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Energy Facts: On-Demand Water Heating

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Demand (Tankless) Water Heaters

Water heaters account for 20% or more of the energy used in the average household. The yearly average operating costs for a conventional gas or electric storage tank heater are about $200 or $450, respectively. Part of this cost is due to "standby losses": the heat lost through the walls of the tank-and in gas-fired water heaters-through the flue pipe, when the hot water is not in use.

Tank-type water heaters raise the water temperature to between 120°-140°F (49°-60°C), and maintain that temperature all day by reheating the water when it cools down. The heater does this even if there was no hot water used. These standby losses represent 10% to 20% of a household's annual water heating costs. One way to reduce this expenditure is to use a demand (also called "tankless" or "instantaneous") water heater.

Demand water heaters are widely used in Japan and Europe and have been since 1940. They have appeared in the United States only in the last 20 to 25 years. Unlike conventional tank water heaters, tankless water heaters do not continuously reheat water to a specified temperature. Instead, they heat water only as needed. A tankless unit 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 of the heater, however, limits the rate of the heated water flow.

Gas and Electric Demand Water Heaters

Demand water heaters are available in propane (LP), natural gas, or electric models. They come in a variety of sizes for different applications, such as a whole-house water heater, a hot water source for a remote bathroom or hot tub, or as a boiler to provide hot water for a home heating system. They can also be used as a booster for dishwashers, washing machines, and a solar or wood-fired domestic hot water system.

You may install a demand water heater centrally or at the point of use, depending on the size of the unit desired. For example, you can use a small electric unit as a booster for a remote bathroom or laundry. These are usually installed in a closet or underneath a sink. The largest gas units, which provide all the hot water needs of a household, are installed centrally. Gas-fired models have a higher hot water output than electric models. As with many tank water heaters, even the largest whole house tankless gas models cannot supply enough hot water for simultaneous multiple uses of hot water (i.e., showers and laundry). Large users of hot water, such as the clothes washer and dishwasher, should have separate demand water heaters installed; or the outputs of two or more units could be connected in parallel to meet the demand for hot water. Alternatively, occupants could avoid taking showers when other large water-consuming appliances are in use. Some manufacturers of tankless heaters claim that their product can match the performance of any 40 gallon (151 liter) tank heater.

Selecting a Demand Water Heater

Select a demand water heater based on the maximum amount of hot water needed to meet your peak demand. Use the following assumptions to find the size of unit that is right for your purposes.

To find the peak demand of hot water in your home, assume that faucets draw 0.75 gallons (2.84 liters) to 2.5 gallons (9.46 liters) per minute; low-flow showerheads draw 1.2 gallons (4.54 liters) to 2 gallons (7.57 liters) per minute; older standard shower heads draw 2.5 gallons (9.46 liters) to 3.5 gallons (13.25 liters) per minute; and that clothes washers and dishwashers each draw 1 gallon (3.79 liters) to 2 gallons (7.57 liters) per minute.

Unless you know otherwise, assume that your incoming groundwater 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 you need, subtract the incoming water temperature from the desired output temperature. In this example the needed rise is 70°F (21°C).

List the number of hot water devices you expect to have open at any one time. Add up their associated flow rates. This is the desired flow rate for the demand water heater. Select a manufacturer that makes such a unit. Most demand water heaters are rated for a variety of inlet water temperatures. Choose the model of water heater 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 demand water 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 with demand water heaters.

Some types of tankless water heaters 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. When combining a solar water heater with a tankless water heater, be sure to use a thermostatically-controlled model. In the above example, the whole-house water heater had to raise the water temperature 70°F (21°C), to provide hot water at the rate of 4.5 gallons (17 liters) per minute. If you connect this same unit 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), to provide close to the fullest flow possible from the hot water pipe-about 2.5 gallons (.5 liters) per minute-for 1/2 inch pipe (1.3 cm) and 6 gallons (23 liters) per minute for 3/4 inch (1.9 cm) pipes.

Cost

Demand water heaters cost more than conventional storage tank-type units. Small point-of-use heaters that deliver 1 gallon (3.8 liters) to 2 gallons (7.6 liters) per minute sell for about $200. Larger gas-fired tankless units that deliver 3 gallons (11.4 liters) to 5 gallons (18.9 liters) per minute cost $500-$1000.

The appeal of demand water heaters is not only the elimination of the standby losses and the resulting lower operating costs, but also the fact that the heater delivers hot water continuously. Gas models with a standing (constantly burning) pilot light, however, offset the savings achieved by the elimination of standby losses with the energy consumed by the pilot light. Moreover, much of the heat produced by the pilot light of a tank-type water heater heats the water in the tank; most of this heat is lost in a demand water heater. The exact cost of operating the pilot light will depend on the design of the heater and price of gas, but could range from $12 to $20 per year. Ask the manufacturer of the unit how much gas the pilot light uses for the models you consider. It is a common practice in Europe to turn off the pilot light when the unit is not in use and on again when hot water is desired.

An alternative to the standing pilot light is an intermittent ignition device (IID). This resembles the spark ignition device commonly used in gas kitchen appliances. Not all demand water heaters have this electrically operated device. You should check with the manufacturer for models that have this feature.

Information Source: Controlled Energy Corporation