners are generally sized in tons. Typical sizes for single family residences are between two and five tons. It is important to note that actual capacity is not constant and will change based on outdoor or indoor temperatures. The published capacity rating of air conditioners and heat pumps is based on performance at the ARI standard temperature levels of 95 F outside, 80 F inside, and 50% relative humidity. The number of tons a system has is the total BTU capacity of the system. The size of the area to be cooled will determine the correct size of the system in tons. While an air conditioner may be called a three-ton unit, it may not produce 36,000 Btu/h in cooling. There is a wide variety of actual capacities that are called “three tons.”

Total air flow rate (QL)
Sum of the primary and secondary air flow rates which are moved in the treated space.

Up flow Furnace
A furnace that pulls cool return air in from the bottom and blows/expels warm air out the top into the duct work. This type of furnace is usually installed in a basement or an out-of-the-way closet.

Components inserted into air ducts or devices which permit modification of the air resistance of the system and consequently a complete shut-off of the air flow (control valves).

VAV (Variable Air Volume)
Variable Air Volume.

VFD (Variable Frequency Drive)
Electronic speed control for motors.

W (Watt)
A watt is a unit of electricity.

Wet Bulb

WC (Water Column)
Common measure of air pressure used in HVAC systems.

Wet-bulb Temperature
When a wet wick is placed over a standard thermometer and air is blown across the surface, the water evaporates and cools the thermometer below the dry-bulb temperature. This cooler temperature (called the wet-bulb temperature) depends on how much moisture is in the air.

1) Conditioned space in a house under the control of a thermostat. 2) A space within a house with a distinct pressure compared to other pressure zones. Also see Buffer zone.

A system in which living areas or groups of rooms are divided into separate spaces and each space’s heating/air conditioning is controlled independently. This can be accomplished by using either multiple independent systems, or a single system using electronic controls and motorized dampers (see Damper). For example, you might prefer the kitchen area of your home be slightly cool, while at the same time keeping the temperature in the bedrooms warmer.

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