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RESIDENTIAL GAS AND ELECTRIC WATER HEATER
SERVICE HANDBOOK
GENERAL SECTION
MISCELLANEOUS INFORMATION
Draw efficiency is the quantity of hot water available to the consumer before the outlet water
temperature decreases 25 degrees F. A 40 gallon water heater will typically provide 70% (28
gallons) of this “usable” hot water. The burner or elements are allowed to operate during this test.
Incoming, cold water mixes the remaining stored water below this 25 degree limitation.
Energy Factor is an indicator of the combined thermal efficiency and standby efficiency of a
water heater. The higher the energy factor, the more efficient the water heater will be.
Recovery rate is the amount of water that is heated to a set temperature, per hour. An example
might be that a water heater has a recovery rate of 30 gallons of water per hour at 80 degree F.
(Fahrenheit) temperature rise.
“R” Value is a measure of the resistance of a substance to heat flow.
Thermal efficiency is approximately the amount of generated BTU (British Thermal Units),
which enters the water. A percentage of the total BTU passes out through the vent piping.
Temperature rise
is the increase in the temperature from its coldest “inlet” water temperature to
the desired hot (outlet) setting. Typically this is assumed to be 40 degrees entering water, 120
degrees desired stored water or 80 degrees “temperature rise.”
Standby efficiency – the water heater’s ability to contain heat in the tank. A minimum of tank
water heat loss per hour is desired.
Sample:
temperature change = Btu/h loss/ square foot of tank surface
“R” value
Water cannot (for all practical purposes) be compressed.
Water expands when it is heated.
Water Hammer – is a concussion of moving water against the sides of a containing pipe or vessel
on a sudden stoppage of flow.
EX: 1/2 “copper pipe, 5GPM flow (7.2ft/sec.) – stop. Pressure rise of approximately 412 PSI.
3/4” copper pipe, 5GPM flow (3.3ft/sec) – stop. Pressure rise of approximately 188 PSI
Minerals and gases will separate from water as temperature increases.
Formulas:
BTU (British Thermal Unit)
is the heat required to raise 1 pound of water 1F
1 BTU = 252 cal = 0.252 kcal
1 cal = 4.187 Joules
BTU X 1.055 = Kilo Joules
BTU divided by 3,413 = Kilowatts
To convert from Fahrenheit to Centigrade: (° F – 32) times 5/9, or .556, equals degrees C.
A.O. Smith Water Products Company Technical Training Department
©2005 Ashland City, TN
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