Water Heating Basics and Facts for Easier Water Heater Selection

Water heating basics and facts help you understand water heaters and water 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, etc.

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
  • Energy Guide
  • Terminology
  • Formulas, conversions, and links 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. This is known as 70% rule.

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

An Energy Factor – EF, or Uniform Energy Factor – UEF shows the efficiency of a water heater. It combines the thermal efficiency and the standby efficiency of the heater. A figher EF factor means that the heater is more efficient and has a lower energy loss.

The 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. The 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 a water heater, or how much 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 peak time.

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 around 50 F.

Input Rating

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

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 a 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

The 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 a faster recovery rate if it has more BTU or kWs 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.

For example, if the hot water temperature 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

So, how much BTU do I need to heat 100 gallons of water to get the temperature of 90 F when the incoming water temperature is 50 F?

The answer is: 100×8.25x(90-50) = 33000 BTUs.


The big yellow EnergyGuide label, found on all residential water heaters sold in the US, allows you to compare different brands and models using the following information:

  • First hour rating (FHR) – Indicates how much hot water you will get from the model. It includes both storage capacity and recovery (in gallons).
  • Estimated energy consumption in a range from the least to the most energy use for similar models.
  • Estimated yearly operating costs (depending on the local utility rates and usage)


Air shutter – controls the amount of primary air intake.

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

Ambient temperature – refers to the air/atmosphere temperature surrounding a 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 to remove the flue gases.

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 level.

Check valve – also known as the non-return valve, allows fluids 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 average 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 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 doesn’t require an electric fan or external power supply, but it uses sealed combustion to move the gases out. More about direct venting.

ECO – Energy Cut-Off – a safety element/switch 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 efficiency and performance.

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

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

Gas input – the amount of gas 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 good the insulation will help prevent the heat loss through the wall of the tank. Aim to buy a unit with a 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 are 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 is a small flame used to ignite the gas on the main burner. Standing pilot burns all the time.

Power venting is a venting system that uses an 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 are water heaters with a small tank capacity and low water flow, used in low-demand applications and supplying hot water mainly to one fixture.

Sealed combustion is 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 is 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 unique feature found on the advanced models mostly and it prevents sediment accumulation at the bottom of the tank and 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 is 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

ohms resistance (ohms) = voltage / amps

amp draw = watts / voltage

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 better understand some frequently seen terms. Moreover, with the provided reference links and formulas, you can quickly find useful info and directions.

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