Electric Heating Systems
Different heating systems use electricity as a source of heat. Each of them employs one or more of the following methods of heat transfer:
1. Radiation
2. Convection
3. Forced air
Radiation is the transmission of heat energy by means of electromagnetic waves (infrared light). In other words, the heat is transferred directly from the surface of the heating element to the people, objects, and furnishings in the room without significantly heating the air.
Convection takes place after the temperature of air is increased by contact with a heated surface. As the air becomes warmer, it becomes lighter and rises. When it touches a cooler object, it gives up some of its heat to that object, becomes cooler and heavier, and sinks, completing the circulation of the air in the space. Now note the difference between radiation and convection. In the radiation method of heat transmission, the heat energy passes through the air to the individual or object in much the same way that the sun transfers its heat to earth through space and through the atmosphere. The air is not used as the medium of heat transfer. The surfaces are warmed, and the air picks up heat from these surfaces (convection), not directly from the infrared radiation passing through it from the heating element.
In the forced-air method of heat transmission, the heated or conditioned air is circulated by motor-driven fans integral to the heating unit or included elsewhere in the system. The heated air is forced directly into the room (as in the case of a wall heater) or through a duct system from a centrally located furnace (central forced-warm-air heating systems).
Some central hydronic systems (i. e., forced-hot-water heating systems) use electric-fired boilers as the heat source. These boilers (Figure 9-1) are compact units consisting of an insulated steel or
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Figure 9-1 Electric-fired boiler. (Courtesy National Better Heating-Cooling Council) |
Cast-iron generator with replaceable immersion heating elements. The expansion tank, circulating pump, valves, and prewired controls are included within the boiler package, and the entire unit is assembled at the factory (Figure 9-2).
An electric-fired boiler is compact, generally having a water capacity of only 2 or 3 gal. Some electric-fired boilers are small enough to be hung on a wall in the home. The small size is possible because water has a much higher heat-carrying capacity than air, and the electric elements are immersed directly in the water. This not only saves space but provides very rapid heating of the water because almost all the heat produced by the elements is transferred directly to the water. The heated water is piped directly to the room convectors, where its stored heat is delivered to the air. Distribution of the water can be easily controlled by zone valves under the control of separate zone thermostats.
The controls for an electric boiler are similar to those found on boilers (e. g., oil — and gas-fired) in other central heating systems. The basic controls are:
1. A low-voltage thermostat
2. Pressure and temperature limit controls
3.
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ADJUSTABLE CONTROL SETS BOILER WATER TEMPERATURE AUTOMATIC AIR VENT BLEEDS OFF TRAPPEDAI |
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PRESET PRESSURE CONTROLS WATER PRESSURE |
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WATER SUPPLY TO RADIATORS |
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ELECTRIC HEATING ELEMENTS |
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DRAIN VALVE |
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THERMOSTAT |
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INDICATIOR LIGHTS SHOW OPERATION OF HEATING ELEMENTS -ELECTRICAL CONTROLS |
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THERMOSTAT CONNECTIONS |
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RETURN LINE |
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Figure 9-2 Interior view of an electric boiler showing the basic components. |
Relay and sequence controls
4. Circulating pump controls
The circulating pump is activated by the low-voltage thermostat. Boiler overheating and pressure buildup is controlled by the boiler high-limit controls and the pressure-temperature relief valve.
More detailed descriptions of electric-fired boilers and their controls are found in other chapters of this book (see, for example, the section on electric-fired boilers in Chapter 15, “Boilers and Boiler Fittings”).
For information about the piping system used in electric hydronic heating, read the appropriate sections in Chapter 7, “Hot-Water Heating Systems” and in Chapters 8 and 9 of Volume 2.
Cooling for an electric hydronic system is generally provided by individual room air conditioners (i. e., through-the-wall units) or by
chilled water pumped from a central refrigeration unit. The latter installation is usually found in multifamily structures, such as apartment buildings.
Central Forced-Warm-Air Heating Systems
Central forced-warm-air heating systems that use electricity to heat or cool the air rely upon the following heat sources:
1. Electric furnaces
2. Duct heaters
3. Heat pumps
Regardless of the heat source, the air is circulated by fans through a duct system to the various rooms. Variations of these ducted air systems are shown in Figure 9-3.
