Backup Electric Water Heaters

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An electric water heater
An electric water heater
There are a lot of choices in the electric water heater market
There are a lot of choices in the electric water heater market; volume and efficiency are important variables.
A threaded electric water heater element
A threaded electric water heater element, covered in stainless steel so it can be immersed in the tank.
The cutaway of a modern tank-style water heater
The cutaway of a modern tank-style water heater reveals a thick layer of foam insulation, an electric heating element, and the cold water fill tube.
An electric water heater
There are a lot of choices in the electric water heater market
A threaded electric water heater element
The cutaway of a modern tank-style water heater

Heating domestic water can consume up to 25% of a home’s energy budget, so the choice of heater and energy types is important. Conventional fossil-fuel choices include natural gas, propane, fuel oil, and electricity, although electricity can also be derived from renewable sources, such as large- and small-scale hydro-electric generation, geothermal, wind, solar, and biomass.

While natural gas and propane-fueled heaters tend to be more economical choices at present, that may not always be the case as prices increase. Regardless of the conventional energy source used for water heating, you can use a solar thermal system for preheating. And pairing that with an electric backup heater powered by a grid-tied PV system can offer longer-term savings and sustainability.

Tank-Style or Tankless?

Conventionally fueled water heaters are available as either storage-tank versions or tankless (instantaneous or demand) models. Storage-tank heaters are the standard in the United States, while instantaneous heaters are more common in other countries. To some extent, this is due to the prevalence of hard (mineral-rich) water that can scale the small waterways in an instantaneous water heater.

Compared to fossil-fuel-fired tanks, which have an uninsulated vent pipe exiting the middle of the tank top, electric tanks typically have lower standby losses. (The exception is super-high-efficient, condensing gas-fired water heaters that use PVC pipe for a flue.) Lower standby losses combined with the possibility of RE-generated electricity make electric tanks attractive for use as a backup for solar hot water systems.

Instantaneous heaters are more efficient than storage-tank-style heaters since they have no tank to incur standby losses, but the heaters themselves are usually more expensive than common tank types. Plus, their retrofit can be expensive. For gas units, a retrofit can require adding gas piping and venting; for electric units, household electrical service may have to be upgraded.

Electric Water Heating Basics

Electric water heaters use electrical resistance heating. If you run too much electrical current through a wire, it will get hot—glowing hot—just before it melts. The current in all modern home electrical systems is limited by a fuse or circuit breaker to ensure the household wiring doesn’t get overheated and melt. Hot wires can cause devastating fires, but the concept is useful for obtaining usable heat.

Elements in heat-producing appliances are usually made from nichrome—an alloy of nickel and chrome with a high resistivity. Nichrome-wire water-heater elements are covered with copper or stainless steel to protect the nichrome, but still allow the heat to be transferred to the water.

Except in the smaller sizes, electrical water heater elements are rated at 4,500 and 5,500 W. The higher wattage and stainless-steel covered elements are associated with more expensive heaters. What does the extra 1,000 W get you? Quicker recovery time. Residential tank-type electric water heaters with storage capacities greater than 20 gallons have a top and bottom element, but the elements are never on at the same time due to thermostatic controls. A 4,500 W element will heat 40 gallons of 60°F groundwater to 120°F in about 80 minutes. A 5,500 W element heater will recover or heat the same amount in about 65 minutes. For an 80-gallon tank, the recovery time would be doubled since it uses elements of the same size. Many larger heaters just use 5,500 W elements (see “Useful Conversions & Calculations” sidebar).

How Much Hot Water Do You Need?

Do you know how much hot water you use each day? If so, you are in a small minority of the U.S. population. The U.S. Department of Energy estimates that the average American uses 15 to 30 gallons of hot water per day. Water heater manufacturers recommend a 40-gallon water heater for households of three or fewer, while 80-gallon tanks are recommended for families of up to five. Residential water heaters are available up to 120 gallons. Although most households can get by with smaller-than-recommended electric tank-style water heaters, some loss of convenience might be encountered because of the additional recovery time.

Most experts agree that the first-hour rate (FHR) is especially important for tank-style water heaters. If hot water is drawn out of the tank faster than the water heater can heat incoming cold water, the water temperature in the tank will start to decrease.

Heat Loss—A Big Deal or Not?

One disadvantage to storage-tank water heaters is heat loss of the tank to the surrounding air. Fossil-fueled tank heaters have additional loss from the central chimney, which cannot be insulated. But how much energy is lost depends mostly on the tank’s level of insulation. Most modern tanks, which are insulated with expanded polyurethane foam, have an insulation rating of about R-6 per inch, as opposed to the 1 inch of fiberglass insulation (R-3) used 30 years ago. Many modern water heater tanks also have little flapper valves at their inlets and outlets to slow heat lost through the system piping.

