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Glossary of Heating, Air Conditioning and Plumbing Terms

Throughout our site we use many technical terms and industry specific language.

It can be confusing. Use this glossary to get a more detailed explanation.

Alphabetical Index of Terms

A  B  C  D   E  F  G  H  I  J  K   L  M  N  O  P  Q  R  S  T  U V   W  X  Y  Z

AFUE This is a percentage measurement of a furnace's heating efficiency. The U.S. Government's minimum efficiency level is 78%. The higher the AFUE, the more efficient the furnace. The initials stand for Annual Fuel Utilization Efficiency.


American Gas Association
Air Handler This is the indoor section of a split system. It can be a dedicated air handler, or could be your furnace. Also known as a fan-coil.

Ambient Temperature

The average temperature of the atmosphere in the vicinity of an appliance

Anode Rod

A sacrificial rod composed of one or more metals installed in the water heater that protects the tank from corrosion helping extend the life of the tank

Anti Scald Valve

Device used to prevent high water temperature


American Society of Heating Refrigeration Air Conditioning Engineers


American Society of Mechanical Engineers

Back Flow

When water travels from one system back into any part of the main distribution system


British Thermal Unit. This is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. You'll see this measurement when you look at heating and cooling capacities - for example, your dealer may re commend a 75,000 Btu furnace and a 24,000 Btu air conditioner for your home.


A heat producing device where the flame or heat is produced

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Check Valve

To allow fluid to flow in only one direction in a pipe; also known as back flow preventer

Closed System

A system where the incoming cold water supply has a device that will not allow water to expand when heated (i.e. check valve, back flow preventer, some pressure reducing valves)

Combustion Chamber

The location where combustion takes place.

Commercial Application

Water heaters used for more than a single family dwelling


Formed by more heat being absorbed into the water leaving less temperature to carry the water vapor produced in gas combustion to pass out of the venting system
Condensing Unit This is the outdoor section of a split system air conditioning system. You'll know it best as the air conditioner that sits outside your home.

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Degree Rise

Difference between the starting water temperature and the ending temperature after heating is complete


A nonconductor of direct electric current

Dip Tube

Tube inside the water heater that sends cold water to the bottom of the tank

Direct Vent

Pulls outside air for combustion and vents combustion gases directly outside

Draft Diverter

A device fitted in the flue way of a gas appliance to prevent updraft, downdraft or the secondary flue blockage from obstructing the escape of products of combustion or otherwise affecting the normal operation of the appliance

Drain Valve

Device designed to allow drainage of stored contents from a water heater

Dual element heater

An electric water heater with an upper and lower element for heating water


A products ability to utilize input energy; expressed as a percentage

Energy Guide Label

The Federal Trade Commission (FTC) requires that gas, electric and oil residential automatic storage water heaters be labeled to show 1.) an estimated annual cost of operation for that particular model, based on a national average cost of fuel specified by FTC and 2.) how the efficiency of that model compares to all other comparable models
Evaporator Coil If your furnace is the air handler section of your split system, then you'll need an indoor coil added to your furnace to complete the system. The coil transfers heat to give you cool air and also aids in dehumidification.  - If your furnace is the air handler section of your split system, then you'll need an indoor coil added to your furnace to complete the system. The coil transfers heat to give you cool air and also aids in dehumidification.

Expansion Tank

Designed to absorb excess pressure due to thermal expansion, e.g. closed system

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First Hour Delivery (Rating)

Combination of the usable stored volume of hot water in tank, plus the recovery capacity for the first hour of operation

Flow control valve

Device designed to reduce water flow (GPM) to a plumbing fixture (i.e. shower head at 5 GPM vs. 2.5 GPM); the use of flow control valves can be cost effective in load calculations, reducing the water usage and the amount of energy used to heat water

Flow rate

Rating in gallons per minute (GPM) or gallons per hour (GPH)


