|
Taking steps to use less hot water and to lower the temperature of the hot water you use reduces the cost of purchasing and running your water heater. Good first steps include installing low-flow showerheads or flow restrictors in showerheads and faucets, insulating your water heater, and insulating any hot water pipes that pass through unheated areas. If you do not have a dishwasher, or your dishwasher is equipped with its own automatic water heater, lower the thermostat on your water heater to 120 ° F.
The most common type of water heating system, the storage unit, is also the least efficient. Twenty to eighty gallons of water are kept at a constant temperature using natural gas, electricity, oil, or propane to provide heating to the water as it is used and/or as heat is lost to the environment. Heat lost through the walls of the tank is known as standby heat loss and occurs whether or not water is being used, resulting in wasteful energy usage. Standby heat loss can be reduced through the use of an insulation jacket around the tank and pipe insulation on the first five feet of the inlet and entire length of outlet water lines from the storage heater. However, energy waste cannot be eliminated in this type of system. The highest efficiency gas-fired storage tank units show recovery efficiencies of 85 percent and Energy Factors as high as 0.72 (reference GAMA directory--see Resources ).
Standby heat losses can be eliminated through the use of demand water heaters. This type of heater is designed to heat water on demand near the point of use instead of storing hot water. This can reduce energy consumption by 20 to 30 percent. These systems are available with either electric heating elements or gas burners; however the gas will be less costly to operate and is preferable from an environmental standpoint. A potential drawback to an electric demand heater is limited flow rate. For a high-power unit, the maximum flow rate may be about 4 gallons per minute (gpm) during the peak of winter in central Texas. However, such a flow rate should be adequate for two simultaneous showers. Additional heaters can be installed in parallel to meet higher demands. Note to Austin Energy customers: Although electric demand water heaters are more efficient than electric storage water heaters they are not allowed in Austin Energy's service area because of the high electric demand while they are operating. The gas units have the advantage of providing greater hot water flow rates than electric models, up to 6 gpm for a single unit. A typical medium-sized gas unit will raise the incoming water temperatures 45 degrees Fahrenheit at a rate of 4 gallons per minute (125,000 BTU input). Incoming water temperatures in Austin range between 50 and 85 degrees Fahrenheit, depending on the time of year. Fifty degrees is a worst-case scenario, and it is not recommended to size a water heater to handle multiple draws at such a temperature. Some units can sense the incoming water temperature, and if the incoming water is already adequately heated these units will not turn on. This is useful for solar backup or circulating systems.
Natural gas demand heaters require a minimum of 10 to 15 psi, are most often wall mounted, and use vertical venting. Limited horizontal venting can also be used. Electronic pilots are now widely available and should be utilized in order to eliminate wasteful energy use. Various freeze protection devices are available and may be necessary for outdoor mounted units. Demand water heaters are relatively small, allowing installation in locations nearby wherever hot water is used.
In situations where electricity is the only option available for water heating, heat pump water heaters are preferable to electric resistance heaters. The same technology that is commonly used to heat/cool living areas can also be used to generate hot water. Heat pump water heaters use electricity to move heat from one location to another instead of direct heating. The unit will extract heat from the air and use it to heat the water in the storage tank. This method is much more efficient than direct heating, however, the initial cost is higher. These units work best when placed in warmer areas, such as furnace rooms or unheated garages. In the winter, they will not work well in temperatures below 40 degrees Fahrenheit. Units can be purchased either in integral units with storage tanks or as a retrofit to an existing storage tank.
Another method of hot water heating utilizes excess heat from an air conditioning unit. The heat that is removed from the living space by the air conditioner is transferred to the hot water. When refrigerant reaches a condenser coil, it is superheated, because its temperature is above its boiling point. A desuperheater removes heat from the refrigerant and brings the temperature down to its boiling point before it enters the condenser coil. This heat is then transferred to the water in the hot water tank through the use of a heat exchanger. Sizing is the most important consideration in the design of this type of system. The water tank should be large enough to accommodate the waste heat from the air conditioner. Systems relying on thermosiphoning should have a tempering valve installed, which can automatically mix cold water when necessary to prevent the hot water from reaching too high a temperature. Systems using pumps and controls can limit the amount of desuperheating when the water is getting too hot; however, this will add cost and additional energy usage.
