A gas water heater is nearly identical to an electric water heater, except that it does not contain the two heating elements, but instead has a gas burner at the bottom, with the chimney running up through the middle of the tank.
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- A heavy inner steel tank that holds the hot water
Typically, this tank holds 40 to 60 gallons. It has to be able to hold the pressure of a residential water system, which typically runs at 50 to 100 pounds per square inch (psi). The tank is tested to handle 300 psi. The steel tank normally has a bonded glass liner to keep rust out of the water.
- - Insulation surrounding the tank
- - A dip tube to let cold water into the tank
- - A pipe to let hot water out of the tank
- - A thermostat to control the temperature of the water inside the tank (Many electric water heaters have a separate thermostat on each element.)
- - Heating elements to heat the water (These are the thick electric elements similar to those you see inside an electric oven.)
- - A drain valve that allows you to drain the tank to replace the elements or move the tank
- - A pressure relief valve (This is an important safety feature that keeps the tank from exploding.)
- - A sacrificial anode rod to help keep the steel tank from corroding
Now let's see how these all these parts work to provide you with hot water.
Heating the Water
The thermostat controls the temperature of the water inside the tank. Normally you can set the temperature between 120 and 180 degrees Fahrenheit (49 to
82 degrees Celsius). It is generally recommended that you keep the temperature between 120 to 140 degrees F (49 to 60 C) -- especially if there are children living in the house -- to prevent scalding. It also saves energy.
Normally, the thermostat is underneath a cover plate and it has a knob or a screw that you can use to set the temperature.
As cold water comes in, it remains at the bottom of the tank because it is denser than hot water. If you use the hot water faster than the heating elements can heat the incoming cold water, and if you consume all of the hot water that the tank holds, you run out of hot water in the middle of your shower. If this seems to happen too often, it could mean that the bottom heating element in an electric water heater has burned out or that your water heater is too small for your house. Or it could mean that you are taking really, really long showers.
Tankless Water Heater
Tankless heaters have pros and cons, just like everything else. They can supply an endless supply of hot water, and can save energy.
However, they are limited in the amount of hot water that can be produced at one time and they are more expensive to purchase than a conventional storage type water heater.
They also make it take longer for you to get your hot water, since they don't start heating the water until you turn on the faucet. This problem can be solved by using a specialized pump, which in c
ombination with the tankless unit can get your hot water to you at less than half the time it would take running the faucet full blast.
The tankless water heaters also cause an increase in water wastage since you have to let the water run longer to get your hot water. This problem is also solved when using the specialized pumping system. Water conservation is an important advantage to the pumping system. One such system is an on-demand hot water pump. With it you get both water conservation and convenience benefits of faster hot water, and, an un-limited amount of hot water.
Tankless hot water heaters save energy and thus money. Water heating accounts for 20% or more of an average household’s annual energy expenditures. The yearly operating costs for conventional gas or electric storage tank water heaters average $200 or $450, respectively.
Storage tank-type water heaters raise and maintain the water temperature to the temperature setting on the tank (usually between 120° -140° F (49° -60° C). Even if no hot water is drawn from the tank (and cold water enters the tank), the heater will operate periodically to maintain the water temperature.
This is due to "standby losses": the heat conducted and radiated from the walls of the tank—and in gas-fired water heaters—through the flue pipe. These standby losses represent 10% to 20% of a household’s annual water heating costs.
Tankless units are common in Japan and Europe. They began appearing in the United States about 30 years ago. Unlike "conventional" storage tank water heaters, tankless water heaters heat water only as it is used, or on demand. A tankless hot water heater has a heating device that is activated by the flow of water when a hot water valve is opened. Once activated, the heater delivers a constant supply of hot water. The output, however, limits the rate of the heated water flow.
Selecting a Tankless Water Heater
Select a based on the maximum amount of hot water to meet your peak demand. Use the following assumptions on water flow for various appliances to find the size of unit that is right for your purposes:
Faucets: 0.75 gallons (2.84 liters) to 2.5 gallons (9.46 liters) per minute.
Low-flow showerheads: 1.2 gallons (4.54 liters) to 2 gallons (7.57 liters) per minute.
Older standard shower heads: 2.5 gallons (9.46 liters) to 3.5 gallons (13.25 liters) per minute.
Clothes washers and dishwashers: 1 gallon (3.79 liters) to 2 gallons (7.57 liters) per minute.
Unless you know otherwise, assume that the incoming potable water temperature is 50° F (10° C). You will want your water heated to 120° F (49° C) for most uses, or 140° F (60° C) for dishwashers without internal heaters. To determine how much of a temperature rise your need, subtract the incoming water temperature from the desired output temperature. In this example, the needed rise is 70° F (39° C).
List the number of hot water devices you expect to have open at any one time, and add up their flow rates. This is the desired flow rate for the water heater. Select a manufacturer that makes such a unit. Most tankless hot water heaters are rated for a variety of inlet water temperatures. Choose the model that is closest to your needs.
As an example, assume the following conditions: One hot water faucet open with a flow rate of 0.75 gallons (2.84 liters) per minute. One person bathing using a shower head with a flow rate of 2.5 gallons (9.46 liters) per minute. Add the two flow rates together. If the inlet water temperature is 50° F (10° C), the needed flow rate through the heater would need to be no greater than 3.25 gallons (12.3 liters) per minute. Faster flow rates or cooler inlet temperatures will reduce the water temperature at the most distant faucet. Using low-flow showerheads and water-conserving faucets are a good idea.
Some types are thermostatically controlled. They can vary their output temperature according to the water flow rate and the inlet water temperature. This is useful when using a solar water heater for preheating the inlet water. If, using the above example, you connect this same tankless water heater to the outlet of a solar system, it only has to raise the water temperature a few degrees more, if at all, depending on the amount of solar gain that day.
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