A passage way for products of combustion

Flue Baffle

A device to deflect, check or regulate flow through the flue

Flue Damper

Minimizes convective heat loss through venting system

Foam Insulation

The insulation surrounding the surface of the water heater tank

Foot Print

The area of space taken up by the water heater

Galvanic Action

If two unlike metals are immersed in an electrolyte, an electrical potential will exist between them; if the two are in electrical contact, an electrical current will flow; the metal which becomes the anode of this cell will corrode and dissolve while the cathode will be protected from corrosion
GPH Gallons Per Hour. You might see this rating if you are looking at an oil furnace or water pump sizing chart. In addition to input and output, an oil furnace also has a rating of gallons per hour, the volume of oil a furnace is capable of burning in 60 minutes.  Most water pumps are sized in Gallons per hour.

Gas Control

Device use to regulate gas pressure on a water heater

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Hard Water

Natural waters contain impurities in various proportions; traditionally hardness is a measure of calcium or dissolved solids in a solution. Below is a list of types of hardness and their referenced parts per million range




0-49 ppm

Fairly soft

50-99 ppm

Fairly hard

100-149 ppm


150-249 ppm

Very Hard

250 plus


The pressure of water as measured at a stated point; it may be measured in feet or in pounds per square inch (PSI)

Heat Exchanger

A heat transfer system
Heat Pump A unit that both cools and heats your home. A heat pump system can be either a split system or a packaged system. A heat pump can be used in conjunction with a gas/oil/LP furnace (using the furnace instead of electric resistance heat when temperatures fall below about 35 F) with the addition of a fossil fuel kit.

Heat Transfer

When heat is passed from one medium to another

Heat Trap

Restricts heat loss through water connections to a tank
HSPF This is a measurement of a heat pump's heating efficiency. There is no governmental minimum rating. The higher the HSPF, the more efficient the heat pump's heating performance. HSPF stands for Heating Seasonal Performance Factor.
Humidistat Wall mounted control which powers and regulates your home's automatic humidifier.


Referring to something submerged in water

Incoming (Inlet) Temperature

Temperature of water entering the tank


The amount of fuel used by a gas appliance in a given period of time: generally rated in 1 hour

Instantaneous Water Heater

A type of water heater that heats water as it flows through a heat exchanger coil

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Junction Box

Utility area where incoming current is connected in a water heater

KW - kilowatt

A measure of the rate of supply of energy or power, and is equal to 1000 watts or 3412 BTU per hour


Liquid Propane - A fuel for gas water heaters

Manifolded Installation

Paralleling of water heaters for large hot water demand applications

Mixing Valve

Mixes hot and cold water to achieve a specified delivery temperature

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Natural Gas

A fuel used with gas water heaters


National Sanitation Foundation

Oil Powered

A water heater that uses oil as its fuel source

Operating cost

The cost of running a water heater for a given time period
Packaged System Packaged units provide both heating and cooling from one unit that is placed outside the home - on the ground, on the roof, or sometimes mounted through the walls of the building. Packaged units come in several combinations of fuel sources - gas heat/electric cooling; heat pump; electric heat/electric cooling; oil heat/electric cooling.

Parallel System

Two or more identical size heaters piped with water connections, that is, cold water travels equal distance into the inlets of the heaters from a "t" connection and hot water travels equal distance from the outlet of the heaters to a "t" connection

Peak Hour Demand

Time when the largest demand for hot water is needed

Peak Load Period

That period of the day when the system has the greatest demand


A small burner used to ignite the main burner

Point of use

Small water heater used for remote locations water heater

Power Venting

Mechanical draft exhaust to outside, usually utilizing room air to support combustion

Preheated water

Water that has been tempered for inlet supply

Pressure Reducing Valve

A valve which automatically reduces inlet water pressure to a specified value at its outlet under static cold water conditions

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Rated Storage Volume

Quantity of water (expressed in gallons) stored in a tank

Recovery (GPH)

