Water heater glossary

A–ZTerms

Air-to-water heat pump

A heat pump that moves heat from air into water. In a heat pump water heater it sits on top of the tank and draws heat from the surrounding room.

Aluminum anode

An anode rod made mostly of aluminum, often used where water is hard. It corrodes more slowly than magnesium in many waters.

Anode rod

A sacrificial metal rod screwed into the tank that corrodes so the steel tank does not. Once it is consumed, the tank begins to rust.

Anti-scald valve

A valve that limits the temperature of water delivered to a fixture, or blends hot and cold at the heater, to reduce the risk of scalding.

Atmospheric water heater

A gas tank that vents naturally: hot exhaust rises through a central flue and a draft hood into a chimney or metal vent with no fan.

B-vent

Double-wall metal vent pipe used for natural-draft gas appliances such as atmospheric water heaters.

Backdraft

Exhaust flowing back into the room instead of up the vent, often caused by house depressurization from exhaust fans. Also called spillage.

Backflow preventer

A device that stops water flowing back from a house into the public supply. It can close the system, making an expansion tank necessary.

Baffle

A twisted metal strip inside a gas tank's flue that slows the exhaust so more heat passes into the water.

BTU/h

British thermal units per hour, the unit for a gas burner's input rate. One kilowatt equals about 3,412 BTU/h.

Burner

The ring or plate under a gas tank, or inside a tankless unit, where gas and air burn to heat the water.

Buyout

A payment to end a water heater rental agreement and take ownership of the tank or remove it, usually set by a schedule in the agreement.

Carbon monoxide (CO)

A colourless, odourless, poisonous gas that fuel-burning appliances can produce if combustion or venting goes wrong. Only a CO alarm detects it.

Check valve

A valve that lets water flow one way only. On a supply line it closes the system, so heated water cannot expand back toward the main.

Closed system

Plumbing in which a check valve, backflow preventer or pressure-reducing valve stops expanding water returning to the main, so pressure rises when water heats.

Cold inlet

The connection where cold water enters the tank, usually with a shut-off valve above it on the cold pipe.

Combi system

A combination system that heats both the house and the domestic water from one appliance, such as a combi boiler.

Combustion air

The outdoor or room air a gas burner needs to burn fuel completely. Starved combustion air causes poor flames and CO.

Condensate

Water that forms when exhaust or air is cooled below its dew point. Condensing gas heaters and heat pump water heaters need a drain for it.

Condensing water heater

A gas heater that cools its exhaust enough to condense water vapour and recover the extra heat, venting through plastic pipe.

Delivered temperature

The temperature of water at the tap or showerhead, which can be lower than the tank temperature when mixing valves are used.

Descaling

Removing mineral scale from a heat exchanger or elements, usually by circulating a mild acid solution. Routine for tankless units in hard water.

Dielectric fitting

A fitting that separates dissimilar metals, such as steel and copper, to slow galvanic corrosion at the tank connections.

Dip tube

A plastic tube inside the tank that carries incoming cold water to the bottom so it does not mix with hot water at the outlet.

Direct vent

Sealed combustion: the heater draws combustion air from outside and sends exhaust outside through dedicated pipes, without using room air.

Discharge pipe

The pipe from the T&P valve that carries released water down toward the floor or a drain. It must stay full size and open.

Draft hood

The open fitting on top of an atmospheric gas tank that mixes room air with exhaust to steady the draft into the vent.

Drain pan

A shallow pan under a tank that catches leaks and can be piped to a drain or fitted with a leak sensor.

Drain valve

The tap near the bottom of a tank used to drain it and flush sediment.

ECO (energy cut-off)

A high-limit safety switch that shuts off heating if the water gets dangerously hot. On electric tanks it usually has a red reset button.

Electric element

A metal-sheathed resistance heater that sits in the water of an electric tank. Most tanks have an upper and a lower element.

ENERGY STAR

An efficiency certification that, in Canada, Natural Resources Canada administers. Certified water heaters meet efficiency criteria above the regulated minimum.

Expansion tank

A small tank on the cold line with an air cushion behind a bladder that absorbs the extra volume of heated water in a closed system.

First-hour rating (FHR)

How much hot water a fully heated tank can deliver in one hour, combining stored water and recovery. The right number to size a tank by.

Flammable vapour sensor

A safety device on many gas tanks that shuts the burner down if flammable vapours are detected near the combustion chamber. A tripped sensor can keep the pilot from lighting.

