In a direct-fired gas water heater, the heat source is a flame burning on the bottom of the water heater. Air enters the combustion chamber, combines with the fuel, and the mixture is ignited. Heat is then transferred to the water through metal surfaces at the bottom of the tank. The vent pipe, an exhaust pipe which carries the by-products of combustion to the outside, also serves as a heat exchanger. This pipe is usually surrounded by the water containment tank, and often contains baffles to slow the escape of these gases through the vent, thus allowing more time for heat to be transferred to the water around the pipe.
Energy is lost in the transfer of heat from the flame to the water, and some heat is lost up the flue.
Oil Water Heaters
A direct-fired oil water heater is similar to a gas water heater. The difference is of course the fuel, which requires a different type of burner. The heat input from the burner is extremely high, making for very fast heat recovery, meaning you can take more showers without running out of hot water. Most of the information presented about gas water heaters applies to oil units. However, there may not be an observation port for the homeowner to view the flame on an oil water heater, and a professional should be hired to inspect the oil burner once a year. Oil water heaters are quite a bit more expensive than gas or electric, but may be cheaper to operate depending on local fuel costs.
Indirect-Fired Tank Water Heater
If you use a hot water boiler to heat your home, your hot water demands may be met by integrating your hot water with your boiler by way of an indirect-fired water heater.
This system consists of a storage tank and a heat exchanger.
The efficiency of an indirect water heater is related to boiler efficiency and how efficiently the heat exchanger transfers heat. While heat exchange efficiencies vary with fluid flow rate and temperature difference, a typical operating efficiency for indirect-fired hot water is about ten percent less than the rated boiler efficiency.
Tankless Water Heaters
On-demand, tankless, or instantaneous water heaters heat water instantly when it is called for, and so there is no need for a storage tank. On-demand water heaters are activated by the flow of water. When there is demand for hot water, a fuel valve opens, igniting the burner. Cold water enters one side of a coil of copper pipe, which then twists through a heat exchanger that is surrounded by flame, and hot water exits the other side of the heat exchanger.
The efficiency of an on-demand water heater is based on the heat transfer efficiency of the heat exchanger and is 75 to 90 percent efficient. One potential problem is pressure drop across the heat exchanger which could produce lower flows when more then one source of hot water is used.
Solar water heaters
use energy from the sun to heat water. Solar water heaters are designed to serve as preheaters for conventional storage or demand water heaters. While the initial cost of a solar water heater is high, it can save a lot of money over the long term. Solar water heaters are much less common than they were during the 1970s and early 1980s when they were supported by tax credits, but the units available today tend to be considerably less expensive and more reliable. At today’s prices, solar water heaters compete very well with electric and propane water heaters on a life-cycle cost basis, though they are still usually more expensive than natural gas.
A heat pump water heater
is a small heat pump that is used to heat water. One type of heat pump water heater removes energy from the air in the room where the unit is located, and uses this energy to heat the water. The vapor-compression cycle is the basis for elevating the temperature and supplying energy to heat the water. The heat pump water heater units presently on the market operate at a Coefficient of Performance of about 2.0 to 2.5, which makes these devices about twice as efficient as ordinary electric water heaters. The removal of heat from the surrounding air makes the selection of the location for installation very important. The heat pump water heater can extract excess heat in a room that is overheated such as a furnace room during the winter. During the summer, the heat pump water heater can cool and dehumidify the room from which heat is extracted.
Some homeowners may be thinking about installing geothermal heat pumps to heat and cool their homes. These pumps draw heat from the ground during the winter and from the indoor air during the summer. To include water heating, you can add a desuperheater to a geothermal heat pump system. A desuperheater is a small, auxiliary heat exchanger that uses superheated gases from the heat pump's compressor to heat water. This hot water then circulates through a pipe to the home's storage water heater tank.
Desuperheaters are also available for demand (tankless or instantaneous) water heaters. In the summer, the desuperheater uses the excess heat that would otherwise be expelled to the ground. Therefore, when the geothermal heat pump runs frequently during the summer, it can heat all of your water. During the fall, winter, and spring—when the desuperheater isn't producing as much excess heat—you'll need to rely more on your storage or demand water heater to heat the water. Some manufacturers also offer triple-function geothermal heat pump systems, which provide heating, cooling, and hot water. They use a separate heat exchanger to meet all of a household's hot water needs.
Tankless Systems
Tankless water heaters operate with a home heating system’s boiler. The water, circulating within the boiler to heat the home, also supplies the home’s hot water needs. This is accomplished through the use of a coil-type heat exchanger immersed in the boiler water within the furnace. When hot water is needed, the cold water passes through this coil and is heated. This heated water is then piped from the boiler directly to its destination.
In northern climates, this type of system is very efficient during the heating season, since boilers operate frequently at that time. There is usually an adequate supply of boiler heat to supply moderate amounts of hot water. When the demand for hot water is high, the boiler may have a poor temperature recovery rate with the possibility of water temperature dropping off from set point.
In warm weather, these systems are less efficient, for the furnace must switch on simply to heat the hot water, even if there is no need to heat the home. This can add heat to the home when it is not needed and add an additional energy demand on the cooling season.
