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Water Heaters and Energy Conservation

Home Energy Magazine Online May/June 1996

At the dump, the authors hold anode rods whose sacrificial metals have been completely eroded away. Most of the now-defunct water heaters visible behind them could have been salvaged had their owners replaced the anode rods in a timely manner.

Water Heaters and Energy Conservation- Choices, Choices!

by Larry and Suzanne Weingarten

"I think I should replace my water heater. What's the most energy-efficient heater I can buy?" "What brand is the best?" "What heater features should I look for?" We get these questions all the time. The only quick answer is one of hydronics wizard Dan Holohan's favorite replies: "It depends."

The most useful answers can be given if the questions tie into the broader context of getting the most from the whole water-heating system. People already know they should look for energy efficiency from the heater itself. But additional savings can be found in water distribution, equipment sizing and selection, and maintenance.

First, find out what's in place already. Look at the heater, distribution piping, fixtures, and appliances, and determine how hot water is used in the house. Try to ascertain the residents' ability and willingness to maintain the equipment. If you see much deferred maintenance when you look over the system, don't recommend high-tech equipment that needs to be programmed, balanced, descaled, and oiled.

During the assessment, find out if there are any complaints about performance, such as hot water delivery time, temperature fluctuations, or amount of hot water. This could be an opportunity to solve those problems, resulting in greater satisfaction with energy conservation measures. And remember always to keep safety in mind. Water heaters still blow up and cause fires, and people continue to get scalded by hot water and poisoned by carbon monoxide.

When you've determined what's already there, consider the following information and select the options for water heating and distribution that best suit the situation.

A cut-away view of a typical gas-fired water heater.

Is a New Water Heater Needed?

If the old heater is leaking, replace it. That's a straightforward decision. But if it's not leaking, what criteria will help you decide whether or not to keep it? Age alone is not the deciding factor. In fact, older tanks often have the advantage of being more sturdily built.

But tanks do deteriorate with age. You will need to inspect the water heater to determine if it is a good candidate to keep and maintain. A tank's age is usually encoded in its serial number. If it begins A-83 or 0183, the tank was built in January 1983. B-83 and 0283 mean February 1983, and so on. If it begins 8301, the tank was built in the first week of 1983; 8352 would mean the last week of that year. Knowing the tank's age is a start.

Inspect all tank fittings to get more information. Do you see any signs of rusting or leakage? Look into the combustion chamber and the flue of fuel-burning units. (A flashlight and an inspection mirror are a big help.) While slight rust or water marking from condensation are not a problem, heavy rust and water streaks are danger signals. A pile of rusty scale on top of the burner suggests that tainted air has damaged the flue.

Another indicator for both gas and electric heaters is the sacrificial anode rod. (This rod "sacrifices" itself to protect the tank.) Although it can be difficult to remove, the effort is worthwhile, because the anode provides the best clue to conditions inside the tank. The sacrificial metal (magnesium or aluminum) is formed around a steel core wire. Normally, the anode slowly corrodes away to protect the heater from rust. When enough metal has corroded away to expose 6 inches of the core wire, it's time to replace the anode. So if you pull out an anode and find little or no sacrificial metal left, some damage will have occurred in the tank.

If your inspection of fittings, combustion chamber, or anode suggests that the tank has undergone substantial damage, replacement is a reasonable choice.

Perhaps you found little or no evidence of tank deterioration. Then, assuming the tank has insulation and is correctly sized, the most cost-effective thing to do is probably to keep it going. But, you may be thinking, what about energy use? The main difference between most old and new heaters in terms of energy performance is the insulation level. Atmospheric combustion and immersion heating elements haven't changed a lot. So insulating blankets, heat traps, and pipe insulation can help keep heat where it should be and help old heaters perform more like new heaters.

Whether you keep the old heater or replace it, maintain it regularly (see "Water Heater Maintenance Combats Tank Failure"). This will avoid the cycle of replacement, saving the energy needed to produce new equipment and dispose of the old.

The two rods on the left show stages of corrosion, the far left one being almost completely dissolved, and the next one fairly corroded. The rods on the right are unused anode rods; the far one, with a hollow PVC section, is designed to be installed as a secondary anode in the hot water outlet port.

New Heater Choices

Choices abound in ways to heat water. In some situations, a tankless heater may be the best option (see "Tank-type versus Tankless"), but tank-type heaters suit most installations.

Gas Underfired Heaters

It's not the most efficient type, but the gas underfired water heater is the most common. It is relatively inexpensive and can be maintained to ensure long service life. New models can include very heavy (up to R-25) internal insulation, multiple flues for greater heat transfer surface, or a submerged combustion chamber to prevent heat loss from the burner area.

Sidearm Heaters

One type of heater has moved the burner and flue outside of the tank (just like the old sidearm heaters) to cut down on standby losses. Because the heating is indirect, the storage tank can be lined with plastic. Another type has moved the gas heating unit even further away from the tank and mounted it on an outside wall. This unit is used to convert an existing electric tank to gas, where space or plumbing problems are restrictive.

