Water Heating

About Water Heating

In the United States, water heating accounted for 9.4% of all residential energy consumed in the in the year 2000.  Virtually every residential unit has a means of heating water for internal use.  Central storage heaters, either electrical or gas-fired, are the norm.  These water heaters are usually specified by capacity, ranging from 19 to 80 gallons.  Size selection is based on the number of occupants in the house.  In 2000, 9.16 million new units were sold 53.5% of them were gas, the 46.5% were electric. 

Construction of a gas-fired water heater is shown below:

The supply of gas is regulated by the gas valve and thermostat control. The thermostat has a dial to allow the homeowner to set the water temperature. The valve shuts the burner on and off as signaled by the thermostat. Above the burner is the bottom of the tank and an exhaust flue for the escape of combustion gases. As the burner heats the bottom of the tank, conduction causes the temperature of the water in the tank to rise. Heat transfer also takes place between the flue and the water in the tank. Convection currents within the tank distribute heat throughout the tank. When the temperature of the water exceeds the set point, the thermostat closes the gas valve.

Cold water enters the tank from the bottom as heated water exits from the top. As a safety precaution, a pressure and temperature relief valve is included on every water heater.

Also included in the water heater is a "sacrificial" anode rod. Its purpose is to inhibit rust on the inside of the tank. Every so often it must be checked and replaced.

Between the tank and the outer shell of the water heater is a layer of fiberglass or foam insulation. The thicker the layer, the more resistant the unit will be to heat loss from the water in the tank to the air outside the unit. Today's water heaters are typically R-6.7. In 2004, DOE requirements will mandate R-20 in all new units sold.

Construction of an electrical unit is shown below.

 

Electric water heaters have two thermostats - one to regulate the temperature at the top of the tank and one for the bottom of the tank. Connected to each thermostat is an electrical heating element. The bottom heating element does most of the work as incoming cold water enters the tank at the bottom. As the water surrounding the bottom heating element increases in temperature, it rises to the top of the tank, from where the outgoing hot water is drawn.

Since no combustion takes place in an electric water heater, there is no need for a flue or exhaust piping. R values of 11.5 are typical.

Service requirements

The expected life of a water heater is 20 years. When a heater fails, it is usually due to leaks in the tank. If the cost of repairing the unit exceeds the cost of a new unit, the homeowner usually decides to replace the unit.

Energy Consumption

The amount of energy consumed by the residential water heater is a function of the hot water usage, the temperature of the incoming water, the temperature setting, the size of the tank, and the amount of insulation around the tank.

While few homes meter the actual gallons of hot water consumed, it is possible to estimate daily or weekly usage through the applications of hot water throughout the home. Typical uses and consumption rates of hot water:

Hand washing - 1 gallons per wash
Showering - 2.5 gallons per minute w/ low flow shower head, 5.0 gallons otherwise
Dishwasher - 14 gallons for full cycle, 10 gallons for short cycle
Laundry - 20 gallons for hot water setting, 10-12 gallons for warm wash, cold rinse

Each water heater sold in the U.S. is required to have an Energy Guide label. The label indicates the amount of energy consumed by the unit annually by a typical household. It also shows a range of energy consumption for all water heaters of comparable size.

For further comparision, Energy Factor (EF) is also marked on the Energy Guide label. This measurement indicates how efficiently a unit converts gas or electricity into hot water. It takes into account heat losses from the tank itself.

For gas water heaters with an R value of 6.7, the EF is typically in the mid-50's, meaning that slightly less than half of the heat value is lost. Most of the heat goes up the flue - the water in the tank simply cannot absorb the heat from the combustion gases quickly enough. Never touch the flue while a gas water heater is operating - you will burn your hand.

Electric water heaters with an R value of 11.5, typically have EF values in the high 80's.

The table below shows minimum current and future EF's as required by the US Dept of Energy. In January 2004, EF's for new gas water heaters sold will increase by five percentage points. For electric water heater's, EF's will increase by four points.

Minimum EF's for Water Heaters Sold in U.S. 

Tank Size

Since April 1991

Starting Jan 2004

(Gallons)

Gas

Electric

Gas

Electric

 

 

 

 

 

20

0.58

0.90

0.63

0.94

30

0.56

0.89

0.61

0.93

40

0.54

0.88

0.59

0.92

50

0.53

0.86

0.58

0.90

60

0.51

0.85

0.56

0.89

70

0.49

0.84

0.54

0.88

80

0.47

0.82

0.52

0.86

 

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