The amount of water in gallons that can be heated in one hour


Water heaters for single family dwellings

Return Circulation System

Tempered water from or near the point of usage which eliminates waste of hot water used for long runs and adds storage to the system


Recovery Factor - Rating based on the efficiency of the product which is input required to raise 1 gallon of water 1 degree F
Gas - 75% to 85% RF = 11 to 9.7 BTU/H
Electric - 100% RF = .0024 KW/H
Oil - 70% to 85% RF = 11.79 to 9.7 BTU/H

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Safety shut off valve

A device on a gas appliance which shuts off the gas supply to the appliance to prevent a hazardous situation. A flame failure safety shut off operates when the actuating flame becomes extinguished. A 100% shut off cuts off all gas including main and pilot burners


A thin coating or layer, usually calcium on the bottom of a tank or interior parts, that may prevent heat transfer

Sealed Combustion

Sealing of combustion chamber to prevent spillage of combustion products
SEER This is a measurement of the efficiency of cooling products. The U.S. Government's minimum efficiency level is 10 SEER for split systems and 9.7 for packaged units. The higher the SEER, the more efficient the cooling product. SEER stands for Seasonal Energy Efficiency Ratio.

Sediment (lime)

The substance that settles on the bottom of a tank


Generally where the primary heater preheats water to a given desired general purpose temperature and feeds into another heater

Sliding Inner Door

A door slides along combustion chamber radius for easy access to the burner and pilot


A coil of wire in the form of a cylinder that when carries a current; resembles a bar magnet

Spark Ignition

Intermittent ignition device that utilizes a spark to light a flame
Split System This describes an air conditioning or heat pump system that is split into two sections - an outdoor section and an indoor section. It won't work without the outdoor section plus an indoor section to move the air.


A threaded opening on the water heater tank

Stand By Loss

The amount of heat lost while unit is in stand by mode

Standing pilot

See pilot

Storage Tank

A tank used to hold a specific volume of water

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T & P Valve

Temperature & Pressure Relief Valve - A safety device used to expel excess pressure or temperature from inside a tank

Tankless Water Heater

Commonly known as instantaneous

Temperature Rise

The number of degrees Fahrenheit (F) the incoming cold water must be raised to reach the desired hot water temperature


A measurement of 100,000 BTU's

Thermal Efficiency

Ability to transfer and absorb heat from fuel source into the water

Thermal Expansion

Water, a non-compressible liquid, expands when heated


A small electric generator. Electron flow between the hot junction of 1200 degrees F and the cold junction of 600 degrees F creates millivoltage

Thermostat Lower

A device which automatically maintains a predetermined temperature in an appliance

Thermostat Upper

A device which automatically maintains a predetermined temperature in an appliance usually equipped with a safety shut off
Ton You'll often see this as a measurement of the capacity of an air conditioning system. Don't panic, it doesn't measure weight! Just like gas and oil furnaces, air conditioners and heat pumps are rated in Btu's. One ton of air conditioning is 12,000 Btu's per hour. This means that a "one ton" air conditioning system has the capability of removing 12,000 Btu's of heat per hour from your home.

Usable Storage

The percentage of hot water that can be drawn from a tank before the temperature drops to a point that is no longer considered hot

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Recommended for installation on all side (bottom) cold inlet heaters;

Relief Valve

prevents internal vacuum conditions that could drain a system by back siphonage, eliminates burned out electric elements and collapsed tanks

Venting Materials

Materials used for venting; i.e. PVC, CPVC, ABS, metal

Waterway Fitting

A channel through which water can flow; connects water heater to inlet & outlet lines

Water Hammer

A concussion or sound of concussion of moving water against the side of a containing pipe or vessel

Watt Density

Amount of watts concentrated per square inch of element blade surface area

Working Pressure

Maximum pressure of the operating system permissible

Yearly Cost

The cost of operating a water heater for a year

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Elyria, OH 44035
Phone: 440-323-2869
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