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Common Heating and Air Conditioning Terms You Need To Know

Annual Fuel Utilization Efficiency – AFUE

This is a rating system used to determine the overall efficiency of your gas furnace, water heater or boiler. The rating system determines how much of the fuel you burn is converted into usable heat, and how much is wasted. Since most gas appliances have to be vented for safety reasons a percentage of the overall heat generated during combustion is sent up the vent (chimney) as wasted energy. For example an 80% A.F.U.E. rated gas furnace converts 80% of the generated heat into useable heat for your home the remaining 20% was sent up the chimney as wasted energy. In other words 80 cents of every dollar you spend comes into your home as useable heat and 20 cents is wasted. By comparison a 96% A.F.U.E. rated gas furnace converts 96 cents of every dollar and only wastes 4 cents. Older furnaces may be as inefficient as 40-50% A.F.U.E. wasting about half of the fuel you paid for.

British thermal unit – BTU

One BTU is the amount energy that is required to raise the temperature of one pound of water, one degree Fahrenheit, at sea level. This form of measure is commonly used to describe how much heat a piece of equipment can transfer in one hour or BTUh.


A capacitor is a small devise that is used to store energy. Our industry uses them to store the energy needed to start a motor from stopped. This is a common part that will fail on an air conditioner or blower motor. The air conditioner uses them to start the compressor and the condensing fan in the top of the A/C condensing unit outside.

Coefficient of Performance – COP

This is how we measure the ratio of heat energy to electrical energy supplied, independently of the units used to measure energy. If $1 dollar of energy is used and you receive $2.50 dollars of heat in return the COP would be considered to be a COP of 2.5. Avg. COP=Heat transferred/electrical energy supplied= (HSPF * 1055 j/BTU)/ (3600 J/watt-hour) = 0.2928104 HSPF. Thus a system which delivers an HSPF of 7.7 will transfer 2.25 times as much heat as electricity consumed over a season, or a COP of 2.25. (Source: Wikipedia the free encyclopedia)

Flue Liner

A flue liner is used to protect the inside lining of a masonry chimney from the corrosive condensation that forms inside of a cold masonry chimney in the winter. This is required by building code anytime an older furnace is removed from a masonry chimney and replaced by a more efficient furnace. There are some exceptions that apply but any time a single gas appliance is venting into a masonry chimney or if the chimney is on an outside wall or wall common with a garage it would require a flue liner. The liner is most commonly a stainless steel flexible liner that is slipped inside the old chimney to reduce the diameter of the chimney so the gasses are concentrated into a smaller space allowing them to stay warm enough to exit the chimney and not condense in the flue.

HSPF stands for: Heating Season Performance Factor as defined by AHRI (Air Conditioning, Heating and Refrigeration Institute).

This is a term used to describe the efficiency of air source heat pumps during the heating season. The higher the HSPF rating the more efficient the unit is. HSPF is a ratio of BTU of heat output measured over the heating season compared to the watt hours of electricity used.

Inverter Compressor

An inverter compressor is a fully variable speed compressor which is used in the most advanced heat pump and air conditioning systems available today.

Line Set

This is a term that is commonly used to describe the copper refrigerant lines that connect the outdoor half (condenser) of your air conditioning system to the inside half (evaporator coil) of the system. One line (the smaller one) carries the liquid refrigerant to the inside coil where it evaporates into a gas. The other line (the larger insulated one) carries the refrigerant in a gas form back to the condenser outside where it is cooled off and compressed back into a liquid.

Reciprocating Compressor

This is an older type of compressor that uses a piston and valve system to compress refrigerant into a liquid from a gas. This used to be the only type available, but has become almost non-existent in newer more high tech equipment.

Return Air

Return air is the air that is returned to the furnace or air handler through what most people call the cold air ducts (return air ducts). Some homes have only one cold air return in the hall way or through the wall with a large grill covering it. Other homes have a more extensive return air system with a main return in the main living area and a system of smaller ones in each bedroom. The return air system needs to be equal to or greater than the amount of supply air (hot air ducts) we need to provide.

Scroll Compressor

A scroll compressor is a type of compressor which uses two plates with a scroll pattern molded into them. The plates are vibrated in a circular pattern to push the refrigerant into a smaller and smaller area, which compresses it into a liquid from a gas. This type of system uses substantially less moving parts that result in improved performance, longer life span and better efficiency. If the path of the refrigerant is ported off partially through the compression process it is referred to as off-loading. This process is used in a two stage scroll compressor.

S.E.E.R. stands for: Seasonal Energy Efficiency Ratio

This is a standard that basically means how many Btu’s of heat can be captured with one watt of electricity under certain operating condition as defined by 2008 standard AHRI 210/240. The SEER number is determined under laboratory conditions and may not reflect the actual efficiency level you receive if the unit is not properly installed. The scale is exponential meaning a 2 SEER is two times more efficient than a 1 SEER but to be two times more efficient than a 2 SEER you need to be at 4 SEER. In other words a 16 S.E.E.R unit is two times as efficient as an 8 S.E.E.R unit and so on.

Single Stage System

A single stage system is a system that has only one operating speed available at a time for both the blower and the heating or cooling source. When this term is used to refer to a furnace it means that the furnace is on full blast or it is off. The same can be said for an air conditioning system. This is very simple technology, and is very common in builder grade systems since it is the least expensive type of system to manufacture. Most single stage blowers have multiple speed settings but must bet set by a technician at the time of installation.

2-Stage System

A 2-stage system is a system with two levels of operation, typically low and high speed. This type of system is used to allow the system to run in low stage as long as possible to provide for better air distribution. The longer run cycles help to deliver more even air temperatures throughout the home, which provides better comfort. An additional benefit is when the system is operating in low stage; the system will usually produce less noise.

Supply Air

The supply air duct system is the network of ducts used in a forced air heating or cooling system that delivers the conditioned air from the furnace or air conditioner to the individual space where the conditioned air is needed. These are commonly referred to by homeowners as hot air ducts or heat ducts. The amount of supply ductwork in a home is in direct proportion to the size of the system. These systems are carefully designed to match the specific amount of air flow the room requires, based on the amount of heat or cooling that is needed in that space (load).

Variable Speed

Variable speed technology has been around for a number of years, but has become more and more popular due to its ability to provide the very best comfort for a reasonably small additional investment. A variable speed furnace or air handler use the fan speed to enhance the dehumidification cycle of a cooling system by slowing down the air flow which allows the indoor coil to get much colder much quicker. Colder more dense air cannot hold as much moisture as warm less dense air can, so the excess moisture or humidity is squeezed out resulting in a much cooler feeling home. The system can also slow the air flow in the winter allowing the air to become much warmer at the register. The slower air also produces less noise and better air distribution since the air is traveling at a slower velocity. This blower can ramp up one rpm at a time producing an unlimited number of speeds. This also results in less energy consumption, using roughly 60 watts of power as compared to 600 watts for a single stage blower.