The electric furnace is a complete unit designed for zero clearance and available in several models suitable for either horizontal or vertical installation. The heat is provided by fast-activating,
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Figure 9-3 Various duct systems for electric heating and cooling. (Courtesy Electric Energy Association) |
Coiled-resistance-wire heating elements. When there is more than one coil, the individual coils are energized at intervals in sequence to prevent the type of current overload possible if all were energized simultaneously. The heating coil sequencer is activated and controlled by a low-voltage thermostat mounted on a wall in some convenient location. Overheating of the furnace is prevented by the same type of high-limit control found in other types of furnaces. The warm air is forced through the duct system to outlets in the room by a blower. Evaporator coils can be added to the furnace for cooling. Additional modifications can provide humidity control (humidification and dehumidification), ventilation, and air filtration.
A more detailed description of electric-fired furnaces is found in Chapter 14, “Electric-Fired Furnaces.” For further information on forced-warm-air heating systems, read the appropriate sections in Chapter 6, “Warm-Air Heating Systems.” Controls for this heating system are described in the section Electric Heating and Cooling Controls in this chapter and in Chapter 4 of Volume 2, “Thermostats and Humidistats.”
Duct heaters (Figure 9-4) are factory-assembled units installed in the main (primary) or branching ducts leading from the blower unit. Heat is provided by parallel rows of resistance wire formed in the shape of spirals, which may be sheathed or left open. The blower is housed in a specially constructed and insulated cabinet to which the ducts are connected. This unit functions as the power source for circulating the air through the duct heater and the rest of the system. Cooling can be provided by adding a cooling unit to the blower. Humidity control is possible by first supercooling the air in
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The coil and then reheating it to the desired temperature and moisture content when it passes through the duct heater.
Some duct heaters are designed to be inserted into a portion of the duct through a hole cut in its side and are more or less permanent units. Other duct heaters are assembled at the factory in a portion of duct flanged for easy installation at the site.
Duct heaters may be installed in more than one duct to provide zoned heating. When this is the case, each duct heater is provided with means for independent temperature control. Another common method is to install a single duct heater in the main (primary) duct leading from the blower unit. Controls for either installation are similar to those used on electric-fired furnaces (see earlier paragraphs in this section).
The third type of heat source used in ducted, central forced — warm-air heating systems is the air-to-air or water-to-air heat pump. These are described briefly in another section of this chapter (see Heat Pumps), and in greater detail in Chapter 10 of Volume 3, “Heat Pumps.”
Any electrical conductor that offers resistance to the flow of electricity generates a certain amount of heat; the amount of heat generated is in direct proportion to the degree of resistance. This method of generating heat is employed in radiant heating systems.
The conductor commonly used in radiant heating systems is an electric heating cable embedded in the floors, walls, or ceilings (Figures 9-5 and 9-6). The cables may be installed at the site (as is often the case with new construction), or they may be obtained in the form of prewired, factory-assembled, panel-type units. The heat generated by the cables is transferred to the occupants and surfaces in the room by low-intensity radiation.
Site-installed heating cables or prewired and assembled panel units are used in the following radiant heating systems:
1. Radiant ceiling panel systems
2. Radiant wall panel systems
3. Radiant floor panel systems
Radiant ceiling heating systems are by far the most commonly used type. The other two have certain disadvantages inherent in their construction. All three radiant heating systems are described in greater detail in Chapter 1 of Volume 3, “Radiant Heating.”
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ELECTRIC HEATING CABLES IN CEILING |
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INSULATE ATTIC |
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SEPARATE THERMOSTATS FOR ZONE HEATING |
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ELECTRICAL SUPPLY |
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DISTRIBUTION BOX FOR SERVICE TO HEATING CABLES IN CEILING AND HOT WATER |
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Figure 9-5 |
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ELECTRIC HOT-WATER SYSTEM Radiant heating system installed in ceiling. |
The electric heating cables are activated and controlled by wall — mounted, low-voltage or line thermostats. Cooling can be accomplished only by adding a separate and independent system.
