HVAC Terminology

There are many acronyms and terms associated with energy efficiency and your Air Conditioning System. This glossary can be used to help you understand the language of the industry when discussing energy efficiency and your HVAC system with your contractor.

 

ACCA – Air Conditioning Contractor of America – Is a group of over 4,000 air conditioning contractors who work together to improve our industry, and keep homes and buildings safe, clean and comfortable.

ACTA – Air Conditioner Trade Association

Active Cooling – A compressor cooling and dehumidifying the air.

Active solar system – A system that uses electrical and/or mechanical devices to help collect, store, and distribute the sun’s energy.

AFUE – Annual Fuel Utilization Efficiency

Air balancing – Adjusting an air conditioning system so that the right amount of air is delivered to the right places in your home in order to achieve the right heating or cooling effect.

Air conditioner – Equipment that conditions air by cleaning, cooling, heating, humidifying, or dehumidifying it. A term often applied to comfort cooling equipment.

Air conditioning – A process that maintains comfort conditions in a defined area.

Air Conditioning and Refrigeration Institute (ARI) – A non-profit organization composed of heating, air conditioning and refrigeration manufacturers.  ARI publishes standards for testing and rating heat pumps and air conditioners.

Air cooled – Using a fan to move heat away from the condenser coil to the outside air.

Air filter – A media that filters out unwanted particles, vapors, or odors from indoor air. It is used to improve indoor air and to protect the HVAC system.

Air handler – The device that moves the air across the heat exchanger in a forced-air system- normally considered to be the fan and its housing.

Air handling unit – A devise used to condition and circulate air as part of a heating , ventilating, and air conditioning system.

Air infiltration – Unwanted entry of air into a conditioned space. Air that leaks into a structure through cracks, windows, doors, or other openings due to less pressure inside the structure than outside the structure. This infiltration can be increased by a significant difference in temperature or high winds. The most common place to finds cracks is around electrical outlets or plumbing.

Air Sensor – A device that registers changes in air conditions such as pressure, velocity, temperature, or moisture content.

Air vent – A fitting used to vent air manually or automatically from a system.

Ambient temperature – The surrounding air temperature.

American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) – They publish standards in the area of HVAC and refrigeration.

Anual Fuel Utilization Efficiency (AFUE) – Annual Fuel Utilization Efficiency applies to furnaces - both gas and oil. It describes the percentage of heat produced for every dollar of fuel consumed.

ARI – Air Conditioning and Refrigeration Institute

ASHRAE – American Society of Heating Refrigeration and Air Conditioning Engineers

American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE) – They publish standards in the area of HVAC and refrigeration.

Automatic control – Controls that react to a change in conditions to cause the condition to stabilize.

Automatic expansion valve – A refrigerant control valve that maintains a constant pressure in an evaporator.

 

Bleeding – Allowing pressure to move from one pressure level to another very slowly.

Bore – The inside diameter of a cylinder.

British Thermal Unit (BTU) – The amount (quantity) of heat required to raise the temperature of 1 pound of water 1°F. In heating, a BTU measures the heat given off during the combustion of fuel. In cooling, it is a unit that represents the capacity to remove that amount of heat from inside space, also known as tons.

British Thermal Units per hour (BTU/h) – BTU divided by hours. One tone of cooling equals 12,000 BTU/h

BTU – British Thermal Unit

BTU/h – British Thermal Units per hour

Burner – A device used to prepare and burn fuel.

 

Capacity – The ability for an air conditioner to cool or heat a specific space. Cooling capacity is usually measured in tons, heating is usually measured in Btu’s.

Carbon dioxide (CO²) – A byproduct of natural gas combustion that is not harmful.

Carbon monoxide (CO) – A poisonous, colorless, odorless, tasteless gas generated by incomplete combustion. CO can enter your living space from A furnace that is not properly maintained or becomes worn.

Central air conditioning system – A home appliance system is used to provided comfort to the entire home in either hot or cold weather. 

CFC – Chlorofluorocarbons

Charge – The quantity of refrigerant in a system. Charging cylinder: A device that allows the technician to accurately charge a refrigeration system with refrigerant.

Chlorofluorocarbons (CFC) – Those refrigerants thought to contribute to the depletion of the ozone layer. These are used in some air conditioning systems and heat pumps.

