2. Basic Equipment for Electric Heating Systems
2. Basic Equipment for Electric Heating Systems
In this chapter, the equipment making up different types of electric heating systems will be described.
Equipment for Forced-Air Systems
DESIGN AND OPERATION
When an electric furnace delivers heated air blown by a fan through a network of ducts, it is called a forced-air system. Because the fan is literally forcing air into each room in the house, this type of system does not depend on natural convection to distribute heated air evenly throughout your house.
Forced-air systems come in a wide range of capacities–generally from 10 kW to 50 kW. The heating elements, circulation fan, air filter, and control devices are contained in a compact cabinet. The equipment would be quite different if a heat pump was being used (see page 30).
Figure 2: Central furnace for an electric forced-air system
If electricity is your only energy source, unused chimney flues can be closed off, insulated, and sealed. Closing off the chimney flues can have an effect on drafts and humidity levels in the house, and will reduce heat loss.
To accommodate different types of houses, there are three main furnace designs for use with forced-air systems. The designs are named according to the way air travels through the system.
- Upflow furnaces are recommended for basement floor locations.
Horizontal flow furnaces are particularly suited for crawl space installations. - Downflow furnaces are recommended for installations in mobile homes or on the main floor of houses on concrete slabs.
If you replace your forced-air furnace with a new electrical furnace, you can usually use the existing ductwork with very few modifications.
Electric plenum heaters can be added to forced-air systems to boost capacity or create dual-energy systems. The plenum heater, consisting of one or more heating coils, is placed in the hot air plenum of the heating system (the plenum is that part of the ductwork immediately downstream from the furnace). The furnace circulating fan blows air through these coils on the way to the warm air registers.
NOTE: Any additions or alterations to an existing furnace involving installation of an electric plenum heater must be done by qualified contractors. The furnace must then be inspected, usually by the local electric utility representative.
MAXIMIZING EFFECTIVENESS IN FORCED-AIR HEATING SYSTEMS
There are several ways to improve the performance of an existing forced-air heating system.
Adjusting the Furnace Fan
Heat output from a forced-air system can often be increased by adjusting the controls that turn the fan on and off automatically. The fan controls are usually located in a metal box, often mounted on the front of the furnace, near the top. Inside the box is a temperature dial with three pointers (Figure 3). (To remove the cover, you must either squeeze it or remove some metal screws.) The lowest setting is the fan OFF pointer; the next one is the fan ON setting. The third and highest pointer is the safety limit control that shuts off the electric elements if the furnace gets too hot. The safety limit is normally set at the factory. Do not adjust this safety limit setting.
Figure 3: Circulating fan control
The ON-OFF fan control pointers have usually been set for an ON temperature of 66oC (150oF) and an OFF temperature of 49oC (120oF). To increase the amount of heat taken from the furnace, most heating experts now recommend changing these to an ON temperature of 49oC (120oF) and an OFF temperature of 32oC (90oF). These changes will cause the fan to turn ON sooner after the electric elements come on and will have the fan stay on longer after they turn off. This allows the circulating air to extract more heat from the furnace.
The fan control dial is spring-mounted, so it must be held firmly with one hand, while you adjust the pointer with the other. Make sure the "Auto/manual" switch is set to "Auto" after replacing the cover of the metal box. If you feel uncomfortable or unsure of about modifying the settings, ask your furnace serviceperson to make the setting changes for you during his next service call.
These modified temperature settings may result in slightly lower air temperatures coming from the room registers at the beginning and at the end of the furnace cycle. If the cooler air at either end of the cycle makes you feel uncomfortable, try raising either the fan ON setting to 54oC (130oF) or the fan OFF setting to 38oC (100oF), or try both.
A two-speed fan will allow you to get more heat out of the furnace, while providing for continuous air circulation and more even temperatures throughout the house when the furnace is off. However, it will be at the cost of an increased electricity bill.
Some of the new high-efficiency furnaces use a more efficient, variable-speed direct-drive commutating motor to run the circulating fan. The speed of the fan varies depending on the heat demand. For extended or continuous fan operation, such a unit can save a significant amount on your electricity bill, while making the delivery of heat more even and comfortable.
Getting the Heat Where You Want It
Uneven heat distribution is sometimes a problem, which often results in the inability to heat some rooms in the house, such as upstairs bedrooms. This can be due to leakage of warm air out through joints in the heating ducts, or heat loss from ductwork passing through the basement or, even worse, through unheated areas such as a crawl space, an attic, or a garage.
