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|HVAC & Water
Water Heating > Sizing & Powering
|The right water heater is one that is both appropriately sized for the household's hot water needs and maximally energy efficient. Given that water heating makes up about 25% of the monthly utility bill, any energy savings that can be achieved are going to make a meaningful difference with respect to on-going expenditures. The "larger is better" mentality definitely does not apply in this case.|
In selecting a water heater system, it is important to not only choose between a storage tank model and a tankless model, but also to decide on the energy type used to operate the system. The specific type of energy utilized by a given water heater will not only affect operating costs, but also may have an indirect bearing on the unit’s size, upkeep, and energy efficiency. Not every option will be equally cost-efficient, or equally practical.
Typically, for consumers who are replacing an old electric water heater with a new model, the least costly option is to purchase another electric unit, as this eliminates the need to run gas lines and make other adjustments. However, if the hot water usage in the household is extensive and the residents may be willing to pay a higher upfront cost in return for lower utility bills down the line, other options should not be discounted.
Even though natural gas is not as efficient in terms of energy yield at the point of use as electricity, it can actually be less expensive for two reasons. First, natural gas prices are substantially lower than electricity prices in many parts of the country. Second, the efficiencies of natural gas water heaters have steadily increased to the point where the latest models lose a relatively small amount of the available energy to combustion and venting.
Natural gas can be used to power both storage tank and tankless water heating systems. In addition, a benefit of natural gas water heaters is that they are not beholden to the electrical grid. In case of a power outage or a prolonged blackout, a natural gas water heater can be expected to continue providing hot water service throughout the home.
A close facsimile of and an alternative to natural gas is propane, which can also be used to power both storage tank and tankless water heaters. Propane has more than twice the amount of energy per cubic foot than natural gas, which means that less than half of the amount of propane is required to produce the same amount of energy as a given quantity of natural gas. Unfortunately, this greater energy capacity is more than offset by propane’s significantly higher price.
When price is taken into consideration, natural gas is still a less expensive fuel source than propane. In addition, propane-fired water heaters are much less common, which means less selection and a reduced level of support from local service providers. However, if propane is more readily available and relatively less costly in a particular area, then a propane-fired water heater may be an attractive option.
Some storage tank water heaters run on fuel oil. Such units are far less common than either electric or natural gas water heaters, but they are out there. The problem with fuel oil is that it is both more expensive and less energy-efficient than natural gas, which means that, from an energy perspective, consumers are paying more to get less. Moreover, fuel oil is less widely available in most parts of the country than natural gas. Taken together, that presents three strikes against fuel oil as an energy source.
Yet another alternative for storage tank water heaters is geothermal energy, which refers to energy transferred from the ground to the home via a geothermal heat pump. This will only apply to households which already have a geothermal heat pump installed. While a geothermal heat pump is one of the most energy efficient heating and cooling options available, it is also one of the most expensive to purchase and install, running between $10,000 and $25,000.
In scenarios where there is already a geothermal heat pump in place, that unit can be used to power the water heater by combining a desuperheater with a retrofitted storage tank water heater. Alternatively, if a new geothermal heat pump is being put in, the residents may want to consider a combination system that provides heating, cooling, and water heating within a single setup.
The last alternative, which is becoming an increasingly mainstream option, is solar heat. Energy captured from the sun’s rays is the most energy-efficient and environmentally friendly alternative. A number of readers may think that they live in areas which are too cold for solar, but that is generally not the case. In fact, the climate in most areas of the country can support some type of solar system.
Unlike any other energy source, the sun’s radiant energy is free, abundant, and does not require piping or delivery of either electricity or fuel. Typically, a solar-based water heating system is comprised of a storage tank which is connected to a set of flat solar collector plates which are installed outside or atop the house. In direct-gain systems, the water circulates through the panels and is heated directly via the sun’s rays. This is a more efficient setup than a system where the sun’s heat is collected and then transferred to the water indirectly via a heat exchanger.
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Any process for generating heat requires the use of energy, whether in the form of electricity or in the form of fuel. In the case of water heaters, a number of different energy sources may be available, as described in the previous section. Energy efficiency refers to how effectively that energy is then converted to water heating by the system.
The energy efficiency associated with a water heater can vary greatly by type, brand, and model. The term used to describe how efficiently energy is transferred from the system to the water is “recovery efficiency”. Generally, electric heaters tend to be more efficient, providing recovery efficiencies that are as high as 98%. However, when the energy losses of the power station that actually produced the electricity are taken into account, electricity can have one of the largest environmental footprints and its overall efficiency, meaning power station efficiency combined with at home usage efficiency, is one of the lowest of any alternative.
Gas-powered water heaters have a maximal recovery efficiency of around 86%, as a proportion of the energy is inevitably lost to venting. However, older and less efficient systems can have recovery efficiencies as low as 50% which, of course, requires a significantly greater usage of gas in order to produce the same level of performance.
In addition to recovery efficiency, a storage tank water heater can also be characterized by its standby heat losses, described above, and by its cycling heat losses, which refers to the heat lost as the water circulates through the tank, including the inlet and outlet pipes. Taken all together, these heat characteristics are described by a number known as the energy factor (EF). The EF is calculated by dividing the amount of useful energy coming out of the water heater by the amount of energy going into the water heater.
Gas water heaters typically have EFs between 0.5 and 0.7, while electric water heaters typically have EFs between 0.75 and 0.95. However, as explained earlier, even though an electric water heater may have a significantly higher EF, it may still be less efficient and more expensive when the power station’s operation and the costs of electricity are taken into consideration. In any event, it is recommended that a gas water heater have an EF of at least 0.6 and an electric water heater have an EF of at least 0.9 in order to maximize energy savings.
