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.

Ge/ Trane American standard electric furnace heating elements

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.



HOME
Copyright © 2007 Scott Meenen
All rights reserved