Seal all joints in the ductwork with a special water-based duct mastic (sealant) to eliminate warm air leaks. Look in the Yellow PagesTM under "Furnaces–Heating" or "Furnaces–Supplies and Parts". (High-temperature duct tape may work, although it tends to degrade or permit air leakage over time).
When the circulating fan is running, heat loss can significantly increase if leaky ducts are located in an exterior wall, an attic or a crawl space and allow the heated air to escape. This is one more good reason to ensure all the ducts are well sealed.
Ducts passing through unheated areas such as a crawl space or an attic should be sealed, then wrapped with batt or duct insulation. The same may be done for long duct runs in the basement. As a minimum, it is recommended that the warm air plenum and at least the first three metres (ten feet) of warm air ducting be insulated. Better still, insulate all the warm air ducts you can access. Use batts of insulation with foil backing or wrap them in insulation between the joists, then add a covering. If your basement is presently heated by the heat loss from the ducts, it may be necessary to have additional registers installed in the basement after you insulate them. This will help to ensure that the heat will go only where you want it, without being lost along the way.
Rooms on upper floors or those far from the furnace are sometimes difficult to heat, because of the heat losses mentioned in the above text, as well as because of friction inside the ducts and other resistances to airflow, such as 90o elbows. This can sometimes be corrected by slight modifications to the ductwork, after the ducts have been well sealed and insulated, and by balancing the airflow in the supply ducts (Figure 4) to redirect the flow of air from the warmer areas to the cooler rooms.
Figure 4: Balancing damper in a supply duct
In some forced-air distribution systems, balancing dampers may be located in the secondary warm air ducts, close to where they branch off from the rectangular main heating duct. Often the dampers can be identified by a small lever on the outside of the duct, as shown in Figure 4. The position of this lever (or sometimes a slot in the end of the damper shaft) indicates the angle of the unseen damper inside the duct. If there are no such dampers, you will have to use the ones in the floor registers. Begin by closing the dampers in the ducts that supply heat to the warmest rooms (even if completely closed, they will probably still supply some heat to these rooms). Wait a few days to see what effect this has on the overall heat balance, then make further adjustments if necessary. Such adjustments may slightly reduce the total airflow through the furnace, but this will to some extent be offset by a slight increase in the temperature of the delivered air.
However, you should be careful. It may be more practical to have an experienced service technician make the adjustments. If you make too large a reduction in the airflow, you could cause an undesirable increase in the temperature of the air inside the furnace plenum. It is a good idea to have your furnace serviceperson check this temperature rise.
Most houses have been designed with inadequate cold air returns. The result is that there is not enough airflow through the furnace. Additional cold air returns in the living areas, particularly in the bedrooms, can improve air circulation and heating system efficiency, while improving comfort and air quality in the house.
For stubborn heat distribution problems that cannot be corrected by damper adjustments and other duct modifications, you should have a qualified serviceperson conduct a thorough check and proper balancing of your distribution system.
Equipment for Electric Hydronic Systems
DESIGN AND OPERATION
Electric hot water or hydronic systems deliver heat to a house by means of hot water. The three main components of such a system are:
| 1. | a boiler to heat the water; |
| 2. |
heating equipment – generally baseboard heaters or radiators – in most rooms, often installed against an outside wall; and |
| 3. |
a pump to circulate water from the boiler to the radiators and ensure that it flows back through the pipes. |
Figure 5: Central boiler for an electric hydronic system
The boiler in an electric hot water heating system is compact. Its heating elements are immersed directly in the water (as in an electric kettle). Where space is limited, the boiler can be installed on a basement wall, in a closet, under a kitchen cabinet–it can even be hung from basement ceiling joists.
If you are replacing a boiler in an existing hydronic system with a new electric boiler, you can probably use the existing heat distribution pipes.
MAXIMIZING EFFECTIVENESS OF HYDRONIC SYSTEMS
As with forced-air furnaces, there are several ways to improve the performance of hydronic heating systems.
Improving Heat Distribution
Old-fashioned gravity systems that circulate the water by natural convection are much less efficient than systems that use pumps. Slow hot water circulation causes home temperatures to fluctuate noticeably, and it takes a long time to restore temperatures after a night-time setback. Also, a gravity system cannot circulate hot water to radiators or baseboard heaters in basement living areas, where they would be below the level of the boiler. All of these problems can be overcome by adding a circulating pump, and replacing the open expansion tank in the attic with a sealed and pressurized expansion tank near the boiler. If you have a gravity system, consult your heating and plumbing contractor about the possibility of improving it.
