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We’ve put together this glossary of commonly used HVAC and thermostat terminology.


Central and local heating and cooling applications are typically controlled or activated using a 24-Volt signal. This low voltage level provides for a much safer method, compared to 120V/240V Line-Voltage.


In a forced-air central heating and/or cooling system, this is the component which houses the “blower fan” or main circulation fan that delivers air from the vents to the room areas. If there is also a furnace present as the heat source, the furnace typically also contains the air handler section as part of it.

This is a secondary heating component found in many Heat Pump systems, especially in medium to colder climates. Auxiliary heat is typically an electric heating element located within your air handler, and is similar to what you have in your kitchen toaster but on a much larger scale. Most heat pump thermostats have an operating mode called Emergency Heat, which simply uses the Auxiliary Heat device as the primary heat source and does not call for the outdoor heat pump to run.


BTU (British Thermal Unit):
The BTU is used as a measure of energy in the heating and air conditioning industry. The term “BTU per hour” is the standard for conveying the output of a heating unit (i.e. 80,000 BTU natural gas furnace).


The internal temperature sensor in a thermostat is accurately calibrated at the factory and usually will not need to be adjusted. The Temperature Calibration feature allows you to manually offset the measured temperature by as much as plus or minus 5°F degrees from its original value. This feature can be useful to match one thermostat to another, if multiple thermostats are used in the same home.

See COMMON WIRE, below.

This is also referred to as the "System Common" wire. In the 24V low-voltage wiring for a heating/cooling system, the connections are made at the thermostat and the equipment using LETTERS. The "C" stands for Common, and this is the negative (-) side of the 24V power source which supplies the positive (+) voltage to the thermostats "R" terminals. BLUE is the most often used color for the “C” wire at the thermostat, however this is not always followed. Most mechanical and battery operated thermostats do not require a common wire, however if one is present it should be used to avoid having to replacing batteries often.

This is the main mechanical component in the outdoor condensing unit which does most of the hard work in a central air-conditioning system or heat-pump system. This component compresses and pumps refrigerant to the indoor unit and through the rest of the components in the system.

This is the main outdoor component in a central air-conditioning system or heat-pump system and acts as the means for expelling heat to or extracting heat from, the outside air.

This is the outdoor component in a central air-conditioning system or heat-pump system. Inside of the condensing unit, there is a compressor, a condenser, and a large fan.

This is generally any type of heating or cooling system that is not termed as a “heat pump”. Examples of conventional systems are: central air conditioning unit, natural gas forced-air furnace or boiler, oil forced air furnace or boiler, electric forced-air furnace, electric baseboard (line-voltage) heating element.


A damper is a movable door or set of louvers which can be closed within the ductwork in a forced-air system, which is used to direct airflow to specific pathways for areas that the air needs to be delivered.

As it relates to electronic devices within the heating and cooling industry, this refers to a temporary alteration of a thermostats set temperature by the electrical power utility company during cases of peak electrical demand, to prevent large scale power outages or system emergencies.

This term varies somewhat throughout the industry. Some brands refer to a Digital thermostat as ANY type of thermostat that is electronic in nature (non-mechanical thermostat). Other brands, including LUX, use the term Digital Thermostat to denote an electronic thermostat that is non-programmable. LUX has four main thermostat classifications or categories: Mechanical, Digital, Programmable, and Line-Voltage.

A heating/cooling system which contains both an electric heat-pump system and the addition of a fossil-fuel furnace (gas or oil) as the auxiliary/emergency heat source. This lets the user choose between different heating methods as desired, in order to respond to fluctuations in utility costs.


The Early Recovery feature affects how the thermostat transitions from an energy saving setback temperature, to a comfort temperature. Normally the change in temperature only starts to occur at an upcoming period’s start time. With Early Recovery Disabled: If you have programmed your thermostat for 70°F at 5:00PM, the thermostat will only begin to heat your home back up when the time reaches 5:00PM. With Early Recovery Enabled: Using the same scenario as above, the thermostat will calculate the best time to turn on the heater, so that the temperature in your home reaches 70°F as close to 5:00PM as possible.

This is a forced-air system which does not use any fossil-fuel (i.e. no gas or oil), and relies solely upon an electric heating grid similar to what is in a kitchen toaster, but large enough to heat an entire home. An electric heating element can either be used alone as the only heat source, or as a supplement or backup to a heat-pump style system.

Also refer to AUXILIARY HEAT. Emergency Heat is a mode of operation that exists on many Heat Pump thermostats which uses the auxiliary electric heating element as the primary heat source. Emergency Heat mode is typically used at times when the outdoor Heat Pump unit has either failed, or when outdoor temperature conditions are cold enough that the Heat Pump is no longer able to provide adequate warmth to your room air.

