Water Heater Glossary
Short, plain-English definitions of the parts, ratings and problems you will hear about when a water heater needs repair or replacement.
This glossary explains the words you will see on quotes, owner's manuals and our guides. Each term links to the page that covers it best. You can also go back to water heater repair in Newark, DE or browse the water heater blog.
A
- Anode rod
A metal rod, usually magnesium or aluminum, that hangs inside a tank water heater. It corrodes on purpose so the steel tank does not. When it is used up, the tank starts to rust.
- Atmospheric (natural-draft) venting
The most common vent for a gas tank heater. Hot exhaust rises up a metal flue and out the chimney with no fan. It needs a proper draft to work safely.
More: Gas water heater repair
B
- Backdrafting
When exhaust from a gas water heater spills back into the room instead of going up the vent. It can bring carbon monoxide into the home and needs a professional check.
More: Gas water heater repair
- BTU (British thermal unit)
A unit of heat. Gas water heaters are rated by burner input in BTUs per hour. A higher BTU rating heats water faster.
- Burner assembly
The gas burner, pilot and thermocouple (or flame sensor) under a gas tank. Rust or soot on the burner can cause weak heat or a pilot that will not stay lit.
More: Gas water heater repair
C
- Check valve
A valve that lets water flow one way only. Many homes have one at the water meter, which creates a closed system and makes an expansion tank important.
More: Why water heaters leak
- Closed water system
A home plumbing system where water cannot flow back to the street because of a check valve or pressure reducing valve. Heated water has nowhere to expand, so pressure rises.
More: Why water heaters leak
- Combustion air
The air a gas burner needs to burn cleanly. A heater closed into a small closet without enough air can make soot and carbon monoxide.
More: Gas water heater repair
- Compressor
The part of a heat pump water heater that pumps refrigerant to move heat from the room air into the water. It makes a low hum, a bit like a dehumidifier.
- Condensate line
A small drain tube that carries water away from a heat pump or condensing water heater. If it clogs, water can pool under the unit.
More: Heat pump water heaters
- Condensing water heater
A high-efficiency gas heater that pulls extra heat out of its exhaust. It makes acidic condensate and vents through plastic pipe.
More: Tankless water heaters
D
- Dedicated circuit (240-volt)
A breaker and wire that feed only one appliance. Most electric and heat pump water heaters need a 240-volt dedicated circuit sized to the unit.
- Descaling
Flushing a mild acid solution through a tankless heater's heat exchanger to dissolve mineral scale. Most makers call for it on a set schedule.
More: Water heater maintenance
- Dip tube
A plastic tube that carries incoming cold water to the bottom of a tank. If it cracks, cold water mixes at the top and showers run lukewarm.
- Direct venting
A sealed vent system that pulls combustion air from outside and sends exhaust outside, often through a side wall. The burner never uses room air.
More: Gas water heater repair
- Drain pan
A shallow pan under a water heater that catches drips. It works best with a pipe to a floor drain, and it matters most for closets, attics and finished basements.
- Drain valve
The spigot near the bottom of a tank used to flush or empty it. Plastic drain valves can drip or break, especially on older tanks.
- Dry firing
Turning on an electric element when the tank is not full of water. The element burns out in seconds, so always refill before restoring power.
E
- Energy factor (EF)
An older efficiency rating for water heaters. It was replaced by the Uniform Energy Factor (UEF), so the two numbers cannot be compared directly.
- ENERGY STAR
The EPA program that labels efficient products. Delaware rebates for heat pump and tankless water heaters usually require an ENERGY STAR model.
- Expansion tank
A small tank on the cold line with an air cushion inside. It absorbs the extra volume of heated water so pressure does not spike in a closed system.
F
- First-hour rating (FHR)
How many gallons of hot water a tank heater can deliver in one busy hour, starting full. It is the best number for sizing a tank to your household.
More: Water heater replacement
- Flame arrestor
A screen under many gas tanks that keeps flames inside the burner chamber. Lint and dust can clog it and starve the burner of air.
- Flame sensor
A rod that confirms a burner flame on tankless and power-vent heaters. A dirty sensor can make the unit shut off and show an error code.
More: Tankless water heaters
- Flushing
Draining water through a tank's drain valve to wash out sediment. A yearly flush can quiet a noisy tank and help it heat better.
G
- Gallons per minute (GPM)
How fast water flows. Tankless heaters are sized by the GPM you need at one time, such as two showers plus a faucet.
More: Tankless water heaters
- Gas control valve
The knob-and-thermostat unit on a gas tank. It sets the water temperature, lights the pilot and shuts off gas if the pilot goes out.