Circuit breaker – A device that opens an electric circuit when an overload occurs.

CO2 indicator – An instrument used to detect the quantity of carbon dioxide in flue gas for efficiency purposes.

Code – The local, state, or national rules that govern safe installation and service of systems and equipment for the purpose of safety of the public and trade personnel.

Cold - The word used to describe heat at lower levels of intensity.

COP - Coefficient of Performance

Coefficient of Performance (COP) – An efficiency ratio that compares the amount of heat delivered to the amount of energy used.  As with MPG on a car, the higher the score the more energy efficient the equipment is.

Coil – A heating or cooling element made of pipe or tubing, usually with plates or fins.

Coil, condenser – A set of tubes outdoors filled with refrigerant that removes heat from the heated gas refrigerant and convert the refrigerant into a liquid form again.

Combustion – A reaction called rapid oxidation or burning produced with the right combination of a fuel, oxygen, and heat.

Compressor – A vapor pump that pumps vapor (refrigerant or air) from one pressure level to a higher-pressure level.

Compressor, scroll – A compressor that uses two scroll like components to compress vapor.

Condenser – (Heat Exchanger) The component in a refrigeration system than transfers heat from the system by condensing refrigerant.

Condensate – The moisture collected on an evaporator coil.

Condensate pan – A pan under evaporator coil that collects condensate “water”, also referred to as drip pan.

Condensate pump – A small pump used to pump condensate to a higher level.

Condenser fan – The fan located in the outside unit that moves air across the condenser coil.

Condenser, air-cooled – A heat exchanger that transfers heat into the surrounding air.

Condenser, water-cooled – A heat exchanger that transfers heat into water.

Condensing temperature – The temperature which vapor or gas will change into liquid.

Condensing unit – A complete unit that includes the compressor and the condensing coil.

Cooling capacity – The ability for a unit to remove heat from an enclosed space.

Cycle – A complete sequence of events (from start to finish) in a system.

 

Damper – A component in an air distribution system (ductwork) that restricts airflow for the purpose of air balance. Dampers can opened and closed to allow more or less air flow into a given space and are used in conjunction with a zone system.

Decibel (dB) – A measurement of sound in reference to loudness. Look for “sound rating number” on your air conditioning equipment.

Degree-day – A measurement of departure form the mean daily temperature. The number of degrees that the mean temperature for that day is below 65°F.

Dehumidify – To remove moisture from the air. Air conditioning helps dehumidify indoor air. It can also be done separately with a dehumidifier.

Dehumidifier – A device that removes excess moisture from the air.

Density – The weight per unit of volume of a substance.

Dew point – The exact temperature at which moisture begins to form.

Drier – A device used in a refrigerant line to remove moisture.

Drip pan – A pan shaped to collect moisture condensing “water” on an evaporator coil in an air conditioning of refrigeration system.

DOE – Department of Energy

Department of Energy – A United States government agency that monitors energy conservation and set the standards for efficiency in the HVAC industry.

Down flow furnace – A type of furnace that takes in air at its top and discharges air at the bottom.

Drain pan – A pan under the evaporator coil that collects condensate “water”.

Duct – A sealed channel used to convey air from the system to and from the point of utilization.  They can be round or rectangular, sheet metal or fiberglass or vinyl tubes.  In air conditioning systems they carry air from the home to the air conditioning system or furnace and back to the home.

Ductwork – A system of ducts used to transfer air from the system to and from the point of utilization.

 

EER – Energy efficiency ratio

Electronic air filter - A filter that changes dust particles using high-voltage direct current and then collects these particles on a plate of an opposite charge.

Electric heating – Heating that relies on electric resistance heaters

Electrostatic filter – A filter device for cleaning or purifying of air that employs electrostatic charge and attracts duct particles.

Energy Audit – An assessment to measure the current energy consumption for an indoor area. Widely used to make “whole house” energy savings.

Energy efficiency ratio (EER) – An equipment efficiency rating that is determined by dividing the output in BTU by input in watts. This does not take into account the startup and shutdown for each cycle. The higher EER the rating the more efficient the system.

Energy Star – An international standard for energy efficient consumer products. It was first created by the United States government programs DOE & EPA, this is where the trade make was developed.