Flue

The tube that carries exhaust out of a gas heater. In a gas tank it runs up through the centre of the water.

Flushing

Draining water through the drain valve to remove sediment from the tank floor.

Gas control valve

The combination valve on a gas tank that holds the thermostat, pilot control and a safety shut-off.

Gas fitter

A tradesperson certified to install and service gas lines and appliances. Who may do gas work is set by each province.

Gigajoule (GJ)

The unit natural gas is commonly billed in. One GJ equals 277.8 kWh of energy.

Glass lining

The porcelain enamel coating on the inside of a steel tank that separates water from steel. Tiny gaps are protected by the anode.

Hard water

Water with a high content of dissolved minerals, mainly calcium and magnesium, which form scale and sediment when heated.

Heat exchanger

The part where heat passes from a flame or refrigerant into water, most often referring to the coil or finned block in a tankless unit.

Heat pump water heater

An electric storage tank with a built-in heat pump that moves heat from the surrounding air into the water, with elements as backup.

Heat trap

A fitting or loop on the hot and cold connections that stops hot water drifting up into the pipes when no tap is open.

Hydrogen sulphide

The gas that smells of rotten eggs. In hot water it usually comes from bacteria reacting with a magnesium anode.

Igniter

An electronic device, hot-surface or spark, that lights a gas burner without a standing pilot.

Indirect water heater

A storage tank heated by a coil connected to a boiler that also heats the house.

Legionella

Bacteria that can grow in warm water and cause Legionnaires' disease when inhaled in mist. Storing water at 60 °C helps control it.

Magnesium anode

The most common anode material. Very protective, but in some waters it contributes to a rotten-egg smell.

Mixing valve

A valve that blends hot and cold water to a set outlet temperature, letting the tank store water at 60 °C while taps deliver about 49 °C.

Modulation

A tankless or condensing burner's ability to vary its firing rate to match demand, rather than switching only on and off.

NOSI

Notice of security interest: a registration on land title for a fixture such as a water heater. Ontario banned them for consumer goods from June 6, 2024.

Orphaned water heater

A gas water heater left alone in a chimney after the furnace that shared it was replaced with a plastic-vented model. The chimney may no longer draft well.

Pilot light

A small standing gas flame that lights the main burner on many gas tanks.

Point-of-use heater

A small tank or tankless unit serving one fixture or a small area, such as a sink or a suite.

Power vent

A gas tank with a fan that pushes exhaust through plastic pipe, usually out a side wall, so it does not need a chimney.

Powered anode

An anode that uses a small electric current instead of sacrificial metal to protect the tank.

Pressure-reducing valve (PRV)

A valve that lowers incoming water pressure. Many PRVs also prevent backflow, which closes the system.

Recirculation pump

A pump that keeps hot water moving through a loop to distant fixtures so hot water arrives faster.

Recovery rate

How much water a heater can reheat per hour by a given temperature rise. Higher heating input means faster recovery.

Scale

Hard mineral deposits, mostly calcium carbonate, that form on hot surfaces such as elements and heat exchangers.

Sediment

Loose minerals and debris that settle on the tank floor, causing rumbling on gas tanks and burying elements on electric ones.

Sediment trap

A short capped pipe on the gas supply just before a gas appliance that catches debris before it reaches the control. Also called a drip leg.

Standby loss

Heat lost from stored water through the tank walls and, on gas tanks, up the flue while the heater is idle.

Stratification

The layering of water by temperature in a tank, hottest at the top and coolest at the bottom.

T&P valve

Temperature and pressure relief valve: opens to release water if tank temperature or pressure gets too high. Never cap or plug it.

Tankless water heater

A heater with no storage tank that fires a burner or elements only while water flows. Also called on-demand.

Temperature rise (ΔT)

How much a heater must warm the water: delivered temperature minus inlet temperature. It drives tankless sizing.

Thermal expansion

The increase in water volume as it heats. In a closed system it raises pressure unless an expansion tank absorbs it.

Thermocouple

A sensor in the pilot flame that generates a small voltage to prove the pilot is lit. A weak one lets the pilot go out.

Thermopile

A group of thermocouples that produces enough voltage to power a gas control without household electricity.

Uniform energy factor (UEF)

The standard efficiency rating for water heaters: the share of energy bought that ends up as useful hot water under a standard test. Higher is better.