RESIDENTIAL ELECTRIC WATER HEATER
SERVICE HANDBOOK
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Technical Training Department
3 of 36
Residential Electric Svc Handbook
Ashland City, Tennessee ©2004
STC082
GENERAL SECTION – cont’d.
TERMS, DEFINITIONS, AND FORMULAS:
Water Expands
when it is heated. Volume increases about 2% from 40° to 140° F.
Minerals and gases
will separate from water as temperature increases
Amperage
(Amps) (1 phase)
= Watts
divided by Volts (Ohm’s Law)
KW
Required
= (GPH X 8.25 X Temp. Rise X 1.0) divided by (3413)
Ohms
= Volts divided by Amperes (Ohm’s Law)
One kilowatt
is equal to 1000 watts
One kilowatt
is equal to 3,413 BTU
Recovery Rate
= (KW X 3413) divided by (Temp. Rise X 8.25)
Rise (F
°
)
= (KW X 3413) divided by (GPH X 8.25)
Supply
electrical fusing
or breakers should be sized at least 125% of expected heater
amperage.
Water weighs
8.25 pounds per gallon at 120
°
F (49
°
C).
% of Hot
water = (Mixed temp. – Cold) divided by (Hot temp. – Cold). This formula
gives the number of gallons drawn from the water heater for each gallon delivered at a
shower head or faucet. If the shower head flow rate (gallons per minute) is known, the
draw efficiency and gallon capacity
of the water heater may be used to calculate the
length of the showering period in minutes.
Watt Density
=
the density of the wattage output of the element compared to the surface area
of the element (i.e. “High Watt Density Element” will have the most wattage per square inch of
element surface
Energy Factor:
Formula: The minimum EF allowed = .97 - .0019 x V (volume of storage)
Example: Volume (50 gal.) x .0019 = .095 gal.x.97 - .095 = .875 (minimum energy factor).
Note:
Since the formula varies by volume, each tank size will have a different minimum
energy factor requirement.
The formula for
gas water heaters
is different than the above example.
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