ELECTRIC FURNACE BASICS
This course is a prerequisite for the course on heat pump defrost
and other controls because all the control voltage and a lot of
junction points originate in an electric furnace, even if no electric
heat is installed which is not uncommon in warmer climates..
Most heat pumps are incorporated into an electric furnace where the
electric heating elements are after the refrigeration coil. Electric
furnaces that were built during the mid 60s to mid 70s may have the
indoor coil (cooling only) on top of them. This was a period of time were electricity
was very inexpensive and thought to be going down in price with the
advent of nuclear power, heat pumps were not very popular until after
this time period. The oil crisis of the 70s changed all that, so much
for the history lesson.
The basic operation of electric furnaces have not changed much in the
last 40 years and is not likely to change anytime soon. An electric
furnace is a box that contains sets of heating elements that are
usually no more than 5,000 watts (5 Kw) each and a blower to force the
air over them. The size (air capacity) of an electric furnace that is
used for comfort cooling is usually chosen at 400CFM ("Cubic Feet per
Minute") per ton (as a
general rule). The electric heating capacity can be stacked in
increments of 5Kw. and sometimes smaller sizes. Most electric clothes
dryers are rated at 5Kw at 240 volts and use heating elements that are
identical to those used in an electric furnace. Many electric furnaces
have model number reflecting
the cooling (blower) capacity in increments of 6 and 12 thousand Btu/hr
(example 36 would be 36,000 Btu/h or 3 tons).
Most manufacturers of HVAC equipment produce an electric furnace with
the option to have a refrigeration coil (indoor coil) ahead of the
heating elements. They can also be added in the duct work in the form
of what will be a duct heater which is a term that mean an electric
furnace that is fitted into the duct work itself.
Above: GE/Trane /American Standard electric furnace heating elements
viewed with blower removed laid on side. This one has 3 sets of elements.
The major difference between different manufacturers is the
configuration and positioning of components. For example one will
control the heating elements with contactors that are identical to
those used to control a compressor and other will use a device called a
sequencer which is a thermal relay. Some will use their own proprietary
circuit boards that have relays on them. The wiring codes and colors
will vary from brand to brand but will be covered in another section.
Above: Images of electric heat sequencers made by Thermodisk. This
particular one was sold for a Carrier electric furnace and has 2 timing
sequences most have just 1 even though they have 2 circuits.
Some manufacturers will have a fuse block with sets of *60 and 30
or *60 and *60 amp fuses. Some will have circuit breakers mounted that
can be accessed from the front of the cabinet. A 5kw heating element
powered by 240 volts draws 5000/240= 20.83 amps and is too much current for a
20 amp circuit so a 30 amp circuit is provided. *2 5kw elements will
draw over 40 amps so a 40 amp circuit will be to small so the next
larger is 50 or 60 amp. Some configurations will have separate circuits
from a main panel and other will have a large circuit like 80 to 100
amps or more feeding a nearby sub panel. More about this later.
When a particular size circuit is mentioned that means the capacity of
the wires supplying it not the circuit breaker or fuse size. For
example a 20 amp circuit will use 12 AWG (American Wire Gauge) wire and
can safely handle 20 amps. For safety margin and convention we will
only load a circuity to 80% of its rated capacity and it is considered
overloaded when the current is at 125% of the circuit capacity. 1.25
and .8 are reciprocals (1/X). So a 20 amp circuit should only be loaded
to 16 amps or 20 X 0.8. = 16 and the circuit breaker that protects it
is expected to trip or the fuse blow at 20 X 1.25 = 25 amps. This
information is useful when trying to understand motor "service factors".
The amount of electric heat that would get installed is a function of
design temperature and building load. The idea is that in the event of
a heat pump failure the electric heat "Emergency Heat" can sustain the
setpoint at design temperature or colder.
When used with a heat pump the electric heating elements are also known
as "auxiliary heat". The idea of auxiliary heat is to make up for low
heat output on a heat pump due to low outside temperatures. The purpose
of auxiliary heat is not to replace the heat pump until the outside
temperature is so low that the "economic balance point" is being
reached. It is the responsibility of the installer or a savvy service
tech to configure the electric heat usage so that on a call for
auxiliary heat only what is necessary to maintain setpoint is used and
not the entire stack (sets of elements), this can be done in many ways
including having the homeowner delete unneeded heating sections with a
circuit breaker or switch.
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Copyright © 2007 Scott Meenen
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