Energy recovery ventilator (ERV) –  a machine that draws fresh air into the home and exhausts stale air from the home.  It uses a process to preheat or pre-cool (depending on the season) to reduce energy costs associated with conditioning the air.

Energy Savings Agreement – A prepaid maintenance performed twice per year, once on the heating and again on the cooling system serving your home. This program is designed to reduce your utility & repair cost, while increasing the life of your system.

Envelope – The separation between the interior and the exterior environments of a building. It serves as the outer shell to protect the indoor environment as well as to facilitate its climate control.

ERV – Energy recovery ventilator

ESA – Energy Savings Agreement

Ethane gas – The fossil fuel, natural gas, used for heat.

Evaporation – The condition that occurs when heat is absorbed by liquid and it changes to vapor.

Evaporator coil – A network of tubes indoors filled with refrigerant that removes heat and moisture from the air as the refrigerant evaporates into a gas again.

 

Fahrenheit scale – The temperature scale that places the boiling point of water at 212'F and the freezing point at 32'F.

Fan – A device that produces a pressure difference in air to move it.

Fan cycling – The used of a pressure control to turn a condenser fan on and off to maintain a correct pressure within the system.

Fan relay coil – A magnetic coil that controls the starting and stopping of a fan.

Field matching – Adding new components to older equipment in an air conditioning system. Proper matching is critical for these components to meet their design specifications.

Filter – A media that filters out unwanted particles, vapors, or odors from indoor air. It is used to improve indoor air and to protect the HVAC system.

Flexible duct – A type on air duct material, often continuous, so less joints. It is easy to route around obstacles in an attic, by bending gently.

Flue – The duct that carries the products of combustion out of a structure for a fossil-or-solid fuel system.

Fluid – The state of matter of liquids and gases.

Flush – The process of using a fluid to push contaminants from a system.

Forced air – A system that uses a fan or blower to move or “force” air through the system and into the intended rooms.

Freeze up – Excess ice or frost accumulation on an evaporator to the point that airflow may be affected.

Freon – A brand name of refrigerant manufactured by the Dupont Company.

Furnace – Equipment used to convert heating energy, such as fuel oil, gas, or electricity, to usable heat. It usually contains a heat exchanger, a blower, and the controls to operate the system.

 

Gage – An instrument used to detect pressure.

GAMA – Gas Appliance Manufacturing Association

Gas – The vapor state of matter.

Gas Appliance Manufacturing Association (GAMA) – Gas Appliance Manufacturing Association is a national trade association serving the interests of manufacturers of gas, oil, and electric appliances and equipment, components and related products used in residential, commercial, and industrial applications.

Gas valve – A valve used to stop, start, or modulate the flow of natural gas.

Ground source heat pump (GHP) – A type of heat pump that uses the stored energy in the soil. Use temperature of the soil to provide heating, cooling and hot water for homes and buildings.

 

HCFC – hydrochlorofluorocarbons

Heat – Energy that causes molecules to be in motion and to raise the temperature of a substance.

Heat anticipator – A device that anticipates the need for cutting off the heating system prematurely so the fan can cool the furnace.

Heat coil – A device made of tubing or pipe designed to transfer heat to a cooler substance by using fluids.

Heat exchanger – (Condenser) The component in a refrigeration system than transfers heat from the system by condensing refrigerant.

Heat gain – The heat that enters conditioned space by infiltration from outside air, solar radiation, human respiration, lighting devices, ect.

Heat loss – The rate of heat transfer from a heated space to the outdoors. The opposite of heat gain.

Heat pump - A refrigeration system used to supply heat or cooling using valves to reverse the refrigerant gas flow.

Heat transfer – Movement of heat energy from one place to another.

Heat recovery system (HRS) – ?

Heat recovery ventilator (HRV) – Brings fresh outside air into an indoor environment and exchanges it with stale indoor air. A transfer core allows heat to be transferred from the outgoing air to the incoming air.

Heating coil – Serves as the heating source and is made of tubing or pipe.

Heating Seasonal Performance Factor (HSPF) – An equipment efficiency rating.  As with MPG on a car, the higher the rating the more fuel efficient the equipment is.

Heating, Ventilation, and Air Conditioning – HVAC

HEPA filtration - High Efficiency Particulate Arresting filters can remove up to 99.97% of particulates 0.3 microns and larger. Highest of all filtration products.