Baseboard heating systems use electric baseboard units located at floor level around the perimeter of each room (Figure 9-7). Each unit consists of a heating element enclosed in a thin metal housing. Zoning is possible within a single wall-mounted line — or low — voltage thermostat located in each room. Heat is provided to the room primarily by convection (some radiation is involved) as the room air moves across the heated elements in the baseboard unit. These baseboard units are described more fully in Chapter 2 of Volume 3, “Radiators, Convectors, and Unit Heaters.”
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Conduit to Power Supply
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As is the case with radiant heating systems, cooling can be provided by installing a separate cooling system (central or room air conditioners). The baseboard heating system represents the most widely used form of electric heating.
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Compartment for: Built In Thermostat if Used Switching Relay if Used Accessories |
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Limit ProtectionLinear Limit or Snap Disc I |
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Heating Element |
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J-Box for Wiring Connections |
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Figure 9-7 Typical electric baseboard heating Unit. (Courtesy Honeywell, Inc.) |
Electric unit ventilators (similar in appearance to electric unit heaters, shown in the next section) are used to heat, ventilate, and cool large spaces that are by nature subject to periods in which there are high densities of occupancy. They are frequently found in offices, schools, auditoriums, and similar structures. The basic components of a typical unit ventilator are:
1. The housing
2. Motor and fans
3. Heating element
4. Dampers
5. Filters
6. Grilles or diffusers
Automatic controls activate the unit ventilator and vary the temperature of the air discharged into the room in accordance with room requirements. Outdoor air is drawn through louvers in the wall and into the unit ventilator before being discharged into the interior of the structure. These ventilators are usually floor — or ceiling-mounted, depending on the design of the room.
In addition to electricity, unit ventilators may also be gas-fired or use steam or hot water as the heat medium. Unit ventilators are described in greater detail in Chapter 2 of Volume 3, “Radiators, Convectors, and Unit Heaters.”
Electric unit heaters (Figure 9-8) are used primarily for heating large spaces, such as offices, garages, warehouses, and similar commercial and industrial structures.
As is the case with unit ventilators, the heat-conveying medium and combustion source may be other than electricity. For example, the air may be heated by either gas-fired or oil-fired units. Steam or hot water can be substituted for air as the heat conveying medium. Selecting electricity as a medium will depend on such factors as:
1. The availability of cheap electrical power
2. The need for supplementary housing
3. The scarcity of other heat-conveying mediums
The typical unit heater contains the following basic components:
1. Fan and motor
2. Heating element
3. Directional outlet
4. Casing or housing
Figure 9-8 Wall — or ceiling — mounted electric unit heater.
Unit heaters may be floor-mounted or suspended from the ceiling, depending upon the design requirements of the structure.
Smaller unit heaters designed and constructed for domestic heating purposes are commonly referred to as space heaters. Like the larger units, they are also designed to operate with other heat-conveying mediums. However, the advantage of using electric space heaters over the fuel-burning types is that they do not have to be vented and require no flue.
Space heaters are available as either portable or permanent types. The latter can be fitted into ducts or mounted on walls or the ceiling.
For further information on space heaters see the appropriate sections in Chapter 2 of Volume 3, “Radiators, Convectors, and Unit Heaters.”
A heat pump (Figures 9-9 and 9-10) is an electrically powered, reversible-cycle refrigeration unit capable of both heating and cooling the interior of a structure.
The heat source is commonly either outside air (in the air-to-air heat pump) or a closed loop of circulating water (in the water-to-air heat pump). The former is the most popular heat pump used for single-family dwellings. The water-to-air heat pump system is most often found in multifamily structures. These are essentially central heating systems, with the heat pump replacing the central furnace or boiler as the heat-generating unit.
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Figure 9-9 Outdoor section of a splitunit heat pump. It is connected to the inside section by refrigerant tubing and Electrical wiring. (Courtesy Electric Energy Association) |
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Figure 9-10 Outside-wall-mounted single-package Heat pump. (Courtesy Electric Energy Association) |
Operating valves in the heat pump unit control the reversal of the refrigeration cycle. It removes heat from the interior of the structure, discharges it outside during hot weather, and supplies heat to the interior spaces during periods of cold weather.
