Water Heating Basics and Facts
For Easier Water Heater Selection
(updated 2017)

Water heating basics and facts helps you understand water heaters and heating for better selection and savings. Find out more info about the Flow Rate, First Hour Delivery, Recovery Rate, efficiency, BTU calculations, energy cost calculator...

Basic water heater definition and terminology

  • Draw efficiency
  • Energy factor
  • First hour delivery
  • Flow rate
  • Inlet temperature
  • Input rating
  • Peak period
  • Recovery efficiency
  • Recovery rate
  • Temperature rise
  • Thermal efficiency and BTU
  • Terminology
  • Formulas, conversions, and link to the energy cost calculator
  • Abbreviations

Draw efficiency

Draw efficiency is the amount of hot water drawn from the tank-type water heater (available to the consumer), at a 3 gallons per minute flow rate and before the outlet water temperature drops 25 F.

In other words; when using a storage type heater for water heating, 70% of the hot water of the tank’s volume may be drawn before hot water dilutes with the incoming cold water. For example, if you are looking to buy a 50-gallon tank-type heater powered by gas, the heater will deliver approximately 35 gallons of hot water (70%); 50 gallons x 0.7 = 35 gallons.

Energy factor - EF

Energy factor (EF) shows the efficiency of a water heater. It combines the thermal efficiency and the standby efficiency of the heater. Higher EF factor means that the heater is more efficient and has a lower energy loss.

Low range EF for tank type gas water heaters is between 0.53 and 0.62. Energy efficiency or energy factor will show you how much energy will be wasted or used to heat the water.

If EF is, let's say, 0.62, from every dollar you spend on water heating $0.62 is being used to heat the water and $0.38 is wasted. High efficient water heaters are usually Energy Star compliant. Minimal EF for one gas tank-type water heater to be Energy Star qualified is 0.67.

First hour delivery (First hour rating)

First Hour Delivery is used to describe the performance capability of the water heater, or how much of hot water a fully heated heater can deliver in the first hour.

Keep in mind that a water heater does not deliver all 100% hot water of the tank's capacity, but 70%, so if you have a water heater with the tank size of:

  • 30-gallon, the available amount of hot water is actually 21 gal.
  • 40-gallon is rated capacity, 28 gallons are available.
  • 50-gallon is the tank's capacity, 35 gallons are for use.
  • 80-gallon is the capacity, 56 gallons are available.

Calculate the First Hour Delivery by using the following formula:

(tank capacity) x 0.7 + (recovery) = FHD

Example: 50 gallons x 0.7 + 36 recovery rate = 71 gallons FHD

Flow rate

Flow rate refers to the maximum hot water flow of all fixtures that might run at the peak time.

In order to calculate the right water heater size, the recommendation is to add up all the flow rates of all the hot-water fixtures and appliances.

Inlet temperature

Incoming water, or the cold inlet water, brings the cool water into the water heater. During the colder months, it is considered to be 40 F, while for southern warmer regions, 50 F.

Input rating

Input rating defines the amount of fuel in BTUs consumed by the water heater, in an hour.

In order to get an equivalent gas input in BTU use the above calculation and divide the result by 0.75.

In order to see what is the heating element wattage equivalent, multiply the above answer by 0.293.

Peak period

Peak period is the term that shows when the highest demand and hot water draw are. In residential water heating, the peak period usually occurs once or twice a day (before school or work and in the evening).

Recovery efficiency

Recovery rate or recovery efficiency refers to the amount of water heated to a set temperature, per hour.

The gas water heater is considered to have 75% recovery rate, which means 75% of the heat produced by the gas burner goes toward heating the water, while 25% is wasted.

With the electric water heaters, equipped with immersion-type heating elements, 99% of the generated heat can heat the water so their recovery efficiency is 99%.

Recovery rate

Recovery rate shows how many gallons of water, per hour, a water heater can raise while the temperature increases by 100 F. When comparing two water heaters, one has the faster recovery rate if it has more BTU or Watts than the other model.

Temperature rise

Temperature rise is the temperature difference between the incoming cold and outgoing hot water and is shown in degrees F or C.

Example: if the temperature of hot water at the faucet is 120 F, and the incoming water temperature is 50 F, the temperature rise is 70 F.

Thermal efficiency and BTU

BTU is short for British Thermal Unit and it represents the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit, for natural and LP water heaters.

Example: 8.25 BTU is needed to raise the temperature of one gallon of water one degree F. Most modern homes today need 20 BTUs per sq.ft. When comparing to electricity, one watt-hour gives 3,413 BTUs.

A formula that is used to calculate required BTUs is:

Gallons x 8.25 x 1.0 x temp. rise = BTU

How much BTU do I need to heat 100 gallons of water to get 90 F hot water, when the incoming water temperature is 50 F? The answer is: 100x8.25x(90-50) = 33000 BTUs.