Information on tank heat loss is sometimes hard to find, but the calculation is fairly easy if you have a few numbers. You’ll need the surface area of the tank, the temperature difference between the tank and the surrounding air, and the R-value of the tank’s insulation.

For example, on a recent visit to the Web site of a well-known appliance manufacturer, models of electric water heaters with 1, 2, and 3 inches of non-CFC (chlorofluorocarbon) foam insulation were listed. While the insulation value wasn’t given, we’ll assume it was R-6 per inch.

The 40-gallon model with 2 inches of insulation had an outside diameter of 20 inches and a height of 48.25 inches. Subtract the insulation thickness (4 inches from each dimension) and we get a tank measuring 16 by 44.25 inches. Since the tank is a cylinder, use the following formula to calculate surface area:

A = 2πr2 + 2πrh = (2 x π x 82) + (2 x π x 8 x 44.25)

= 402.12 in.2 + 2,224.25 in.2 = 2,626.37 in.2 or 18.24 ft.2

The difference in the tank and air temperature can vary depending on the tank’s location and its thermostat set point: If located inside, water heaters typically experience air temperatures between 60°F and 80°F. An electric water heater thermostat can be set from about 110°F to 140°F.

Example: A 40-gallon water heater with 2 inches of foam insulation is located inside, where the temperature averages 70°F. The heater’s thermostat is set to 120°F.

First, find the difference in temperature between the heater and room air:

120°F - 70°F = 50°F

The 40-gallon tank’s surface area is 18.24 square feet (see above). The insulation is 2 inches of foam with a factor of R-6 per inch, or R-12 total, which translates into 12 hr.·ft2·°F/Btu.

18.24 ft.2 ÷ 12 hr.·ft.2·°F/Btu = 1.52 Btu/hr.·°F x 50°F = 76 Btu/hr. heat loss

The same tank with 1 inch of insulation would be:

18.24 ft.2 ÷ 6 hr.·ft.2·°F/Btu x 50°F = 152 Btu/hr. heat loss

The same tank with 3 inches of insulation:

18.24 ft.2 ÷ 18 hr.·ft.2·°F/Btu x 50°F = 51 Btu/hr. heat loss

Assuming an electricity cost of 10 cents per kWh, a tank with 1 inch of foam insulation will have a daily standby loss of a little more than 1 kWh, or 10 cents. The tank insulated with 2 inches of insulation would have a daily loss of about 0.5 kWh, or 5 cents, and the tank insulated with 3 inches of foam would have daily loss of about 0.35 kWh, or 3.5 cents. Annual costs for standby losses would be about $37 for the 1-inch insulated tank, about $19 for the 2-inch insulated tank, and $13 a year for the 3-inch tank.

If a water heater has a expected life of 20 years, the cost differences of our simple calculations give us a little guidance. Assuming foam insulation with an R-6 per inch and a 40-gallon tank, and the cost of electricity at 10 cents per kWh, the standby loss costs can be estimated for a period of 20 years and will depend on the insulation thickness (see “Standby Loss Costs Over 20 Years” table).

This is a simplified way of calculating the heat loss, but it has some merit for calculating the value of insulation. It can also give some guidance when considering a tankless water heater retrofit. Heat loss is a moving target, since tank temperature, air temperature, and groundwater temperatures change over time. More heat is lost out of the top of the tank than the bottom due to stratification. Higher electricity rates will increase the standby energy savings of a tank with more insulation. And, keep in mind that a larger tank will have more standby loss because the surface area is greater.

A 40-gallon electric water heater with 2 inches of insulation will pay for itself in life-cycle costs if its initial cost is less than $360 more than a tank with 1 inch of insulation (at 10¢ per kWh). Increasing to 3 inches of insulation is not as clear as a money-saver, but it will surely save more energy. As with home insulation, there is a point of diminishing economic return with added tank insulation—2 inches of foam may be close to that line.

Both electric-resistance storage and tankless water heaters present limited individual energy savings potential and are rated by energy factor (EF)—a measurement of water-heater energy efficiency. According to the DOE, the perfect electric-resistance water heater could not exceed an EF of 1.0 due to this technology’s physical limitations. The highest-efficiency, electric tank-style heaters achieve a 0.95 EF and the best electric-resistance tankless water heaters achieve a 0.99 EF.

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Solar thermal editor Chuck Marken is a licensed plumber, electrician, and heating and air-conditioning contractor in New Mexico, installing and servicing solar thermal systems since 1979. He is an instructor for Solar Energy International and the Department of Energy’s Solar America Cities Program, and teaches solar workshops throughout the United States.

Further Reading:

“Solar Hot Water: A Primer,” by Ken Olson, HP84

“Solar Hot Water Simplified,” by John Patterson, HP107

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