The basic components of a heat pump installation are (1) the compressor, (2) the condenser, (3) the evaporator, and (4) a low- voltage thermostat. These and other aspects of heat pumps are described in greater detail in Chapter 12 of Volume 3.
Electric Heating and Cooling Controls
In an electric heating and cooling system, the best results are obtained by finding a comfortable thermostat setting and leaving it there. Constantly changing the thermostat setting results in consuming more energy (resulting in increased operating costs) and creates additional wear and tear on the equipment. Experts estimate that for each degree the thermostat is raised above the normal setting, there is a 3 percent increase in heating costs. A comfortable setting for the thermostat depends upon a number of variables, including (1) the type of system, (2) environmental conditions, and (3) your personal requirements. For example, 70°F is usually adequate for a radiant heating system (i. e., baseboard, panel-type heating installations, etc.) in a properly insulated structure. A comfortable indoor temperature setting for air conditioning is usually 76 to 78°F.
A variety of control methods are used in electric heating and cooling systems to maintain kilowatt demand at a level both economical in operation and suitable in performance. The basic controls used for these purposes are:
1. Thermostats
2. Sequence switching devices
3. Load-limiting controls
4. Time-delay relays
Thermostats can be located in each room or on each heating unit to provide decentralized control. A manual switch can be provided with each thermostat to shut off the current if the room is to be unoccupied for long periods of time.
Sequence switching devices are used to switch electrical current in sequence (rotation) from one room or circuit to the next. A load — limiting control is designed to shut off the current to one or more circuits when total electric demand exceeds a preset value. Time — delays are used to restore service after an interruption nonsimulta- neously over a period of several minutes so that overloading is avoided. Each of these controls is examined in considerable detail in Chapter 14, “Electric Furnaces.”
Insulation for Electrically Heated and Cooled Structures
Any structure electrically heated and cooled must be properly insulated, or operation costs will be unacceptable. Converting an older structure to electric heat is therefore not recommended unless you have no objection to the additional expense of improving the insulation or you feel you can live with the higher operating costs. Consequently, electric heating is more often considered for new construction.
Insulating a structure that is to be electrically heated and cooled requires greater attention to construction details than do other types of systems; however, the results are well worth the efforts because reduced operating costs and other gains (e. g., the reduction of outside noise and quiet operating characteristics) are soon readily
Apparent. Because electric heating and cooling systems require an especially well-insulated structure for efficient operation, recommendations for the required minimum levels of insulation are available from the Electric Energy Association (e. g., its publication Electric Space Conditioning in Residential Structures).
The recommendations found in the following paragraphs will contain references to U-factors and R-values. The U-factor is the overall coefficient of heat transfer and is expressed in Btu per hour per square foot of surface per degree Fahrenheit difference between air on the inside and air on the outside of a structural section. The R-value is the term used to express thermal resistance of the insulation. The U-factor is the reciprocal of the sum of the thermal resistance values (R-values) of each element of the structural section. Table 9-1 gives the insulation recommendations for electrically heated and cooled structures both in terms of R-values and in terms of U-factors (overall coefficients of heat transfer).
Table 9-2 lists heat loss limits for electrically heated structures as recommended by the Electric Energy Association. The values that are listed are expressed in watts and Btu/h (1 watt = 3.413 Btu/h) per square foot of floor space measured to the exterior walls. The assumed infiltration rate on which this table is based is approximately three-quarters air change per hour.
The maximum summer-heat-gain limits for electrically air conditioned homes are indicated in Figure 9-11. These figures are adapted from the FHA Minimum Property Standards for single — and multifamily structures.
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Table 9-1 Maximum Winter Heat Loss for Electrically Heated Homes
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Table 9-2 Insulation Recommendations (R-Values) and Overall Coefficients of Heat Transfer (U-Factors) for Major Areas of Heat Loss and Heat Gain in Residential Structures
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Courtesy Electric Energy Association
Note: Insulation R-values refer to the resistance of the insulation only.
