Basics and Facts about Water Heating

Basics and Facts about Water Heating
June 28, 2012 Lorelie

Are you recently looking for that perfect gas or electric water heater for your home?  If you are serious enough in getting a gas or electric water heater unit that is of very good quality and is worth every penny you spend for it, then you need to know a few basics and facts about water heaters.  In order to acquire an excellent water heater unit, you need to be familiar with some common terminologies about water heaters.

Flow Rate

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

When you are looking for a water heater that is the right size for your needs, it is recommended that you add up all the flow rates of all the hot-water fixtures and appliances that are present in your home.

Draw Efficiency

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

In simple terms, when you use a storage-type water heater for your water heating needs, about 70% of hot water of the tank’s volume may be drawn prior to the hot water diluting with the incoming cold water.  If in case you are looking for a 50-gallon tank-type water heater, this heater will then deliver approximately 35 gallons of hot water (70%) – resulting to 50 gallons x 0.7 = 35 gallons.

Energy Factor (EF)

The energy factor (EF) is the one that shows the efficiency of a water heater unit.  It combines the standby efficiency and the thermal efficiency of the water heater.  A higher EF means that the water heater is more efficient and has lesser energy loss.

Low-range EF for gas-powered tank-type water heaters is typically between 0.53 and 0.62.   The energy efficiency or energy factor will show you how much energy will be used and/or wasted in order to heat the water.

For instance, if the EF is 0.62, then it means that from every dollar that you spend on water heating, $0.62 is being used to heat the water, and the remaining $0.38 is wasted.  The high-efficient water heater units are generally Energy Star compliant.  The minimal EF for one gas-powered tank-type water heater to be Energy Star qualified is about 0.67.

Recovery Rate

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

Gas-powered water heaters are considered to have 75% recovery rate, meaning that 75% of the heat produced by the gas burner goes toward heating the water, while the remaining 25% is wasted.

Since electric-powered water heaters are equipped with immersion-type heating elements, 99% of heat is produced to heat the water, thus, their recovery rate is 99%.

In simpler terms, recovery rate shows how many gallons of water, per hour, a water heater unit can raise while the temperature increases by 100°F.  A particular water heater unit can be considered to have a faster recovery rate is it has more Watts or BTU than the other units.

Thermal Efficiency and BTU

BTU stands for British Thermal Unit, and it refers to the amount of heat needed to raise the temperature of 1 pound of water 1 degree Fahrenheit.

For instance, 8.25 BTU is needed in order to raise the temperature of 1 gallon of water 1 degree F.  Nowadays, many modern homes require 20 BTUs per square foot.  When comparing to electricity, 1 watt-hour gives about 3,413 BTUs.

The formula below is used to calculate the required BTUs:

Gallons x 8.25 x 1.0 x temperature rise = BTU

Sample Question:  How much BTU is needed in order to heat 100 gallons of water to get 90°F hot water, if the incoming water temperature is 50°F?

Solution: 100 gallons x 8.25 x (90°F – 50°F) = 33,000 BTUs

Input Rating

Input rating refers to the amount of fuel in BTUs that is consumed by the water heater unit in an hour.

To get an equivalent gas input in BTU, use the formula above and divide the result by 0.75 (44,000 BTUs with the above inputs).

To see what is the heating element wattage equivalent, multiply the above answer by 0.293 (9,669 W with the above inputs).

Inlet Temperature

The inlet temperature refers to the temperature of the incoming water (or otherwise known as the cold inlet water) which is the cool water coming into the water heater.  During the colder months, it is considered to be at 40°F; whereas in southern regions where it is warmer, it’s 50°F.

Peak Period

The peak period refers to the time when the highest demand and hot water draw are.  In residential home water heating, the peak period typically occurs once or twice a day (before school or work, and in the evening).

Temperature Rise

The temperature rise refers to the difference in temperature between the incoming cold and outgoing hot water, and is shown in C or F degrees.

For instance, if the hot water temperature at the faucet is 120°F, and the incoming cold water temperature is 50°F, then the temperature rise is 70°F.

First Hour Delivery (or First Hour Rating)

The First Hour Delivery (FHD) or First Hour Rating (FHR) is used to describe the performance capability of the water heater unit, or how much of hot water a fully heated water heater can deliver in its first hour of operation.

Remember that a water heater unit does not deliver all 100% of the hot water of a tank’s capacity, but only 70%.  Thus, if you have a water heater with a tank size of:

  • 30 gallons, then the available amount of hot water is essentially 21 gallons.
  • 40 gallons (which is rated tank’s capacity), then 28 gallons is available for use.
  • 50 gallons (which is the tank’s capacity), then 35 gallons is available for use.
  • 80 gallons (which is the tank’s capacity), then 56 gallons is available for use.

You can easily calculate the First Hour Delivery by using the formula below:

(Tank Capacity) x 0.7 + (Recovery Rate) = FHD

For instance:  50 gallons Tank capacity x 0.7 + 36 Recovery rate = 71 gallons FHD

Glossary of Terms

Closed System – A system wherein water, when it is heated, will not expand (if the following elements are installed such as the back-flow valve, check valve, or some pressure-reducing valves).

Hard Water – incoming cold water that has a particular percentage of impurities such as calcium and other minerals, which can cause water to have a hard characteristic.

Heat Trap – is an element that restricts loss of heat through the water connections to a tank.

Hydronic Heating – is the most pleasant form of central heating.  It operates by re-circulating hot water, heated by a gas-fired boiler or water heater.

Standby Heat Loss – the heat that is lost from hot water, wherein most of it is through the surface of the tank-type water heater.

Pilot Light – a small flame that is used to ignite the gas in the main burner.  The standing pilot burns all the time.

Scale – a buildup layer of lime, calcium or bicarbonates that are typically found at the bottom of a tank and internal elements that may prevent heat transfer.

Simultaneous Heating Operation – both heating elements in an electric-powered water heater operates at the same time.

Non-simultaneous Heating Operation – both heating elements in an electric-powered water heater are not designed to operate at the same time.


The basics and facts about water heaters is a simple yet useful guide when you are looking for a new water heater or you would like to do some DIY installation or troubleshooting.  This will help you to understand better some of the basic terms and calculations used in water heaters, which will be tremendously useful and will help save you time and money should you decide to install or troubleshoot your water heater unit by yourself.

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