This is the main indoor component in a central air-conditioning system or heat-pump system and acts as the means for delivering heat to or extracting heat from, the home’s air flow as it passes through the air handler and ductwork.


This is a system which uses a centralized blower fan to deliver heated or cooled air to the home through ducts and vents.


This setting changes how the system’s blower fan (if applicable) is controlled while in HEAT mode, and with the Fan switch in the AUTO position. This setting does not affect fan operation while in COOL mode. When set to “Gas”, the fan is controlled solely by the heating system itself and not by the thermostat unless the fan is set to manual fan on. When this option is set to “Electric”, the fan terminal is always controlled directly by the thermostat in both HEAT and COOL mode. NOTE: If your blower fan does not operate properly immediately after installation of a new thermostat, try the Gas/Electric option to “Electric”.

This is a feature provided on the LUX/GEO which allows you to save energy by automatically setting back temperature when you are away from home. With the Lux App installed on your iPhone or Android mobile device you can set a “radius” around your home. When you are inside the radius area your GEO will go to your programmed comfort temperatures. However when you are outside the fenced area your GEO will go to the “away” temperature.


A heat pump looks like what most people would recognize as a central air-conditioning unit, with the main difference being that the outdoor unit runs during heating in addition to just running in cooling only. The system contains additional components such as a “Reversing Valve” to enable it to generate either heating or cooling depending upon the signals from your room thermostat. When in the Reverse mode, the outdoor “Condenser” becomes the “Evaporator” and the indoor “Evaporator” becomes the “Condenser”.

A high-efficiency furnace (also referred to as a condensing furnace), is a heating unit that is able to convert the vast majority of its fuel source to heat energy, with very little wasted or lost. High-efficiency furnaces can achieve from 90% to 98% fuel efficiency. One of the ways this is done is by running their combustion exhaust through a secondary heat exchanger to extract additional heat that would have otherwise been exhausted and lost. So much heat is extracted that the combustion exhaust generates a significant amount of condensation, up to 5 to 6 gallons a day, which must be pumped outside by a small condensate pump.

The term HVAC stands for Heating, Ventilation, and Air Conditioning, and simply refers to all of the components that make up your entire heating and/or cooling system.

This uses a boiler (gas or oil) or water heater to circulate hot water through a continuous loop of pipes to either radiators or wall mounted baseboard elements that act just like long miniature radiators.


JUMPER OPTION (also referred to as a SHORTING CAP):
This refers to a small plastic rectangle sleeve (with metal inside) that shorts two metal pins together. Shorting Caps are typically found on the circuit board of a thermostat for selecting user options or operational setup configurations.


The Watt is the standard unit of measuring electrical power. A Kilowatt is simply 1,000 Watts, and would be equivalent to the amount of electricity required to illuminate about (17) 60-Watt incandescent light bulbs. Most electric space heaters are 1500 Watts, or 1.5 Kilowatt. A Kilowatt hour is the sum of how much power it would take to power a 1 Kilowatt device, for a duration of 1 hour.


This term relates to anything which connects to or directly uses the 120 AC Volts or 240 AC Volts that is provided by your main electrical circuit breaker panel or fuse panel. Electric (resistive) Baseboard heating elements are an example of a LINE-VOLTAGE heating application.

Even replaceable batteries come in different shapes and use different chemical technologies. For correct operation of the LUX/GEO thermostat, you should always use AA size batteries made using “lithium” technology and we recommend the Energizer brand.

This term represents the majority of heating and cooling systems, such as: central air conditioning, natural gas forced air furnace, most hot water boiler systems, etc… The actual heating/cooling equipment does run on either 120 or 240 volts, however they are controlled or signaled by low voltage, typically 24 volts.



A mechanical thermostat both senses and controls temperature without requiring either batteries or electrical power from the system in order to function. With this type of thermostat there is generally no ability to set programs for time of day operation, or any similar advanced features.

These are also called “Ductless Mini-Splits”, and are simply smaller versions of regular central Split Systems air-conditioners, however they do not have any ductwork for their air delivery. These units have a small condensing unit outside, and typically a ceiling or wall-mounted air handler component with the evaporator built-in. The only thing that connects the indoor/outdoor components is a pair of copper tubing lines, which makes this preferred in locations where ductwork cannot physically be installed.

This is any heating or cooling system that is capable of varying its output levels depending upon the amount of demand currently on the system. This is accomplished by having a low and high output; initially stage 1 is called for, and if stage 1 alone cannot satisfy the user’s target temperature, then stage 2 is added to it so both stages can run together.


