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The Basics of Heating, Ventilation and Air Conditioning

Air Conditioners: How Does It Work?

An air conditioner cleans, circulates, cools and dehumidifies (removes undesirable moisture from) indoor air. A filter cleans the air by trapping dust and other small particles. An air handler (blower built into the system) circulates it, while the cooling and dehumidifying are accomplished by a process called refrigeration.


Refrigeration cools a home by transferring heat inside a home to the outdoors. All central air conditioners employ two main units in this process - the indoor unit and the condensing unit.

The Indoor Unit

This unit removes undesirable indoor warmth and humidity. It includes the filter, the air handler and the evaporator coil. The air handler blows filtered air through the evaporator coil. The evaporator coil is kept cold by the circulation of a substance called a refrigerant. Air that travels across the evaporator coil gives up heat (the colder coil absorbs it) and humidity (moisture condenses upon contact with the cold surface of the coil). The cooler, drier air that continues through the air ducts is vented throughout your home to maintain your desired comfort level. Depending on the structure of your home, the ductwork may be above the ceiling or below the floor.

The Condensing Unit

Outdoors, at the condensing unit, an air conditioner releases the heat that was captured indoors. The same refrigerant that absorbed the heat indoors at low pressure is now pressurized by the compressor and is circulated through another coil, the condensing coil. In the condensing coil, under high pressure, the refrigerant releases its heat very quickly, making the coil itself hot. A fan blows across the coil, cooling its temperature down and transferring the heat to the outside air.

Furnaces: How Does it Work?

The furnace is the most important component of a central heating system. It houses all the working parts. So when you replace the furnace, you replace the vital operating parts of your heating system. It is by choosing from among the different models and brands of furnaces available that you determine the quality and cost of your business’s heating for years to come.


The Furnace is part of a forced-air system. Warm air is forced, or blown, through a system of air ducts to each of the rooms in the office. Office air drawn into the furnace passes through a filter, where dust and other small particles are trapped. A blower unit blows the filtered air through the furnace, and the air absorbs heat.

Gas Heat

If it is a gas furnace, the heat is supplied by the burning of natural gas. A mixture of gas and air flows into the burner and is ignited by the pilot. Combustion occurs, and warm air from the burner flame rises to fill a chamber known as a heat exchanger. The heat exchanger becomes hot. Office air passing around the heat exchanger absorbs that warmth, continues into the air ducts and the heat is distributed through the business.The by-products of combustion pass upward through a venting system and escape through a vent in the roof.

Electric Heat

If the furnace is electric, heat is generated by an electric heating element. Electric current traveling through the element creates heat. By the heat transfer processes called conduction and convection, heat is transferred into the air stream and flows through the air ducts into the rooms of the business.

The Thermostat

Whether you heat your office with gas or electricity, a wall thermostat will be installed. This measures room temperature and turns the central heating system off or on as the temperature rises or falls to designated levels. Careful location of the thermostat is an essential consideration in maintaining maximum comfort levels in your business.

       HVAC Terminology


Annual Fuel Utilization Efficiency. Used to express the efficiency of gas furnaces. The higher the AFUE rating, the SUITE more efficient the unit. Federal law has required that all new residential furnaces built after January 1992 operate with an AFUE of 78% or higher. All furnaces are manufactured to exceed these requirements with a minimum AFUE of 80%.

If your furnace was built before 1992, chances are it is operating with an average efficiency of around 60%. Most of the heat is lost up the chimney or out the exhaust vent - devices used by older furnaces to expel dangerous fumes created by the furnace, such as carbon monoxide, carbon dioxide, aldehydes, and even soot.


Modern furnaces use more technologically advanced venting techniques to achieve greater energy efficiency. An important note: Be sure to ask your installing contractor if your chimney or exhaust vent is suitable for use with your new equipment.


The Air Conditioning and Refrigeration Institute is an organization created by HVAC manufacturers to ensure an acceptable level of quality within the industry. ARI is a voluntary, nonprofit organization which publishes ratings standards and benchmarks for testing cooling and heating products.


Short for British Thermal Unit. The amount of heat required to raise or lower the temperature of one pound of water by one degree Fahrenheit.


The heat transfer rate of HVAC equipment is measured in British Thermal Units per Hour.


Usually measured in BTUs or tons, capacity refers to an air conditioning or heating unit’s ability to cool or heat a space. For instance, a 20-ton air conditioning unit has twice the capacity of a 10-ton unit.


A unit to express movement of volume, including air, in Cubic Feet per Minute. A 400 CFM air handler moves 400 cubic feet in one minute.


The compressor plays an integral role in cooling your home. It is the device responsible for pumping refrigerant through the refrigerant lines and the coil, making the transfer of heat from inside your house to the outdoors possible.


The coil responsible for dissipating heat to the surrounding, outside air. Also called the condenser coil, or outdoor coil, its role is reversed when a heat pump is used in heating mode.


