8.20   Electric Heat

Electric heaters are usually 100% efficient at converting the elec­tricity to heat in the room where they are located. However, electric heat is inherently inefficient because of the low effi­ciency of fossil-fuel electric power plants, which is why electric­ity is comparatively expensive as a heating option.

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 may not provide good comfort because they allow the room temperature to vary by 2°F or more. Newer, more accurate line-voltage thermostats are available with a positive-off feature that prevents unintentional heating during mild weather.

Programmable thermostats for electric resistance increase and reduce the temperature automatically.

8.20.2   Electric Furnaces

An electric furnace is usually the most expensive way to heat a building because electricity is relatively expensive and because of furnace duct losses.

Electric furnaces heat air moved by its fan over several electric-resistance heating elements. Electric furnaces have two 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 multi-speed fan switches to higher speeds as more elements engage to keep the air tempera­ture stable.

8.20.3   Central Heat-Pump Energy Efficiency

Heat pumps move heat with refrigeration rather than converting it from the chemical energy of a fuel. 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.

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 compete with combustion furnaces for comfort and value, but only with exemplary installation.

Heat pumps contain auxiliary electric resistance heating coils, known as strip heat. The energy efficiency of a heat pump depends on how much of the heating load the compressor pro­vides compared to 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.

Although we can generally evaluate the heat pumps refrigerant charge in the winter, it may be necessary to return in warm weather to more accurately charge the system. This summer ver­ification is required with new heat-pump installations.

Consider these steps to evaluate heat pumps during the winter.

✓     Measure the airflow of the air handler by temperature rise method, flow plate, or flow hood. Heat pumps must have 400-450 CFM per ton.

✓     With only the heat pump operating and the supplemental heat off, look for a temperature rise of approximately 20°F when the outdoor temperature is 32°F. Add or remove 1° of temperature rise for every 3° that the outdoor tempera­ture is over or under 32°F.

✓     Check for strip-heat operation 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 blowers. Lower external static pressure promotes higher airflow.

✓     Seal supply and return ducts and insulate them after you’ve verified the airflow as adequate. Measure airflow again after duct sealing if the ducts were very leaky.

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.

In the summer, use the same procedures to evaluate central heat pumps as to evaluate central air conditioners, described on page 378.

The illustration shows features of an energy-efficient heat pump installation.

8.20.4   Room Heat Pumps

Room heat pumps can provide all or part of the heating and cooling needs for small homes and apartments. 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 the space requires neither heating nor cooling. 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 are somewhat higher than gas space heaters, but they are safer than combustion appliances.

Room heat pumps have an efficiency over central furnaces or central boilers because they heat a single zone and don’t have the delivery losses. If room heat pumps 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 electric 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 operate room heat pumps with exten­sion cords or plug adapters.

8.20.5   Ductless Minisplit Heat 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 consume.

Ducted mini-splits have become popular for retrofitting multi­family buildings. Variable refrigerant flow (VRF) systems employ a computerized valving unit to distribute the correct amount of refrigerant to each terminal unit or head in large buildings.

Consider minisplits heat pumps or VRF systems as replacement HVAC solutions when they are appropriate, for example.

•       Buildings currently having no ducts.

•       Buildings with poorly designed or deteriorating ducts out­side the thermal boundary or located in areas, such as floor cavities and on roofs.

•       Isolated parts of building such as additions or previously unconditioned areas.

•       Very well-insulated, airtight, and shaded buildings.

•       Rooms needing cooling in buildings with no central air conditioning.

•       Masonry buildings, often multifamily, being retrofitted to replace obsolete central space-conditioning systems, for example: steam or obsolete packaged systems.