2
Draw ef
fi
ciency
is the quantity of hot water available to the
consumer before the outlet water temperature decreases
by 25F° (14C°). A 40 gallon water heater will typically
provide 70% (28 gallons) of this “usable” hot water (60%
is the minimum). The burner or elements are allowed to
operate during this test. Incoming, cold water mixes the
remaining stored water below this 25F° (14C°) limitation.
Energy Factor
is an indicator of the combined thermal
ef
fi
ciency and standby ef
fi
ciency of a water heater. The
higher the energy factor, the more ef
fi
cient the water
heater will be.
Minerals and gases
will separate from water as
temperature increases.
“R” Value
is a measure of the resistance of a substance
to heat
fl
ow.
Recovery rate
is the amount of water that is heated to a
set temperature, per hour.
Standby ef
fi
ciency
is the water heater’s ability to contain
heat in the tank. A minimum of tank water heat loss per
hour is desired. e.g. temperature change/“R” value = Btu/h
loss/square foot of tank surface
Temperature rise
is the increase in the temperature from
its coldest “inlet” water temperature to the desired hot
(outlet) setting. Typically it is assumed that the entering
water be 40°F (5°C), stored water desired to be 120°F
(49°C) resulting in a “temperature rise” of 80F° (44C°).
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.
Water cannot (for all practical purposes) be
compressed.
Water expands when it is heated.
Formulas and Conversions:
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.
One gallon of (120°F, 49°C) water weighs approximately
8.25 pounds.
Pounds X .45359 = Kilogram
Gallons X 3.7854 = Liters
% of Hot = (Mixed Temp. – Cold) divided by (Hot Temp.
– Cold)
% Thermal Ef
fi
ciency = (GPH X 8.25 X Temp. Rise X 1.0)
divided by BTU/H Input
BTU Output = GPH X 8.25 X Temp. Rise X 1.0
GPH = (BTU/H Input X % Eff.) divided by (Temp. Rise X
8.25)
One cubic foot of Natural Gas contains about 1000 BTU
of heat.
One “therm” is equal to 100,000 BTU
One cubic foot of Propane Gas contains about 2500 BTU
of heat.
One gallon of Propane gas contains about 91,250 BTU
of heat.
One pound of Propane gas contains about 21,600 BTU
of heat.
One pound of gas pressure is equal to 27.7 inches water
column pressure
Inches of Water Column X .036091 = PSI
Inches of Water Column X .073483 = Inches of Mercury
(Hg.)
Centimeters = Inches X 2.54
mm (millimeters) = Inches X 25.4
Meters = Inches X .0254
Doubling the diameter of a pipe will increase its
fl
ow
capacity (approximately) 5.3 times.
Construction: Tank is constructed of steel. The inside of the
tank is constructed of a glass lining bonded to the steel.
This prevents water to metal contact and rusting of the
tank. An anode rod will be installed within the tank. The
hex-head plug end of the anode is visible on the top of the
water heater. This metal rod offers secondary protection of
the tank against corrosion where the application of glass
is not possible (threaded tank openings). These areas will
have small areas of water to metal contact.
All water heaters will contain at least one thermostat (to
operate the heater) and one high limit (to prevent water
from overheating).
GENERAL INFORMATION