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Air shutter - Control the amount of primary air intake.

Ampere or AMP - a measure of the flow of electric current and is calculated: Watts divided by the Voltage.

Ambient temperature - refers to the temperature of the air/atmosphere that surrounds the water heater (i.e. the temperature of the room where the unit is installed).

Atmospheric vent - uses the chimney or other vertical -type pipe and natural draft for removing the flue gases out.

Backflow valve or backflow preventer - is the valve that allows water to flow only in one direction.

Carbon monoxide - generated when natural and LP gas are burning. It is dangerous above a certain amount.

Check valve - also known as the non-return valve allows a fluid to flow in only one direction.

Closed water heater system - A system where water, when heated, will not expand (if the following are installed: check valve, back-flow valve, some pressure reducing valves...) The expansion device is usually installed in the closed system to reduce the pressure when it exceeds the normal values.

Coaxial vent - represents the venting system that is designed as the pipe inside the pipe. The bigger or outer pipe is for the air-intake while the inner is for the exhaust, outside the house.

Condensation - it usually occurs when the tank is filled with water for the first time, during heavy water draw and cold water inlet.

Dry firing - refers to the heating element in electric water heaters that is charged but is not fully submerged in the water (in the tank). This causes the element burnout.

Direct venting - it doesn't require an electric fan or external power supply, but it uses the sealed combustion to move the gases out. More about direct venting.

ECO - Energy Cut-Off - a safety element/switch that is built into the thermostats of a heater to protect the unit from overheating.

Flammable Vapor Ignition Resistance (FVIR), used to prevent the accidental burning of the flammable vapors outside the combustion chamber.

Hard water - incoming water has a certain percentage of impurities, calcium and dissolved solids, which can be defined as hardness; from soft to very hard. Hard water is often a problem because the sediments will build up at the bottom of the tank and elements, reducing the efficiency and performance.

Heat traps are the elements that are installed at the water inlet and outlet connections to restrict the heat loss.

Hydronic heating - heating that uses circulated hot water for heating.

Input - the amount of gas or electricity consumed by a water heater, usually expressed in BTU per hour.

Insulation R-Value - explains how well the water heater, for example, is insulated, or how well the insulation will help prevent the heat loss through the wall of the tank. Aim to buy a unit with the higher R-value.

Low-watt vs high-watt density heating elements - low-watt density elements are better as they have a larger surface area to transfer the heat to water so they operate more efficiently and last longer than high-watt density elements.

Mobile home water heaters - heaters designed to meet HUD requirements for manufactured housing and mobile homes.

Non-simultaneous heating - both heating elements in an electric water heater are not permitted to operate at the same time.

Open water heater system - cold water, when heated, increases its volume and pressure and since there are no valves the hot water freely goes from the storage tank into the cold water supply line and further into the municipal water system.

Pilot light - a small flame used to ignite the gas on the main burner. Standing pilot burns all the time.

Power venting - a venting system that uses the electrically operated device such as the blower to force the products of combustion outside while forcing the fresh air in. Read more about power-vent heaters.

Point-of-use heaters - water heaters with the small tank capacity and low water flow, used in low demand applications and supplying hot water mainly to one fixture.

Sealed combustion - a combustion system in gas water heaters where the products of combustion (flue gases) are forced outside while the fresh air for combustion is drawn from the outside.

Scale - a buildup layer of lime, bicarbonate or calcium that is usually found on the bottom of a tank and internal elements, which may prevent heat transfer.

Self-cleaning system - this is a special feature found on the advanced models mostly, that prevents sediment accumulation on the bottom of the tank and units elements, by creating the turbulent water flow inside the tank.

Simultaneous operation - both heating elements in an electric water heater operate at the same time.

Standby heat loss - the heat that is lost from hot water, most of it through the surface of the water tank heater.

Formulas and conversions

BTU to Watts - 1 BTU X 0.293 = watts

BTU to Kwhr- 3,412 BTU equals 1 kilowatt hour (Kwhr)

Fahrenheit to Centigrade - (°F - 32) * 0.556

Centigrade to Fahrenheit - (°C*1.8) + 32

Energy Costs - Kwhr (or cubic feet) x fuel costs

Energy Cost Calculator on energy.gov


A.G.A. - American Gas Association

ASME - American Society of Mechanical Engineers

ANSI - American National Standard Institute

CSA - Canadian Standards Association

GAMA - Gas Appliance Manufacturers Association

ASHRAE - American Society of Heating Refrigeration Air Conditioning Engineers

LEED - Leadership in Energy and Environmental Design is a third-party, nationally accepted standard for green building design.

UL - Underwriters Laboratory

NEC - National Electric Code


Water heating basics and facts article is a great assistant when shopping for a new water heater, installing and troubleshooting one, as you will have better understanding of some frequently seen terms. And with the provided reference links and formulas you will easily find useful info and directions.

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