The OFFSET is the number of degrees below (above) the set temperature at which the 2nd stage of heating (cooling) will start. Having a larger number for the offset will mean that your 2nd stage of heating (cooling) will run less frequently but when it does run it will run for a longer period of time.
In most LUX thermostats, this setting is adjusted as a number of degrees, and only effects the operation of the second heating stage (W2 wire terminal) or second cooling stage (Y2 wire terminal), if present. The setting range for Offset is from 0 to 9 degrees. When set to 0 degrees, the second stage is disabled.


Most commonly found on rooftops of small businesses and commercial buildings, these units contain both the heating and cooling components within the same physical enclosure. These are made specifically for outdoor installation, and usually only require the local attachment of the supply/return air ducts and the utility connections (electric, natural gas).

This is a method which some thermostats use as an alternative to that of a) replaceable batteries, or b) using a dedicated 24V Common Wire to supply the power needs of the thermostat itself. This was more feasible in the earlier years of electronic thermostats, but most modern thermostats require too much power from the circuit to be used in a reliable fashion.

A programmable thermostat is designed to let the user adjust the temperature settings at specific times throughout the day, to accommodate the user’s lifestyle and temperature needs. If used properly, a thermostat that varies the set temperature to modest values while the user is away can save a significant amount of money, as compared to remaining at a single consistent temperature.

This is a wireless networking term, which refers to the act of preparing or configuring a device (thermostat in this case) to communicate with an existing exterior network. After a connected thermostat is provisioned, it can then sync with an internet server and be controlled by a web browser or a mobile device at a remote location.

This stands for Packaged Terminal Air Conditioner, which is a type of air conditioner and heater unit combined into a single, electrically powered enclosure, typically with its own built-in thermostat controls. These types of units are often found in hotel rooms, and are typically installed through an outside wall at floor level.


See “Geofencing”


This stands for Seasonal Energy Efficiency Ratio, and is an energy efficiency rating for air conditioners and heat pump units. The higher the SEER number, the better the energy performance and the more you save.

See “multi-stage system”

This refers to the target temperature that a user “wants” the room temperature to be. When a user adjusts or changes the Set Point or set temperature of a thermostat, this will signal the heating or cooling equipment to turn on as needed to achieve the desired Set Point.

SHORTING CAP (also referred to as a JUMPER OPTION):
This refers to a small plastic rectangle sleeve (with metal inside) that shorts two metal pins together. Shorting Caps are typically found on the circuit board of a thermostat for selecting user options or operational setup configurations.

This is any type of cooling (and sometimes heating) system that contains a combination of indoor components and outdoor components which are separate from each other. The majority of common central air-conditioning and heat pump heating systems are all considered split systems.

A thermostat works by turning your heating or cooling system on and off whenever the room temperature varies from the set-point temperature. In a LUX thermostat, the amount of this variation is called the swing. A smaller swing number means the room temperature will not be allowed to vary a lot from the target set temperature, so the room temperature will remain more stable and constant. A larger swing number allows the room temperature to vary both above and below the target set temperature which will mean that the system will run for a longer duration and then be off for a longer duration between on/off cycles. This can generally save energy in most cases. For most LUX thermostats, the Swing setting will be a number value between 1 and 9, with number 1 being plus/minus 0.25F from the set temperature and number 9 being plus/minus 2.25F from the set temperature (each higher number is a 0.25F degree increment).



This is an advanced feature which can restrict how high the heating set temperature can be set to, or how low the cooling set temperature can be set to. Temperature limits are typically put in place to prevent excessive or wasteful usage of the heating or cooling equipment.

These are the connection points on a thermostat for attaching your heating and/or cooling wires.

The thermostat is the control unit that instructs the heating or cooling system components to cycle on and off at regular intervals to maintain the desired interior set point temperature that is set by the user.


With regards to thermostats, this term implies compatibility with the vast majority of the most typical systems that exist in the marketplace.


In the 24V low-voltage wiring for a heating/cooling system, the connections are made at the thermostat and the equipment using LETTERS, not necessarily the wire colors. The ones that are most often used are R (24v power), W (heating signal), Y (cooling signal), and G (fan signal). The "R" may also be separated into "RH" (24v power from heating components) and "RC" (24v power from cooling components). If there is a "C" this stands for Common, and this is the negative (-) side of the 24v power source which supplies the positive (+) voltage to the "R" terminals. Most of the time, the wire colors will indeed match the letters for the main four: R=RED, W=WHITE, Y=YELLOW, and G=GREEN, but this cannot always be relied upon without first confirming the letters match them. Beyond that, any other terminals are much less followed as a standard, however C=BLUE for System Common at the thermostat is the most often color usage.


This method divides a home into separate areas or comfort zones which are each independently controlled by their own thermostat. The benefits are that areas not being used can remain set back to a modest temperature to conserve energy, and one single heating/cooling unit can be shared among the separate zones, instead of needing to have many smaller units for each area.