ASince the thermal resistance of sandwich construction depends upon its composition and thickness, the amount of additional insulation required to obtain the maximum U-factor must be calculated in each case.
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20 25 30 35 40 45 Maximum allowable heat gain (in thousands of Btu/h) Figure 9-11 Maximum summer heat-gain for air conditioned Homes. (Courtesy Electric Energy Association) |
Note
The insulation details, recommendations, and illustrations included in Chapter 3, “Insulating and Ventilating Structures” of this volume meet the minimum requirements for an electrically heated house.
Further and more detailed information about insulation, the problems of heat loss and gain, and methods for calculating heat loss can be found by reading the appropriate sections of Chapter 2, “Heating Fundamentals”; Chapter 3, “Insulating and Ventilating Structures”; and Chapter 4, “Sizing Residential Heating and Air Conditioning Systems.”
Advantages of Electric Heating and Cooling
Among the principal advantages of using electric heating and cooling are:
1. Greater safety
2. Quiet operation
3. Economy of space
4. Reduction of drafts
5. Reduction of outside noise
6. Uniform temperature
7. Structural design flexibility
Although the chances of an explosion in gas — or oil-fired heating systems have become very small because of the safety features built into these systems, an explosion simply cannot occur in total electric heating. A properly installed electric heating and cooling system will last for years without any problems. If a problem occurs (e. g., a defect in the heating unit or wiring), resistance builds up in the line to the fuse or circuit breaker box. At a certain point, the fuse will blow or the circuit breaker will trip, which will automatically shut off the electricity before any damage occurs.
Electric heating units are very compact and therefore utilize very little space. In baseboard systems, there is no need for ducts or pipes to carry a heat medium from its source to the space being heated. These factors offer a great degree of structural design flexibility because duct and pipe arrangements do not have to be taken into consideration. In addition, no chimney or flue arrangement is required.
In a structure properly insulated for electric heating and cooling, there is a marked reduction of drafts and the degree of outside noise penetration. Moreover, uniform temperatures will prevail.
Finally, an electric heating system is generally quieter than other types because fewer mechanical parts are involved. This quietness of operation is particularly characteristic of baseboardtype installations.
Disadvantages of Electric Heating and Cooling
An electrically heated and cooled structure offers certain inherent disadvantages when compared with other types of heating and cooling systems. Of course, this is true of any system regardless of the energy source. When choosing an energy source for a heating and cooling system, it is always necessary to weigh the advantages and disadvantages of the system and its energy source carefully against the requirements you demand from them.
An electrically heated or cooled structure must be well insulated against heat gain or loss. If it isn’t, the cost of heating or cooling it will be extremely high. For that reason, electric systems are seldom installed in existing structures, except in situations where a room is added or a basement is finished.
Electric heating and cooling systems generally have higher operating costs. These energy costs are dependent upon a number of variables, such as the amount of insulation, the orientation of the structure, the number of windows (total glass area), the cost of electricity where the structure is located, and the energy use habits of the occupants.
One disadvantage of electric furnaces is that they are frequently oversized. An oversized electric furnace heats up and cools down too rapidly to maintain acceptable comfort levels in the rooms of a structure. However, oversizing is not a problem limited to electric furnaces. See Chapter 4, “Sizing Residential Heating and Air Conditioning Systems” for additional comments about the problem of oversizing.
Troubleshooting Electric Heating Systems
Table 9-3 suggests remedies for troubleshooting electric heating systems.
Never attempt to service or repair the electric controls inside an electric furnace cabinet unless you have the qualifications and experience to work with electricity. Potentially deadly high-voltage conditions exist inside these furnace cabinets.
Electric Heating Systems 271 Table 9-3 Troubleshooting Electric Heating Systems
Symptom and Possible Causes
No heat
Power may be off. Check fuse or circuit breaker panel for blown fuses or tripped breakers.
Check thermostat (programmable type) for dead batteries.
Not enough heat
Check thermostat setting.
Suggested Remedies
Replace fuses or reset breakers. If the problem repeats itself, call an electrician or an HVAC technician.
Replace batteries and reset thermostat.
Thermostats in electric heating systems must be set several degrees higher than the desired room temperature.
Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers