Electric Domestic Water Heaters
Electric Water Heater
Electric water heaters supply hot water for household use in many Canadian homes. An electric water heater usually consists of a tank, thermostats, two electric resistance elements (which are submerged inside the tank), and inlet and outlet pipes for cold and hot water. Internal thermostats regulate the temperature of the water.
Tanks are typically covered with foam insulation and lined on the inside with a ceramic glass layer. When cold water replaces the water withdrawn from the tank and the temperature of the water falls below a certain level, the elements are activated, reheating the water to the correct temperature. Essentially, electric hot water heaters are large closed electric kettles.
The submerged electric resistance heating elements in water heaters are very efficient, providing about 99 per cent of the available heat to the surrounding water. Even so, older water heaters lose heat as a result of standby losses.
Reducing Energy Losses
Most direct heat loss from water heaters is made up of:
- losses by heat conducted through the tank walls and base
- losses by hot water convection through the hot and cold water feed pipes
The operating efficiency of a domestic hot water system can be improved significantly by designing the system carefully and selecting equipment that generates hot water more efficiently and reduces stack and standby losses. For starters, you might want to lower the temperature of the water in the tank.
Note: In the past, tap water was usually set to 60°C (140°F), and homeowners were told to lower the set temperature to prevent scalding. However, a set temperature of 60°C (140°F) is required for the reduction of legionella bacteria. If you wish to protect children and others in your home from high water temperatures, use a mixing valve or add some other sort of scald protection downstream of the water heater.
Reducing Standby Losses
Standby loss is heat lost to the surrounding air from the water in a domestic water heater and connecting pipes. It is a function of:
- the temperature difference between the water and the surrounding air
- the surface area of the tank
- the amount of insulation encasing the tank
To reduce standby losses, consider the following options:
- Cover the tank with an insulating blanket. Caution: It is extremely important not to insulate over any controls or obstruct the vent connections or combustion air openings. The insulation should not come in contact with the vent connector.
- Install a heat trap above the water heater. This is a simple piping arrangement that prevents hot water from rising up in the pipes, thereby minimizing the potential for this loss.
- Insulate hot water pipes to reduce heat loss from the pipes themselves. Pipe insulation is available in a variety of materials and thicknesses, with easy application to most hot water pipes. Use insulation with an RSI (insulation value) of at least 0.35 (R-4) over as much of the pipe as you can easily access.
- Place the water heater over a layer of rigid thermal insulation to reduce heat loss through the bottom of the tank. This is particularly applicable to electric water heaters and external storage tanks for integrated space and water-heating systems.
- Buy a tank with bottom inlet of cold water
Before carrying out any of these steps, check with your local installer or electricity utility representative to ensure that you will not compromise the safety or operation of the appliance.