Benefits from a desuperheater are twofold. On the water heating side, the entire demand for hot water heating can be met through the use of this technology while the air conditioning unit is operating. Thus the cost and energy use to provide hot water is completely eliminated at these times. On the air conditioning side, the lower temperature and pressure of the refrigerant allows the compressor and condenser to expend less energy.
Water heaters should be strategically located near the center of all the points to which it services, or nearest to the points it services most often. The average American household wastes more than 10,000 gallons of water per year with the practice of allowing water to run down the drain while waiting for the hot water to reach the faucet. This waste can be eliminated by the installation of hot water circulating systems. Both passive or active and continuous or demand systems are available. Continuous circulation systems constantly circulate hot water through all hot water pipes, which can have enormous standby heat losses. All passive and some active systems are included in this category. The penalty for this type of system has always been the heat lost through the pipes as the water is circulated. These heat losses cause both the water heater and air conditioner to work overtime in the summer months. For these reasons, continuous circulation systems are not recommended in warmer climates such as Austin's unless controls are used to activate the system during the winter only.
Active circulating systems use an electric pump in the hot water line to circulate hot water through the tank and pipes, making it available at all faucets. Timers and temperature sensors can limit the amount of circulation time, but a new type of system can effectively eliminate losses altogether. Hot water remains contained in the storage tank and is only circulated when needed. Buttons near each tap activate the circulation pump, bringing hot water to all taps before the taps are opened. The cooler water in the lines is sent back to the hot water tank for reheating. A temperature sensor deactivates the pump when a certain temperature is reached. This type of system can be configured to work with either conventionally plumbed houses, houses with dedicated hot-water-return piping, and demand type water heaters.
Graywater heat recovery systems
A low technology system that reclaims the heat from hot water as it travels down the drain can be a cost effective method of improving the overall efficiency of a water heating system. As water flows down a vertical drain pipe it clings to the walls of the pipe and raises the temperature of the pipe wall. This heat is transferred to cold water circulating in a coil of copper tubing wrapped around the drain. The preheated water is then fed into the hot water heater, which reduces the amount of heat the water heater must produce to raise the temperature of the water. In addition to saving energy, this type of system has the added benefit of increasing the capacity of an existing hot water system by as much as three times its current capacity. In homes, a shower drain proves to be the most effective application, while almost any industrial process would benefit from this technology.
A gas combination heating system (combo or hydronic heat) uses the hot water from the water heater to heat the air in a building. The water heater operates like any conventional water heater. When space heat is needed, the thermostat energizes a small pump that circulates hot water from the water heater through a coil in the air handler. The fan in the air handler comes on and blows air over the coil where it absorbs the heat. The warm air (105-110 degrees Fahrenheit) is then circulated through the ductwork and into the building. The hot water loses only 15-25 degrees and is returned to the water heater to be reheated. This technology has been in use for more than 25 years in the United States. Major manufacturers such as Lennox and Rheem, as well as several smaller companies, produce combo heating systems. At least one manufacturer produces a combo heat system with cooling coils which, when coupled with a condensing unit, makes the unit an air conditioner. Combo heating systems are most commonly used on apartments and small homes but can work just as well in large homes and buildings. A combo heating system eliminates the need for a separate furnace. This can save money on the up front costs of installation, as well as space, compared to separate space- and water-heating systems. The air handler and the ductwork needed for heat distribution is the same system used for air conditioning. Though the water heater can be located away from the air handler for convenience, it should be located as close as possible to avoid heat loss. Energy costs are lower for a combo heating system. Since gas combo heaters involve only one cost for two heating jobs, overall energy costs are reduced by more than one-third over electricity. Smaller, but still substantial savings can be expected over using separate gas water heaters and gas furnaces. Maintenance costs are also lower for a combo heating system. Water heaters are fairly simple appliances to maintain and repair and no furnace is required for this system. Sediment build up in the water heater is reduced because of the greater frequency of water circulation. This will increase the life of the water heater.
The combo heating system needs only 120v electrical supply. This can reduce electrical installation costs and the size of the electrical panel. The cost of installing a combo heat system in conjunction with air conditioning is usually only a $300 or $400 more than installing a conventional system. The additional cost will be repaid in two or three years by the energy savings resulting from this type of system. |