HFC – Hydrofluorocarbon

HRS – Heat recovery system

HSPF – Heating Seasonal Performance Factor

HRV – Heat Recovery Venilator

Humidifier – A device that adds moisture to air for your home.

Humidity – The concentration of moisture in the air.

Humidify – To add moisture to the air.

HVAC – “H-V-A-C” stands for “heating, ventilating, and air conditioning”

HVACR – Heating, ventilating, air conditioning, and refrigeration

Hydrochlorofluorocarbons (HCFC) – Refrigerants containing hydrogen, chlorine, fluorine, and carbon, thought to contribute to the depletion of the ozone layer although not to the extent of chlorofluorocarbons.

Hydrofluorocarbon (HFC) – A chlorine-free refrigerant containing hydrogen, fluorine, and carbon with zero ozone depletion potential.

 

ICM motor – Is a variable-speed motor. It adjusts its speed to deliver the right airflow needed by the system.

Ignition transformer – Provides a high-voltage current, usually to produce a spark to ignite a furnace fuel, either gas or oil.

Indoor Air Quality (IAQ) – This term generally refers to the study or research of air quality within buildings and the procedures used to improve the air quality.

Indoor coil – Evaporator coil

Induced draft motor – The draft developed in the heat exchanger of a gas-fired furnace by a fan located at the outlet of the heat exchanger, May be used with a natural-draft vent, or with a direct vent system: also called fan-assisted or mechanical draft.

Inert gas – A gas that will not support most chemical reactions, particularly oxidation.

Infiltration – Air that enters your home through holes, gaps, and cracks, (e.g., plumbing or electrical holes, the heating and air conditioning system, doors, and windows).

Insulation – Helps you save energy by sealing air leaks. Consider factors such as your climate, building design, and budget when selecting insulation R-values for your home.

Intermittent ignition – Ignition system for a gas furnace that operates only when needed or when furnace is operating.

IQA – Indoor Air Quality

 

Junction box – A metal or plastic box within which electrical connections are made.

 

Kilowatt (kW) – Equal to 1,000 watts.

Kilowatt hour (kWh) – A unit of electrical consumption. It is the equivalent of one kilowatt (1 kW) of power expended for one hour (1h).

 

Latent Heat – The energy that suspends moisture vapor in the air.  

Limit control – A control used to make a change in a system, usually to stop it when predetermined limits of pressure or temperature are reached.

Line set – A term used for tubing sets furnished by the manufacturer.

Liquid – A substance where molecules push outward and downward and seek a uniform level.

Liquid line – A term applied in the industry to refer to the tubing or piping from the condenser to the expansion device.

Liquid receiver – A container in the refrigeration system where liquid refrigerant is stored.

Load – The required rate of heat removal.

Load calculation – A mathematical determination of how much cooling and heating (BTUs) an HVAC system must deliver for occupant safety and comfort.  It is based on a variety of factors: square footage, building orientation, number of occupants, size and placement of rooms, number and size of windows and doors, amount of insulation, number of floors, and climate.

Load estimate – Studies to determine the heating or cooling requirements of a home or building.

 

Manual D® – An ACCA procedure covering the proper design, installation, maintenance, and repair of ductwork.

Manual J® – An ACCA procedure covering the method for calculating heating and cooling requirements (load calculation) for single-family detached homes and mobile homes. 

Manual RS® – An ACCA publication covering the design, installation, and commissioning of a residential HVAC system.

Matched System – An air conditioner or heat pump system composed of equipment that has been certified by ARI to work together to deliver the specified heating and cooling capacity at the stated efficiency rating.

Media – As used in air conditioning and heating, the material in an air filter that traps and holds the impurities until it is cleaned or replaced.

Melting point - The temperature at which a substance will change from a solid to a liquid.

Mold – A natural byproduct of the fungi family that thrives when organic substances and water combine under certain circumstances.  Mold reproduces via spores that can remain dormant, yet viable, for years.  Many molds are beneficial.  For example, they are the “bleu” in bleu cheese, and we use them to make wine, penicillin, and antibiotics.  However, some molds can cause health problems.

Modulator - A device that adjusts by small increments of changes.

Molecule - The smallest particle that a substance can be broken into and still retain its chemical identity.

