Glossary
Plain-language definitions of water heater terms, from anode rods and FVIR to UEF and expansion tanks, written and reviewed by the people who install them.
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Flammable Vapor Ignition Resistant (FVIR)
FVIR is a design standard for gas water heaters that keeps flammable vapors near the floor from igniting outside the combustion.
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First-Hour Rating (FHR)
First-hour rating is the number of gallons of hot water a tank water heater can deliver in one hour of use starting…
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Expansion Tank
An expansion tank is a small tank with an air cushion separated from the water by a bladder. It absorbs the extra…
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Direct Vent
A direct vent water heater draws combustion air from outside and exhausts outside through a sealed, usually concentric, vent. It does not…
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Dip Tube
The dip tube is a plastic tube that carries incoming cold water to the bottom of a tank so it does not…
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Condensing Water Heater
A condensing water heater captures extra heat from its exhaust, cooling it enough that water vapor condenses. That recovered heat raises.
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Closed Plumbing System
A closed plumbing system is one where water cannot flow back into the city main because of a pressure-reducing valve, check valve…
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BTU (British Thermal Unit)
A BTU is the amount of heat needed to raise one pound of water by one degree Fahrenheit. Gas water heaters are…
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Atmospheric Vent
An atmospheric vent water heater relies on the natural rise of hot exhaust gases to carry them up a metal flue and…
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Anode Rod
An anode rod is a metal rod inside a tank water heater that corrodes on purpose so the steel tank does not.…
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