In addition to the EF rating, the federal government has a program in place to help consumers identify the most efficient systems. This program is called ENERGY STAR and it has a two-pronged purpose: “(1) to reduce greenhouse gas emissions and other pollutants caused by the inefficient use of energy; and (2) to make it easy for consumers to identify and purchase energy-efficient products that offer savings on energy bills without sacrificing performance, features, and comfort.”
The ENERGY STAR program is run by the Environmental Protection Agency (EPA). The agency identifies specific products that it deems to be energy efficient and then assigns an ENERGY STAR label to those products. In order to qualify for the ENERGY STAR label, a product must be cost-effective, provide significant energy savings, and deliver the features and performance demanded by consumers. Before the label can be assigned, each product is tested to ensure that it not only works as stipulated, but also delivers the requisite level of energy efficiency.
In addition to identifying products which meet ENERGY STAR’s guidelines, the program also provides tax rebates for consumers who purchase energy efficient water heating systems. These rebates allow consumers to recoup a portion of the purchase costs of a new or upgraded water heating system. In order to determine whether a particular product qualifies and also to calculate the amount of the possible rebate, consumers should visit the ENERGY STAR web site at www.energystar.gov.
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An even more important issue with respect to water heater selection than energy efficiency is unit sizing. A properly sized water heater will ensure that all of the household’s hot water needs are met at all hours of the day or night while, at the same time, not overshooting the mark and heating a far larger volume of water than necessary. For example, a storage tank water heater that is too large for the residence will consume far more energy than necessary and run up utility bills, no matter how efficient the unit.
The methodology used to calculate the appropriate size of a water heater varies depending on water heater type. There are three basic calculations which may be used, with one for storage tank water heaters, one for tankless water heaters, and one for solar water heaters.
For storage tank water heaters, the calculation relates the hot water needs of a household to a number known as the First Hour Rating (FHR). This value refers to the amount of hot water, in gallons, that the heater can supply per hour, assuming that it is starting with a full tank. Every storage tank water heater has an FHR rating.
In order to apply the FHR rating for a particular heater to the household, the residents need to approximate the peak hour demand for hot water in their home. This approximation is not especially difficult and the following short-hand assumptions can be used: 30 gallons each for a washing machine run; 15 gallons each for a shower or bath; 12 gallons each for a dishwasher run; and 4 gallons each for a shaving, a hand washing, a face washing, a shampooing session, a cooking session, or a dish-washing by hand. The residents should then take each of these values and multiply them by the maximum number of times that they may occur in a single hour within the household. Then, the resulting values are simply added together to arrive at the peak hour demand number for hot water.
Consider the following illustrative example: in a particular home, there lives a family of four consisting of a father, a mother, a teenage son, and a teenage daughter. In the morning, the son and the daughter may both be taking showers in separate bathrooms while the father shaves and the mother prepares food and runs the dishwasher. The calculation for this peak hour load would be: 2 showers multiplied by 15 gallons, which equals 30 gallons; plus 1 shave multiplied by 4 gallons, which brings it to 34 gallons total; plus 1 cooking session multiplied by 4 gallons and 1 dishwasher run multiplied by 12 gallons, which brings the grand total to 50 gallons all together. Thus, the peak hour demand for this household of four would be 50 gallons. This means that the appropriate FHR for a storage tank water heater to service this household would be in the 48 to 52 range. A substantially lower FHR could lead to insufficient hot water availability, while a substantially higher FHR would lead to an unnecessary waste of energy as the water heater would constantly be heating a higher volume of water than was ever being used.
For a tankless water heater, the calculation is different as there is no storage tank whose volume needs to be properly sized. Instead, there are two values which are important: gallons per minute (GPM), which is the flow rate which the tankless water heater can facilitate; and temperature rise (degrees Fahrenheit) which the number of degrees by which the unit is able to heat the water from its starting temperature point of around 50 degrees.
Let us consider the flow rate first. To calculate the minimum flow rate which a tankless water heater needs to support, residents need to determine the maximum number of hot water devices which they may need to run at the same time, and then simply add together the flow rates of each device. Typical flow rates are as follows: 3 GPM for a bath tub, 2 GPM for a shower, 1 GPM for a kitchen faucet, and 2 GPM for a dishwasher.
Using the example of the household of four above, there are five devices being operated during the peak hour: two shower heads, a bathroom faucet, a kitchen faucet, and a dishwasher. That comes to 2 GPM for each of the shower heads, which sums to 4 GPM; another 1 GPM for each of the two faucets, which brings the sum to 6 GPM; and, finally, 2 GPM for the dishwasher, which comes to a grand total of 8 GPM. Thus, the residents of this household would want a tankless heater that can support a flow rate of 8 GPM at the desired level of heating.
With respect to the desired level of heating, the residents would need to figure out the necessary temperature rise. This is simply the difference between the desired temperature of the hot water and the regular temperature of the water at the point of entry into the home. Since most municipal water is delivered at around 50 degrees and the typical hot water requirement is around 112 degrees, the requisite temperature rise will be approximately 62 degrees. That means the household in the example would want a tankless water heater that can support around 60 degrees of heating at 7 GPM to 9 GPM.
In the case of a solar water heater, there may be yet another set of calculations which should be taken into consideration. The majority of solar water heaters utilize a storage tank and therefore an approximation of the FHR value can be utilized as described above for storage tank water heaters. However, an additional consideration is the area of solar collector plates necessary to support the desired water heating output.
The greater the collector area of solar panels, the more energy is able to be gathered by the system. A workable rule of thumb is 10 square feet of collector area for each person in the household. Using our example household of four, the residents would want a 40 square foot collector area available should they opt to install a solar water heater.
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