Balancing the Heat
Balancing the heat delivered to different areas of the house is as important with hydronic heating as it is with a forced-air system. Radiators are often fitted with simple manual valves that can be used to control the amount of water flowing through them. Such valves can be used to vary the heat delivered to different rooms in the same way that balancing dampers are used in a forced-air system.
One device that can vary the heat output automatically is a thermostatic valve (Figure 6) that can be set to control the temperature in any room. However, this will not work on radiators or baseboard heaters installed on what is called a "series loop"system. In a "series loop", the water must pass through all the heating units on its way back to the boiler. If there is more than one loop in the system, some balancing of the heat output can be achieved by adjusting the valves that control the water flow through each loop. The same type of baseboard radiators are equipped with built-in air dampers which allow heat output to be regulated to some extent.
Figure 6: Thermostatic radiator valve
Conventional hydronic systems usually have the boiler temperature set at 82oC (180oF). It is possible to reduce energy consumption in a number of hydronic systems by means of a regulator valve that causes the temperature of the water circulating in the system to vary in relation to the temperature outside. As it becomes warmer outside, the temperature of the water is reduced.
Electric room heaters can be installed in each room and individually controlled. The thermostat controls can be located in the units themselves or be mounted on a nearby wall. All room heaters have built-in controls to prevent overheating if airflow is restricted. A wide variety of moderately priced room heaters are available.
The most common type of room heater is the baseboard heater (Figure 7). These units are installed permanently, preferably under windows, along outside walls in an unobstructed space. They rely on the natural convention of heated air to distribute heat. These units are available in different lengths, suitable to the heating requirements of a room.
Figure 7: An electric-resistance baseboard heater
Electric baseboard heaters consume a lot of electricity. Each baseboard heater normally requires its own dedicated electric circuit. The easy installation of this wiring is a factor in evaluating the cost of the system. In newly constructed structures or buildings, baseboard heating usually has the lowest initial cost compared to other systems, but often has very high operating costs.
Other Types of Electric Heaters
If a baseboard heater installation is difficult, impossible, or expensive other types of electric heaters can also do the job. Portable heaters (Figure 8), whether convection or fan-assisted types, range from 500- to 1500-watt capacity and come in many models, shapes, colours and sizes. They are small enough to plug into regular house circuits and are useful for auxiliary or temporary heating. Make sure, however, that your house wiring can handle the additional electric load.
Portable units should be considered as supplementary units to the existing heating system. They may not be appropriate for wet locations.
Figure 8: A portable electric room heater
Wall convection heaters can be mounted onto a wall or recessed into it (see Figure 9 below).They are suitable as primary or auxiliary heat sources for confined areas such as hallways, entrances, landings and bathrooms. Portable convection heaters, of the same type, are available for auxiliary heating. Some units have small fans to distribute heat more quickly.
Floor insert units (see Figure 10) are designed for use in front of stairways, floor-level windows or sliding glass doors. These units are installed into the floor.
Figure 9: Wall convention heater
Figure 10: Floor insert unit
Other types of electric heaters, such as oil-filled rods, quartz heaters and duct heaters are also available. Check with a local distributor, your local electric utility, or a local contractor for more information. Some products are designed as booster heaters for existing forced-air heating systems. The installation of duct heaters requires a qualified contractor.
"Combination Systems"
Wood-electric combination furnaces (Figure 11) are common in rural areas. These are wood furnaces that contain built-in heating elements that are only activated if the wood furnace cannot meet the heating requirements of the home. Electric baseboard heaters can also be used to supplement a central wood furnace, a wood-oil combination furnace, or a wood stove.
Figure 11: A combination wood-electric furnace
NOTE: An electric plenum heater cannot be added to a forced-air wood furnace.
OIL-ELECTRIC
An oil-electric combination system (see Figure 12 below) consists of an oil furnace with factory-installed electric heating elements. The electric elements supply a large part of the heating requirements, with the oil burner kicking in only during very cold weather.
Figure 12: An oil-electric combination
Another option is to add an electric plenum heater to a forced-air oil heating system. In milder weather, the furnace fan and plenum heater are used to heat the house. During colder weather, higher electricity costs turn oil into a lower-cost heating source. Dual energy rates exist in the province of Quebec to encourage such usage. This rate may also apply when heat pumps are used in conjuction with fuel fired furnaces.
Heat Pumps
Heat pumps produce useful heat by transferring or pumping heat from one place to another. Since it normally takes less energy to transfer heat than to generate it, heat pumps can be very energy efficient.