More: Gas water heater repair
H
- Hard water
Water with dissolved calcium and magnesium. Heat makes the minerals settle as sediment in tanks and scale in tankless heat exchangers.
More: Water heater maintenance
- Heat exchanger
The coil or plates inside a tankless heater where burner heat passes into the water. Scale on it lowers flow and can trigger overheat errors.
More: Tankless water heaters
- Heat pump water heater
An electric tank heater with a small heat pump on top. It moves heat from the surrounding air into the water and uses electric elements only as backup. Also called a hybrid water heater.
- Heating element
A metal rod inside an electric tank that heats water when power runs through it. Most tanks have an upper and a lower element.
- High-limit switch (ECO)
A safety cutoff on an electric water heater, usually with a red reset button. It cuts power if the water gets too hot.
L
- Legionella
Bacteria that can grow in warm, still water. Setting a tank too low raises the risk, which is one reason 120°F is a common middle-ground setting.
More: Water heater maintenance
- Low-NOx burner
A gas burner designed to make less nitrogen oxide. Many newer gas tanks use one, sealed behind a combustion chamber with a flame arrestor.
O
- Operating modes
Settings on a heat pump water heater, such as heat pump only, hybrid, high demand and vacation. Hybrid mode is the usual everyday setting.
P
- Permit (plumbing permit)
Approval from the local building office to install or replace a water heater. In New Castle County, the county, the City of Newark or another town issues it, depending on where you live.
More: Water heater replacement
- Pilot light
A small, steady gas flame that lights the main burner on many gas tanks. If it goes out and will not relight, the thermocouple or gas supply is often the cause.
- Point-of-use water heater
A small heater installed near one sink or shower. It cuts the wait for hot water at a far fixture but cannot serve a whole house.
More: Tankless water heaters
- Power venting
A gas water heater vent that uses a fan to push exhaust out through plastic pipe, often a side wall. It needs an outlet nearby to run.
More: Gas water heater repair
- Powered anode rod
An electric anode that uses a tiny current instead of wearing away. It can stop rotten-egg odor and does not need regular replacement.
- Pressure reducing valve (PRV)
A valve near the water meter that lowers street pressure to a safe level for the house, often about 50 to 60 psi. It also makes the system closed.
More: Why water heaters leak
R
- Recirculation pump
A pump that keeps hot water moving to far taps so it arrives faster. It saves water but adds some energy use.
More: Tankless water heaters
- Recovery rate
How many gallons a tank can heat per hour after it runs low. Gas tanks usually recover faster than electric ones.
- Rotten-egg smell (hydrogen sulfide)
A sulfur odor in hot water caused by bacteria reacting with a magnesium anode. An aluminum-zinc or powered anode often fixes it. A sulfur smell near a gas heater can also mean a gas leak, so leave and call the gas company if you are unsure.
S
- Scale (limescale)
A hard mineral coating that forms when hard water is heated. In tankless units it lines the heat exchanger and cuts flow.
More: Water heater maintenance
- Sediment
Minerals and grit that settle at the bottom of a tank. Sediment causes popping or rumbling noises, slows heating and can overheat the tank bottom.
- Shutoff valve
The valve on the cold water pipe that feeds the heater. Closing it stops new water from entering a leaking tank.
- Standby heat loss
Heat that leaks out of a tank while it waits between uses. Tankless heaters avoid it because they store no hot water.
T
- Tankless water heater
A heater that warms water only as it flows, with no storage tank. Also called an on-demand heater. It is sized by flow rate and temperature rise.
More: Tankless water heaters
- Temperature and pressure relief valve (T&P valve)
A safety valve near the top of every tank. It opens to release water if temperature or pressure gets too high. Never cap or plug it.
- Temperature rise
How many degrees a tankless heater must warm incoming water. Cold Delaware winters raise it, which lowers the flow a unit can deliver.
- Thermal expansion
Water grows in volume when heated. In a closed system that extra volume raises pressure, which can make a relief valve drip.
More: Why water heaters leak
- Thermocouple
A heat-sensing probe in the pilot flame. It tells the gas valve the pilot is lit. A worn thermocouple is a top reason a pilot will not stay lit.
- Thermostat
The control that sets tank temperature. Electric tanks have one per element; gas tanks have one built into the gas control valve.
- Thermostatic mixing valve
A valve that blends hot and cold water to a set, safer outlet temperature. It lets you store water hotter while lowering scald risk at the taps.
More: Water heater maintenance
U
- Uniform Energy Factor (UEF)
The current federal efficiency rating for water heaters. Higher is better. Heat pump models often rate above 3, while standard gas tanks rate well under 1.
W
- Water leak detector
A small alarm that sounds or sends a phone alert when it gets wet. Some models also close an automatic shutoff valve.
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