Montreal Protocol - This landmark agreement initially required a production and consumption freeze. The Montreal Protocol called for a stepwise reduction and eventual production phase-out of various ozone depleting substances in developed countries.

 

NATE – North America Technician Excellence

Natural gas - A fossil fuel formed over millions of years from dead vegetation and animals that were deposited or washed deep into the earth.

Neutralizer - A substance used to counteract acids.

Nitrogen - An inert gas often used to "sweep" a refrigeration system to help ensure that all refrigerant and contaminants have been removed.

North American Technician Excellence (NATE) – headquartered in Arlington, VA. was founded in 1997 and is the nation's largest non-profit certification organization for heating, ventilation, air conditioning and refrigeration technicians. NATE is the only technician certification organization governed, owned, operated, developed and supported by the HVACR industry.

 

OEM – Original Equipment Manufacturer

Open-loop heat pump – A Heat pump system that uses the water in the earth as the heat transfer medium and then expels the water back to the earth in the same matter.

Oxidation - The combining of a material with oxygen to form a different substance. This results in the deterioration of the original substance.

Ozone - A form of oxygen. A layer of ozone in the stratosphere that protects the earth from certain of the sun's ultraviolet wave lengths.

 

Package Unit – A refrigeration system where all major components are located in one cabinet.

Pilot light - The flame that ignites the main burner on a gas furnace.

Plenum - A sealed chamber at the inlet or outlet of an air handler. The duct attaches to the plenum.

Pressure - Force per unit of area.

Pressure limiter - A device that opens when a certain pressure is reached.

Pressure switch - A switch operated by a change in pressure.

Programmable thermostat – Can help save energy by automatically raising or lowering temperatures at night and when you're away.

Propane - An LP gas used for heat.

PSI - Abbreviation for pounds per square inch.

Pump - A device that forces fluids through a system.

Purge - To remove or release fluid from a system.

PVC ( Polyvinyl choride) - Plastic pipe used in pressure applications for water and gas as well as for sewage and certain industrial applications.

 

R-22 – A refrigerant containing chlorine used in air conditioning systems. The EPA has mandated that R-22 cannot be manufactured after 2010 because it has been linked to the depletion of the ozone layer and global warming. Most commonly referred to by its trademarked name, Freon.

R-410A – The refrigerant that replaces R-22. It does not contain chlorine and is not hazardous to the environment.

R-factor – A rating of insulating or thermal resistance value. The ability to slow heat flow rather than to transmit it. A higher “R” number means better insulating value. 

Radiant barrier – A layer of thin material, usually made of metallic foil, that reflects heat in a specific direction.   Among their many users, these are used inside attics and in roofing materials to reduce heat gain or heat loss. 

Radiant floor heating – The use of embedded tubing within concrete floors through which Hot water is pumped for heating the floor and space around it.

Radiant heat – Heat that passes through air, heating solid objects that in turn heat the surrounding area.

Reciprocating compressor – A compressor that uses a piston in a cylinder and a back-and-forth motion to compress vapor.

Refrigerant – The fluid in a refrigeration system that changes from a liquid to a vapor and back to a liquid at practical pressures.

Refrigerant charge – The procedure an HVACR technician performs to ensure that the system has enough of the right kind refrigerant for peak operating performance.

Refrigerant recovery – To remove refrigerant in any condition from a system and store it in an external container without necessary testing of processing it in any way.

Refrigerant recycling – To clean the refrigerant by oil separation and single or multiple passes through devices, such as replaceable core filter-driers, which reduce moisture, acidity and particulate matter. This term usually applies to procedures implemented at the job site or at a local service shop.

Refrigeration – The process of removing heat from a place where it is not wanted and transferring heat to a place where it makes little or no difference.

Relative humidity (RH) – The amount of humidity the air is holding verses what it could hold at any given temperature.

Relay – A small electromagnetic device to control a switch, motor, or valve.

Relief valve – A valve designed to open and release liquids at a certain pressure.

RETURN, RETURN AIR, RETURN SIDE – The path the air takes to get to an air-handling unit or furnace so it can be cooled or heated.  It is the “return” path.  The return side should be “balanced” with the supply side to ensure proper air flow and comfort.

Rollout switch – A heat-sensitive protective device that opens the circuit if flame migrates away from the burner box.

Rotor – The rotating or moving component of a motor, including the shaft.

