Electric heaters are usually 100% efficient at converting the electricity to heat in the room where they are located.
8.20.1 Electric Baseboard Heat
Electric baseboard heaters are zonal heaters controlled by thermostats within the zone they heat. Electric baseboard heat can help to minimize energy costs, if residents take advantage of the ability to heat by zones.
Baseboard heaters contain electric resistance heating elements encased in metal pipes. These are surrounded by aluminum fins to aid heat transfer. As air within the heater is heated, it rises into the room. This draws cooler air into the bottom of the heater.
• Make sure that the baseboard heater sits at least an inch above the floor to facilitate good air convection.
• Clean fins and remove dust and debris from around and under the baseboard heaters as often as necessary.
• Avoid putting furniture directly against the heaters. To heat properly, there must be space for air convection.
The line-voltage thermostats used with baseboard heaters sometimes don’t provide good comfort. This is because these thermostats allow the temperature in the room to vary by 2°F or more. Newer, more accurate thermostats are available. Programmable thermostats for electric baseboard heat use timers or a resident-activated button that raises the temperature for a time and then automatically returns to the setback temperature. Some baseboard heaters use low-voltage thermostats connected to relays that control baseboard heaters in rooms.
Electric furnaces heat air moved by its fan over several electric-resistance heating elements. Electric furnaces have three to six elements — 3.5 to 7 kW each — that work like the elements in a toaster. The 24-volt thermostat circuit energizes devices called sequencers that bring the 240 volt heating elements on in stages when the thermostat calls for heat. The variable speed fan switches to a higher speed as more elements engage to keep the air temperature stable.
8.20.3 Central Heat-Pump Energy Efficiency
An air-source heat pump is almost identical to an air conditioner, except for a reversing valve that allows refrigerant to follow two different paths, one for heating and one for cooling. Heat pumps move heat with refrigeration rather than converting it from the chemical energy of a fuel.
Like air conditioners, air-source heat pumps are available as centralized units with ducts or as room units. Heat pumps are 1.5 to 3 times more efficient than electric furnaces. Heat pumps can provide competitive comfort and value with combustion furnaces, but they must be installed with great care and planning.
Heat pumps are also equipped with auxiliary electric resistance heat, called strip heat. The energy efficiency of a heat pump depends on how much of the heating load the compressor provides without using the strip heat.
Evaluating Heat Pumps During the Heating Season
Heat pumps should have two-stage thermostats designed for use with heat pumps. The first stage is compressor heating and the second stage is the inefficient strip heat. Evaluating heat pumps in the winter is more difficult than a summer evaluation. Consider these steps to evaluate heat pumps during the winter.
• Look for a temperature rise of around half the outdoor temperature in degrees Fahrenheit.
• Check for operation of strip heat by measuring amperage. Then use the chart shown here to find out if strip heat is operating.
• External static pressure should be 0.5 IWC (125 pascals) or less for older, fixed-speed blowers and less than 0.8 IWC (200 pascals) for variable-speed and two-speed blowers. Lower external static pressure promotes higher airflow.
• Seal supply and return ducts and insulate them after you’ve verified the airflow as adequate. Return ducts should be sealed too.
Most residential central heat pumps are split systems with the indoor coil and air handler indoors and outdoor coil and compressor outdoors. Individual room heat pumps are more efficient since they don’t have ducts, and are factory-charged with refrigerant. The illustrations show features of an energy-efficient heat pump installation.
In the summer, use the same procedures to evaluate central heat pumps as to evaluate central air conditioners, described on page 332.
The illustration shows features of an energy-efficient heat pump installation.
Room heat pumps can provide all or part of the heating and cooling needs for small homes. These one-piece room systems (also known as terminal systems) look like a room air conditioner, but provide heating as well as cooling. They can also provide ventilation air when neither heating nor cooling are required. They mount in a window or through a framed opening in a wall.
Room (or unitary) heat pumps can be a good choice for replacing existing unvented gas space heaters. Their fuel costs may be somewhat higher than gas furnaces, though they are safer and require less maintenance than combustion appliances. Room heat pumps also gain some overall efficiency because they heat a single zone and don’t have the delivery losses associated with central furnaces or central boilers. If they replace electric resistance heat, they consume only one-half to one-third the electricity to produce the same amount of heat.
Room heat pumps draw a substantial electrical load, and may require 240-volt wiring. Provide a dedicated circuit that can supply the equipment’s rated electrical input. Insufficient wiring capacity can result in dangerous overheating, tripped circuit breakers, blown fuses, or motor-damaging voltage drops. In most cases a licensed electrician should confirm that the house wiring is sufficient. Don’t run portable heat pumps or any other appliance with extension cords or plug adapters.
8.20.5 Ductless Minisplit Pumps
Ductless minisplit heat pumps contain an outdoor condenser and one or more indoor fan-coil units that heat or cool the rooms. Mini-split heat pumps are among the most efficient heating and cooling systems available, providing 2-to-4 watt hours of heating or cooling for each watt hour of electricity they use. Specify minisplits heat pumps as replacement HVAC solutions when they are appropriate, for example.
• Homes currently having no ducts.
• Homes with poorly designed or deteriorating ducts outside the thermal boundary or located in inaccessible areas, such as floor cavities.
• Isolated part of a building such as an addition or a bonus room.
• Very well-insulated, airtight, and shaded homes.
• Bedrooms needing cooling in homes with no central air conditioning.
• Masonry buildings being retrofitted to replace obsolete central space-conditioning systems (often steam).