The Coefficient of Performance rates a heat pump’s ability to efficiently use electricity in its operation. The Air Conditioning and Refrigeration Institute provides the Coefficient of Performance at 47 degrees Fahrenheit and 17 degrees Fahrenheit. This is because a heat pump is more efficient at higher, outside-air temperatures.


The relative loudness of a sound is expressed in dB, short for decibel. As an example, the sound of a human voice talking is around 70 dB. (See also SRN.)


A federal agency, the Department of Energy, sets the standards for efficiency throughout the HVAC industry and monitors consumption of energy sources.


A term used to describe the direction of airflow through a furnace. A downflow furnace takes return air from the top, heats it and then delivers the warm air from the bottom


A central heating and air conditioning system uses many components to heat or cool air This warm or cool air is then transferred to different registers throughout the house via special flexible large-diameter pipes or ducts The system of ducts throughout your house is often referred to as ductwork or ducting


Energy Efficiency Ratio The ratio of the cooling capacity of the air conditioner in BTUs per hour to the total electrical input in watts This measure is determined by comparing test units to the Air Conditioning and Refrigeration Institute specifications


A general term used to describe how effectively a heat pump air conditioning system or furnace converts incoming energy to outgoing energy The higher the number the more efficient the unit and the lower the operating costs


An integral part of the indoor unit of a heat pump or air conditioning system. So called because when warm air passes over a coil filled with liquid refrigerant, the refrigerant itself evaporates and absorbs some of the heat. This gas refrigerant is then pumped to the outdoor coil, where it releases heat into the surrounding air and returns to its liquid state.


Responsible for transferring heat from furnace burners to the blower. Brian’s Plumbing and Heating uses some of the finest heat exchangers in the industry with limited lifetime warranties on some models.


A term used to describe the direction of airflow through a furnace. A horizontal flow furnace takes return air from one side, heats it, arid then delivers the warm air from the other side.


Heating Seasonal Performance Factor. It measures the efficiency of the heating portion of your heat pump. The Department of Energy minimum is 6.8. (Similar to SEER.)


Usually available as an optional accessory, a humidifier is used to inject water vapor into the dry, heated air expelled from a furnace/air handler. The benefits can be improved efficiency and a more comfortable living environment.


Heating, Ventilating and Air Conditioning. Used to refer to the industry at large, particularly dealers of heating and air conditioning equipment.


Split-system home comfort systems use two main components to deliver air for a comfortable living environment. The indoor coil is the device responsible for transferring heat from indoors to the outdoors (or the reverse in the case of a heat pump in heating mode) Most modern systems are designed to achieve maximum efficiency when the indoor unit (coils and blower) is properly matched with the outdoor unit (air conditioner or heat pump). For best results, be sure to replace both the indoor and outdoor units at the same time.


A unit used to express 1,000 Watts. Denoted as kW.” Note that the W in ‘kW’ is always capitalized because the Watt unit was named after a person.


If a unit uses 1,000 Watts in 1 hour, it is said to have an energy rating of lkWh.


Furnaces are designed to deliver maximum heat for comfort on the coldest of days In most cases those days account for fewer than three percent of winter days. The rest of the time, your furnace is providing more heat than necessary. Because conventional furnaces are either providing no heat or at full capacity the temperature in your house goes up and down by several degrees adversely affecting your comfort and your energy bills Modulating furnaces solve this problem by varying the amount and temperature of air delivered between different capacities so that the air flowing out of the registers is always at the temperature you determine This results in lower operating costs, more comfortable temperatures throughout the house and quieter operation.


Equipment in which all heating and cooling components are located in one cabinet. Installed either beside or on top of a home or business.


The liquid used to absorb and transfer heat from one part of the home comfort system to another.


Copper lines used to transfer the refrigerant between the outdoor unit and the indoor unit.


Seasonal Energy Efficiency Rating. Used to express the efficiency of an air conditioning unit, or a heat pump in cooling mode. The higher the SEER rating, the more efficient the unit. The Department of Energy minimum is 13


A home comfort system that uses an indoor and an outdoor component to deliver comfortable air to a living environment.


The Air Conditioning and Refrigeration Institute performs tests and assigns a Sound Rating Number (SRN) to units. A lower SRN rating indicates a quieter unit with average SRNs of between 74dB and 80dB.


A temperature-measuring device used to control the operation of home comfort systems to maintain a comfortable temperature within the house. Programmable thermostats allow you to program different temperatures for different times of the day.


The ton ratings you see here have nothing to do with the weight of the unit. In fact a ton is simply 12,000 BTUs (see BTU definition on this page). A typical home cooling/heating system uses heat pumps or air conditioners with a capacity of between 1.5 and 5 tons.


A term used to describe the direction of airflow through a furnace. An upflow furnace takes return air from the bottom, heats it, and then delivers the warm air from the top.


Electrical power also expressed as ‘W’ For example a 100W globe consumes 100 Watts of electrical power The W in Watt is always uppercased, because it is named after a person.


A home may be divided into several different areas, or zones, to better control the temperatures throughout the house The process of dividing your home into different zones is called zoning

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