A water heater uses a heating source to raise the temperature of incoming cold water from a municipal main or well. The heated water is stored in a tank and distributed on demand to showers, bathtubs, sinks and other water-using equipment in the home. Several types of water heaters are available:
- Storage tank water heaters
- Tankless water heaters
- Integrated space/sater heating systems
- Solar water heaters
- Heat pump water heaters
Storage tank water heaters are by far the most common type used in Canada. These systems heat and store water in a tank so that hot water is available to the home at any time. As hot water is drawn from the top of the tank, cold water enters the bottom of the tank and is heated. The heating source can be electricity, gas or oil.
More efficient storage tank water heaters can perform as much as 40 percent better than conventional models. An energy-efficient model will typically have one or more of the following features:
- extra tank insulation for better heat retention and less standby loss (loss of heat through the walls of the tank)
- a better heat exchanger to transfer more heat from the energy source to the water
- factory-installed heat traps, which allow water to flow into the tank but prevent unwanted flow of hot water out of the tank
Energy-efficient gas-fired storage tank water heaters may include additional design features, such as:
- electronic ignition, which saves energy by eliminating the need for a continuous pilot light
- powered exhaust
- improved control of flue baffle and flue damper, which reduces heat loss through the flue vent
- condensing heat exchangers, which greatly improve the overall efficiency
- oil-fired water heaters with state-of-the-art burners, which offer high-efficiency performance and minimal stack losses.
These systems (also known as demand or instantaneous water heaters) do not have a storage tank. They heat water only when it is needed, thus avoiding standby heat loss through tank walls and water pipes. The most basic units consist of either an electric element or a gas burner surrounded by flowing water.
Tankless water heaters are usually installed to serve a specific need near the point of use, such as under a kitchen sink. Depending on overall water usage, they may not have the capacity to supply an entire home with hot water. For this reason, they are often used as booster heaters to supplement another water heating system.
A relatively new tankless technology – the low-mass water heater – is capable of supplying much more hot water to the home. These systems are typically gas-fired with electronic ignition and power exhaust. This makes them more efficient than conventional tankless heaters. They can be connected to an external storage tank if necessary.
Integrated space/water heating systems combine the household heating requirement with the household hot water needs, saving money on total system installation. A single boiler is used, requiring only one combustion burner and one vent. Often these systems employ an insulated external storage tank with a high-efficiency low-mass boiler to heat the water, which then passes through a fan coil (as in a car radiator). The system then blows the heat around the house in a warm air distribution system, like a conventional furnace.
For integrated systems that do not use high efficiency boilers, the initial cost saving is soon eliminated by very low seasonal efficiency. The heater is sized to produce enough heat to warm a house on the coldest winter day. However, in the spring, summer and fall, when no heating is required, the same heater heats domestic hot water only. The effect is an oversized water heater that operates for several months of the year with a low heating demand – and low efficiency, as a result.
One type of integrated system that has been around for many years, particularly in the Maritime provinces, is a fuel-fired hot water boiler with a tankless coil water heater that uses a heat exchanger in the boiler to heat tap water but without a separate storage tank. The water flows through a coil inside the boiler whenever a hot water faucet is turned on. The drawback is that this system is dramatically less efficient in warmer months, when space heating is not required, as the boiler water must be kept hot all the time.
Solar water heaters use the sun's energy to heat water. Active solar systems, on the other hand, use pumps and controls to move the heated water from the collector to the storage tank. In areas where the temperature drops below freezing, the fluid in the collectors is usually antifreeze, which is then run through a heat exchanger to heat the household water.
Solar systems can supply up to 50 percent of the energy needed to heat water for an average household (depending on climate conditions and water use). Since energy from the sun is free, solar water heaters can significantly reduce a household's water heating costs – savings that in turn can offset the higher purchase and installation costs of a solar system.
Heat Pump Water Heaters
Heat pump water heater (HPWH) technology uses electricity to move heat from one place to another instead of generating heat directly.
To understand the concept of heat pumps, imagine a refrigerator working in reverse. While a refrigerator removes heat from an enclosed box and expels that heat to the surrounding air, a HPWH takes the heat from surrounding air and transfers it to water in an enclosed tank.
A low-pressure liquid refrigerant is vaporized in the heat pump's evaporator and passed into the compressor. As the pressure of the refrigerant increases, so does its temperature. The heated refrigerant runs through a condenser coil within the storage tank, transferring heat to the water stored there. As the refrigerant delivers its heat to the water, it cools and condenses, and then passes through an expansion valve where the pressure is reduced and the cycle starts over.