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Water Heaters

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Randon Campbell

on 21 April 2010

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Transcript of Water Heaters

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. A water heater consists of the following parts, as shown in the figure above:

* 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 water heater sizing A properly sized water heater will meet your household's hot water needs while operating more efficiently. Therefore, before purchasing a water heater, make sure it's the correct size.
Sizing a demand (tankless or instantaneous) water heater

Demand water heaters are rated by the maximum temperature rise possible at a given flow rate. Therefore, to size a demand water heater, you need to determine the flow rate and the temperature rise you'll need for its application (whole house or a remote application, such as just a bathroom) in your home. First, list the number of hot water devices you expect to use at any one time. Then, add up their flow rates (gallons per minute). This is the desired flow rate you'll want for the demand water heater. To determine temperature rise, subtract the incoming water temperature from the desired output temperature. Unless you know otherwise, assume that the incoming water temperature is 50ºF (10ºC). For most uses, you'll want your water heated to 120ºF (49ºC). In this example, you'd need a demand water heater that produces a temperature rise of 70ºF (39ºC) for most uses. For dishwashers without internal heaters and other such applications, you might want your water heated at 140ºF (60ºC). In that case, you'll need a temperature rise of 90 ºF. Sizing a solar water heating system

Sizing your solar water heating system basically involves determining the total collector area and the storage volume you'll need to meet 90100% of your household's hot water needs during the summer. Solar system contractors use worksheets and computer programs to help determine system requirements and collector sizing. Collector area

Contractors usually follow a guideline of around 20 square feet (2 square meters) of collector area for each of the first two family members. For every additional person, add 8 square feet (0.7 square meters) if you live in the U.S. Sun Belt area or 1214 square feet if you live in the northern United States. Storage volume

A small (50- to 60-gallon) storage tank is usually sufficient for one to two three people. A medium (80-gallon) storage tank works well for three to four people. A large tank is appropriate for four to six people.

For active systems, the size of the solar storage tank increases with the size of the collector typically 1.5 gallons per square foot of collector. This helps prevent the system from overheating when the demand for hot water is low. In very warm, sunny climates, some experts suggest that the ratio should be increased to as much as 2 gallons of storage to 1 square foot of collector area.
3 showers 20 × 3 = 60
1 shave 2 × 1 = 2
1 shampoo 4 × 1 = 4
1 hand dishwashing 4 × 1 = 4
Peak hour demand = 70

To properly size a storage water heater including a heat pump water heater with a tank for your home, use the water heater's first hour rating (FHR). The first hour rating is the amount of hot water in gallons the heater can supply per hour (starting with a tank full of hot water). It depends on the tank capacity, source of heat (burner or element), and the size of the burner or element.

The EnergyGuide Label lists the first hour rating in the top left corner as "Capacity (first hour rating)." The Federal Trade Commission requires an EnergyGuide Label on all new conventional storage water heaters but not on heat pump water heaters. Product literature from a manufacturer may also provide the first hour rating. Look for water heater models with a first hour rating that matches within 1 or 2 gallons of your peak hour demand the daily peak 1-hour hot water demand for your home.

Do the following to estimate your peak hour demand:

* Determine what time of day (morning, noon, evening) you use the most hot water in your home. Keep in mind the number of people living in your home. The energy factor (EF) indicates a water heater's overall energy efficiency based on the amount of hot water produced per unit of fuel consumed over a typical day. This includes the following:

* Recovery efficiency how efficiently the heat from the energy source is transferred to the water
* Standby losses the percentage of heat loss per hour from the stored water compared to the heat content of the water (water heaters with storage tanks)
* Cycling losses the loss of heat as the water circulates through a water heater tank, and/or inlet and outlet pipes. For gas and oil water heaters

You need to know the unit cost of fuel by Btu (British thermal unit) or therm. (1 therm = 100,000 Btu)

365 X 41045/EF X Fuel Cost (Btu) = estimated annual cost of operation


365 X 0.4105/EF X Fuel Cost (therm) = estimated annual cost of operation

Example: A natural gas water heater with an EF of .57 and a fuel cost of $0.00000619/Btu

365 X 41045/.57 X $0.00000619 = $163 Calculating Annual Operating Cost To estimate the annual operating cost of a storage, demand (tankless or instaneous), or heat pump (not geothermal heat pump) water heater, you need to know the following about the model:

* Energy factor (EF)
* Fuel type and cost (your local utility can provide current rates)
Sizing storage and heat pump (with tank) water heaters US Sun Belt Area
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