 

Saturated – Refrigerant liquid and vapor existing together.

Scroll compressor – A compressor that uses two scroll type components to compress vapor.

SEER – Seasonal Energy Efficiency Ratio

Seasonal Energy Efficiency Ratio (SEER) – An equipment efficiency rating that measures how much energy it takes to cool the air.  As with MPG on a car, the higher the number the more efficient the unit.

Sensible Heat – The temperature of the air.  This type of heat is measured with a thermometer.

Sensor – A component for detection that changes shape, form, or resistance when a condition changes.

Service valve – A manually operated valve in a refrigeration system used for various service procedures.

Short cycle – The term used to describe the running time (on time) of a unit when it is not running long enough.

Single package – A central air conditioner unit that contains both the condenser and air handling components in one packaged unit.

Solenoid – A coil of wire designed to carry an electrical current producing a magnetic field.

Sound rating number (SRN) – A unit of sound based on ARI standard tests for air conditioners and heat pumps. SRN is a measured in bels (one bel equals 10 decibels). A lower rating means quieter equipment. Most air conditioners operate at 8 to 9 bels. The quietest units are rated to operate at 6.8 bels. A small difference in numerical rating translates into a large sound difference.

Source temperature – The temperature of the medium from which a refrigeration system extracts its heat.

Split system – A refrigeration or air conditioning system that has the condensing unit remote from the indoor (evaporator) coil.

SRN – Sound Rating Number

Steam – The vapor state of water.

Sub-cooled – Refrigerant liquid cooled below its saturation temperature.

Subcooling – The temperature of a liquid when it is cooled below its condensing temperature.

Superheated – Refrigerant vapor heated above its saturation temperature.

Supply side – The part of an HVAC system that takes (supplies) the conditioned air from the air-handling unit or furnace to your home.  The supply side should be “balanced” with the return side to ensure proper air flow and comfort.

 

Temperature – A word used to describe the level of heat of molecular activity.

Thermal Expansion Valve (TXV) – A metering device that maintains constant superheat, and controls the evaporator load over a wide range of conditions. Usually maintains 8-12°F of superheat at the tail end of the evaporator.

Thermocoupler – A device made of two unlike metals that generates electricity when there is a difference in temperature from one end to the other. Thermocouples have a hot and cold junction.

Thermometer – An instrument used to detect differences in the level of heat.

Thermostat – A device that senses temperature change and changes some dimension or condition within to control an operating device.

Ton – The amount of heat required to melt a ton (2,000 lb) of ice at 32°F, 288,000 Btu/24 h, 12,000 Btu/h, or 200 Btu/min. One ton of air conditioning removes 12,000 Btu of heat per hour.

Transformer – A coil or wire wrapped around an iron core that induces a current to another coil of wire wrapped around the same iron core.

Two-stage – Describes equipment that operates at more than one speed. This feature usually associated with higher efficiency compressors, because they operate at low speed (thus lower energy usage) most of the time. However, when more air conditioning is needed, the equipment switches to the higher stage for increased comfort.

TXV – Thermal Expansion Valve

 

Ultraviolet – Light waves that can only be seen under a special lamp.

Upflow furnace – This furnace takes in air from the bottom or from sides near the bottom and discharges hot air out the top.

 

Vacuum pump – A pump used to remove some fluids such as air and moisture from a system at a pressure below the earth's atmosphere.

Valve – A device used to control fluid flow.

Vapor – The gaseous state of a substance.

Variable-speed motor – A motor that runs at a wide range of speeds rather than just one, reduces noise and energy consumption.

Vent – Port where air enters or exits a duct, room, or area.

Ventilation – The process if supplying and removing air by natural or mechanical means to and from a particular space.

 

Whole house approach – A system plan for indoor climate control and comfort that considers the interaction between humans, the structure, the building orientation, climate, and other elements or components. Other considerations include appliance, insulation, lighting, space heating and cooling efficiency, water heating, and more.

Window air conditioner – An air conditioner installed in a window that rejects the heat outside the structure.

 

Zoned System – A single HVAC system that can meet different heating and cooling needs in different areas (zones).  Each zone of a home has its own thermostat with which it can regulate the temperature and humidity in its area.  One “zoned air conditioner” could be set for a high temperature in one zone and for a lower temperature in the other zone.  Zone systems have two or more zones.