Although a heat pump is technically similar to a household refrigerator, it can be used for both heating and cooling houses. In the summer, it removes heat from the air inside the house and transfers it outside, much like a conventional air conditioner. In the winter, the heat pump operates in reverse, removing heat from the cold outside air or ground, and transferring it inside the house.
Residential heat pumps are divided into two major groups: air-source (air-to-air) and ground source. Each type will be briefly described here. For more information, refer to a companion booklet in this series entitled Heating and Cooling with a Heat Pump.
AIR-SOURCE HEAT PUMPS
A typical residential air-source heat pump ( Figure 13) resembles a residential central air conditioner. In fact, the only difference between a heat pump and an air conditioner is the heat pump's ability to reverse the flow of refrigerant so that the equipment can provide heating in the winter as well as cooling in the summer. Even cold air contains heat. Because heat is absent only at absolute zero (-273oC), heat pumps can operate even during the coldest Canadian winters.
However, both efficiency and capacity decrease with significantly lower outside temperatures.
Mini-split heat pumps
"Mini-split" heat pumps, which have a small air-handler mounted on an inside wall to supply heating and cooling to a single room, have recently become available. These systems work equally well as a window air conditioner, but are much quieter and have higher efficiencies. However, they can be expensive.
Figure 13: An air-source heat pump during the heating cycle
Because the output of air-source heat pumps declines with decreasing outdoor temperature, the house's heat load increases. Thus, air-source heat pumps are normally equipped with supplementary or auxiliary heating equipment, such as electric plenum heaters, or oil- or gas-fired furnaces to meet the heat load of the house in colder weather.
The actual performance of a heat pump is indicated by The heating seasonal performance factor (HSPF) which is the quotient of amount of heat delivered, divided by the amount of electricity consumed by the heat pump over its period of use during the heating season. The higher the HSPF, the more efficient the heat pump.
GROUND-SOURCE HEAT PUMPS
(EARTH-ENERGY SYSTEMS)
Ground-source heat pumps are different from air-source heat pumps. Heat is extracted from the ground or underground water instead of air. For this reason, ground-source heat pumps have come to be known as "earth-energy systems" (EESs).
Since the temperature of the ground or ground water is much higher than the ambient air in winter and is fairly constant, the output of GSHP does not fall with colder outside temperatures. Thus, in colder climates, EESs are effective.
The New Technologies
In recent years, the heating industry has concentrated more on oil and natural gas heating systems than electric ones. Nevertheless, in addition to ground-source heat pumps there have been some real breakthroughs in electric heating.
ELECTRIC THERMAL STORAGE
Electric Thermal Storage (ETS) heating was developed in Europe in the 1940s, and was introduced to the United States market in the 1980s. This type of space-heating system is capable of providing all of a home's heating requirements by storing heat produced during the night, when utilities generally offer lower off-peak rates. Most of the ETS systems now available can provide 24 hours of on-peak heat from as little as eight hours of off-peak charge.
The ETS central furnace consists of a storage medium (usually called the core) and controls, which detect when it is necessary to accumulate a charge during the off-peak period. Elements within the storage core heat ceramic bricks, crushed rock, or water to a predetermined temperature level to provide the heating requirements for the entire on-peak period. Room storage units are smaller versions of central ETS furnaces. They come in a variety of sizes, from 2.0 to 7.2 kilowatts and supply the heating for individual rooms. Larger rooms in the home may sometimes require more than one storage heater.
These systems can offer savings of up to 30 per cent on heating costs, if there are significantly lower off-peak (night) electricity rates.
| Condensation Problems |
| Electrically heated homes may experience problems of high indoor humidity because of a lack of a chimney, and therefore, have lower rates of air exchange. |
| Heavy condensation on the inside of windows, stains or mould on walls or ceilings are indicators of too much moisture. If this problem is not remedied, serious structural damage can occur. Fortunately, indoor condensation problems can be solved. Because most of the indoor humidity comes from regular household activities, such as showering and cooking, your first step should be to reduce the amount of moisture from these sources. For example, ensure that your dryer vents to the outside, that you cover your pots with lids when cooking, and that you take shorter showers. You should consider installing exhaust fans in the bathroom and kitchen to vent moisture directly outside. Check the setting of the humidifier of your forced-air furnace as well, if you have one. In more air-tight houses, humidifiers are not necessary. As a last resort, you could talk to a contractor about installing a Heat Recovery Ventilator (HRV) that will increase the house's ventilation and decrease its humidity without wasting energy. |
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Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency