Mechanical (HVAC) Terms & Definitions

(Tired of being confused when you talk to a mechanical contractor, or maybe you want to know the correct term to use, or even look up a term or acronym you found on a piece of equipment. This article should help you with each and every one of those situations. If you ever come across a term or acronym that is not included in this article, email us at info@revivedrealestateva.com.)

HEATING, VENTILATING & AIR CONDITIONING TERMS & DEFINITIONS

A

AC – Voltage for alternating current, a type of electric current in which the polarity is constantly reversing causing the electron flow to reverse.

Air Conditioner – Any device that can remove heat from one space and deliver it to another therefore changing the temperature, humidity or general conditioning of the air.

Air cleaner – Any device that removes undesirable particles from air stream.

Air Flow – The amount of air that circulates through your Duct system, expressed in cubic feet per minute (CFM). Proper airflow depends on the indoor unit, the sizing of the ductwork and most importantly whether the filters are clean.

Air Handler – A unit that moves air for the purpose of heating or cooling the structure.   Residential air handlers include a blower, a coil (if equipped with A/C), some type of expansion device(A/C), some type of heating element or heat exchanger and a filter. Heating elements (Electric Heat) for air handlers are sold as accessories, sometimes installes at the factory.

B

BTU -  British thermal unit. The amount of heat required to raise or the temperature of one pound of water one degree Fahrenheit. The heat added (Heating)or removed (A/C) from your home by an is measured in BTUs. British thermal units per hour. 12,000 BTUh equals one ton of cooling (A/C).

Burner – A device that uses some type of fuel to support combustion rated in BTU’s.

Burner Orifice – The opening through which gas flows to the air/gas mixing chamber of the burner.

C

Capacity – The amount of heat removed (A/C) or heat added to your home referred to in BTU,

Celsius -  The metric temperature scale in which water freezes at zero degrees and boils at 100 degrees, designated by the symbol (C)

CFM  – Cubic feet per minute, a standard measurement of airflow. A typical cooling (A/C) system requires 400 cfm per ton.

Charge – Adding refrigerant to a system. This is refrigerant contained in a sealed system or in the sensing bulb to a thermostatic expansion valve.

Compressor - This is the heart of an air conditioning or heat pump system. It is part of the outdoor unit and pumps refrigerant to meet the cooling requirements of the system.

Condensate – Vapor that is cooled below it’s Dew Point condenses due to the lowering of its temperature. It is usually run to a drain or pump for removal.

Condenser Coil – In an air conditioner or Heat Pump (in A/C mode) it is the coil of the outside unit. The coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid. This process removes unwanted heat from the structure and dissipates it to the outdoors.

Condenser Fan -  The fan that circulates air over the condenser coil in the outdoor unit.

Contactor -  A switch (relay) which turns the Unit on and off by interrupting an electrical cicuit.

D

DC – Direct current electricity. This type of electricity (as opposed to Alternating Current, or AC) flows in one direction only, without reversing polarity.

Damper – Found in ductwork, this movable plate(manual or Automatic) opens and closes to control airflow. Dampers can be used to balance airflow in a duct system. They are also used in zoning to regulate airflow to certain rooms.

Defrost – To melt frost; from heat pump coil outdoor coil.

Degree Day – A equation that calculates the amount of heating or cooling needed for a building. A degree day is equal to 65 degrees Fahrenheit minus the mean or outdoor temperature.

Dehumidifier – The removal of humidity (Moisture) in the air by a mechanical system (A/C). Usually accomplished by lowering the conditioned air below the dew point or temperature the moisture will condense.

Diffuser – A grille over an air supply duct having vanes to distribute the discharging air in a specific pattern or direction.

Down flow Furnace – A furnace that return air to the air filter is at the top and discharges or conditioned air at its bottom. Most furnaces in the U.S. are of the upflow configuration.

Drain Pan – It is the pan that catches the condensate, removed by the A Coil reaching the dew point of the conditioned air. This condensate is then removed by dumping it in a drain or pumping it outside.

Ductwork – A pipe or conduit through which air is delivered. Ducts are typically made of metal, fiberboard or a flexible material. In a home comfort system, the size and application of ductwork is critical to performance and is as important as the equipment.

E

EER – M Energy Efficiency Ratio

Expansion Valve – A refrigerant metering valve with a pressure or temperature controlled orifice.

Evaporator Coil – The indoor coil located inside the indoor unit. This is a coil made tubing in which a low pressure liquid evaporates and absorbs unwanted heat from the structure. This is where the refrigerant evaporates as it absorbs heat from the indoor air that passes over the coil.

F

Fahrenheit – The temperature scale on which water freezes at 32 degrees and boils at 212 degrees; designated by the letter F.

Filter – Any device that removes impurities through a straining process.

Flue – Any vent or passageway that carries the products of combustion from a furnace.

Furnace – That part of the heating system in which the combustion of fossil fuel and transfer of heat occurs.

Fuse – A metal strip in an electrical circuit that melts and breaks the circuit when excessive current flows through it. The fuse is designed to break in order to save more expensive electrical components

G

Gas Furnace Heat Exchanger – Located in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout your home.

H

Heat Exchanger – An area, box or coil where heat flows from the warmer to the colder fluid or surface.

Heat Gain – Heat added to the conditioned space by infiltration, solar radiation, occupant respiration and lighting.

Heating Coil -  Any coil that serves as a heat source.

Heat Loss – The rate of heat transfer from a heated space to the outdoors.

Heat Pump – A mechanical compression cycle refrigeration system that can be reversed to either heat or cool the controlled space.

Heat Transfer – The movement of heat energy from one point to another. The means for such movement are conduction, convection, and radiation.

Hertz – In alternating current (AC electricity), the number of cycles per second.

HSPF -  Heating Seasonal Performance Factor, This rating is used in measuring the heating efficiency of a heat pump. The higher the number, the more efficient the heat pump system.

Humidifier – A devise that adds water vapor to the condition air to increase humidity.

Humidistat – A humidity sensing control that cycles the humidifier on and off to keep the conditioned space at a preset humidity level.

Humidity – The presence of water vapor in the air.

HVAC – Heating, Ventilating and Air Conditioning.

K

Kilowatt – kW 1,000 watts.

L

Latent Heat -  A type of heat, which when added to or taken from a substance, does not change the temperature of the substance but changes its state (ice, steam, vapor). Instead, the heat energy enables the substance to change its state.

M

Media – The material in a filter that traps and holds the impurities.

O

OEM  – Original equipment manufacturer.
P

Package Unit – A heating and cooling system contained in one outdoor unit. A package unit is typically installed beside, on the roof, or sometimes in the attic of a home.

PSI – Pounds per square inch.
PSIA – Pounds per square inch, absolute.

PSIG – Pounds per square inch gauge.

PVC – Polyvinyl chloride; a type of plastic.

R

Reciprocating Compressor -  A compressor whose piston or pistons move back and forth in the cylinders.

Refrigerant -  A chemical that produces a refrigerating effect while expanding from a liquid state to vaporizing. Most residential air conditioning systems contain R22 refrigerant. R22 is regulated in United States by the Environmental Protection Agency. R22 is scheduled to be in production until the year 2020.R134 & R410 is one of the replacements.  It’s used in approximately 95 percent of air conditioning equipment manufactured in the U.S. today.

Refrigerant Charge – The total amount of refrigerant required for a specific system.
S

SEER  – Seasonal Energy Efficiency Ratio. A measure of cooling efficiency for air conditioners and heat pumps. The higher the SEER, the more energy efficient the unit. The U.S. Government’s minimum SEER rating is 10.

Self Contained System -  A refrigerating system that can be moved without disconnecting any refrigerant lines; also know as a package unit.

Set point – The temperature or pressure at which a controller is set with the expectation that this will be a nominal value depending on the range of the controller.

Split System – The combination of an outdoor unit (air conditioner or heat pump) with an indoor unit (furnace or air handler). Split systems must be matched for optimum efficiency.

T

Thermostatic Expansion Valve -  A refrigerant metering device that maintains a constant evaporator temperature by monitoring suction vapor superheat. Also called a thermal expansion valve.

Thermostat – A series of sensors and relays that monitor and control the functions of a heating and cooling system.

Ton – A unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.

Two stage heating / Two stage cooling – Two stage heating and cooling is considered to be more efficient, because it operates at low speed (first stage) most of the time. However, on days when more heating or air conditioning is required, it switches to the next stage for maximum comfort.

U

U Factor – The factor representing resistance to heat flow of various building materials.

UL – Underwriters Laboratories.

Upflow Furnace – A furnace in which return air is drawn in through the sides or bottom and conditioned discharged out the top. This is the most common style furnace in the US.

V

Vacuum – A pressure below atmospheric pressure. A perfect vacuum is 30 inches Mercury (periodic symbol “Hg”).

Volt – The unit of measure used to describe a difference in electrical potential. Abbreviated by the symbol “v”.

Voltage – The force that pushes electrical current along wires and cables.
W

Watt  – The unit of electrical power equal to the flow of one amp at a potential difference of one volt.

Wet Bulb Thermometer – A thermometer whose bulb is covered with a piece of water soaked cloth and spun in the free air. The lowering of temperature as a result from the evaporation of water around the bulb indicates the air’s relative humidity.
Z

Zoning System – A method of dividing a home into different comfort zones so each zone can be independently controlled depending on use and need; an air conditioning system capable of maintaining varying conditions for various rooms or zones.