Heating Systems 315
thermal units (Btus). One Btu is the amount of heat required to raise the
temperature of one pound of water one degree Fahrenheit. Heat energy
in the amount of 3.4 Btus per hour is equivalent to one watt of electrical
energy.
Another basic factor to be considered in the study of electric heat-
ing is design temperature difference. This is the difference between inside
and outside temperatures in degrees Fahrenheit. The outside tempera-
ture is considered to be the lowest temperature that is expected to occur
several times a year. The inside temperature is the desired temperature
(thermostat setting).
A factor used in conjunction with design temperature difference is
called degree days. The degree-day factor is used to determine the aver-
age number of degrees that the mean temperature is below 65°F. These
data are averaged over seasonal periods for consideration in insulating
buildings.Importance of Insulation
The insulation of a building is a very important consideration in elec-
tric heating systems. Insulation is used to oppose the escape of heat. The
quality of insulation is expressed by a thermal resistance factor (R). The
total thermal resistance of a building is found by considering the thermal
resistance of the entire structure (wood, concrete, insulation, et cetera).
The inverse of thermal resistance is called the coefficient of heat transfer
(U), which is an expression of the amount of heat flow through an area,
expressed in Btus per square foot per hour per degree Fahrenheit. The fol-
lowing formulas are used in the conversion of either U or R to electrical
units (watts):1
thermal resistance = —————————————
coefficient of heat transfercoefficient of heat transfer
watts = ————————————
3.4
or:
watts = 0.29 × UThe manufacturers of insulation can supply various tables that can
be used to estimate the heat loss that can occur in buildings of various
types of construction. Heat loss occurs particularly through the windows