Direct-Vent Heaters

Direct-vent heaters have their own combustion air inlet and vent routed through a sidewall. They are used where vertical venting is not possible or where tight construction may cause various appliances to compete for the same air. These units can save energy because they do not rob conditioned indoor air from the house. Also, they cannot backdraft combustion gases into the house.

Power Vent Heaters

Power vent heaters use a fan to assist venting of combustion gases. Therefore they can use a long horizontal or vertical vent pipe, which allows them to be located almost anywhere, and they cannot backdraft while the burner is firing. The disadvantages are that the fan robs conditioned air from the house, and the water heater needs electricity to operate (so a power outage means no hot water).

Tank-Type versus Tankless
While tank-type heaters are the norm in the United States, a tankless heater is sometimes the better choice. Tankless heaters (also known as instantaneous or demand water heaters) heat the water as it is called for at the tap. Here's how they compare with tank-type heaters.
Cost

Tankless heaters, ranging from about $300 to $1,000, usually cost two to three times as much as comparable tank-types.

Installation

Tankless heaters, being smaller, can fit into spots where you'd never get a tank.

If you replace a tank with a tankless, both gas line size and venting or power supply (if electric) will probably need to be increased. The heating elements or burners are typically much larger than in storage units. (A tankless gas unit has a capacity up to 170,000 Btu/h versus 40,000 Btu/h typical for gas storage.)

Ability to Serve

Tank heaters (with adequate distribution plumbing) can serve multiple taps simultaneously. Tankless heaters are limited. Smaller gas models and essentially all electric models are meant for one-person households or point-of-use applications. All but the largest gas units have difficulty serving multiple taps.

When the flow is kept within range, tankless heaters can, however, provide unlimited hot water (if it is needed). However, if minimum flow isn't met, the heater will quit heating. If maximum flow is exceeded, water at the tap will cool down. In addition, temperatures can fluctuate with changing water demand.

Maintenance

Upkeep on tank-type heaters is simpler. They're less complex, and it's easier to find people who can work on them. Parts for tankless heaters are often difficult to get.

A tankless heater will generally outlast an unmaintained tank-type.

Other Considerations

Tank heaters are less likely to be damaged by freezing weather.

Tankless heaters pose less risk in an earthquake (less weight to restrain).

Tankless have no anode and so are not susceptible to sulphur odor.

Tank-type heaters are more tolerant of hard water conditions. Lime buildup in tankless heaters (essentially mini-boilers) cuts efficiency significantly.

Having tankless heaters in easily accessible locations in kitchens and baths would be a big change from current practice and codes.

What About Energy? When their standing pilot lights are shut off between uses, tankless heaters are easily more efficient than tank-type heaters. Whether that energy savings is enough to offset equipment, installation, and maintenance costs will depend on heater use and owner involvement. Since most Americans have been spoiled with the install-and-forget ways of tank heaters, they will probably not opt for the level of involvement needed to make tankless heaters energy savers.

Electric Tank-Type Heaters

The second most common type of heater is the electric tank. This is usually the most expensive way to heat water. It is best used where demand is small, electric rates are low, or no other options exist. Direct immersion heating elements allow plastic tanks and plastic-lined steel tanks to be used (both available with lifetime warranties). Plastic tanks don't need anodes; in cases where smelly water is a problem, tanks without anodes can help to control the odor.

Heat Pump Water Heaters

Public acceptance of heat pump water heaters has been slow, probably because they are expensive and relatively difficult to maintain. A new unit, the E-Tech, by Crispaire Corporation, is about twice as efficient as electric tank-type heaters and should cost less than previous heat pumps.

Combined Systems

If the resident uses a boiler for space heating, it can usually be combined with an indirect storage heater to provide hot water (see "Once Heated, Twice Used," HE July/Aug '92, p. 14). This combined system offers reduced operating costs and improved efficiency over tankless coils or conventional water heaters. Unlike tankless coils, it does not require the boiler to come on each time you draw hot water (this is particularly unpleasant in summer), and the superinsulated storage tank retains heat very well. The higher the efficiency of the boiler, the more attractive this method looks.

Making the Decision

Many gimmicks are used to sell water heaters, but they don't necessarily give you a better tank. When you select a new tank-type heater, size it appropriately using the manufacturer's guidelines. Don't just automatically put in the same size heater as before-appliances and fixtures are replaced over time, and this may have reduced the demand for hot water. The efficiency (indicated by the Energy Factor on the label) gets lower as the tank gets bigger, so a smaller tank will use less energy per gallon of water heated. Insist upon a minimum of R-16 internal insulation. Internal insulation thoroughly covers the tank, including the top, which must be left exposed when using an external blanket on a fuel-fired heater.

Look for ease of maintenance and anode accessibility. Make sure the anode's hex head is visible on top of the tank, or that the tank has a combination anode/hot outlet. Don't pay extra for a longer warranty. Tanks with longer warranties are usually equipped with two anodes instead of one, but you can add a second anode yourself for a fraction of the cost.

Heater Location

Indoors or out? In cool climates, any heat lost by an electric water heater located indoors might be a welcome addition, while gas heaters vent most of their heat loss up the flue and take conditioned house air with it. In warmer climates, consider locating the heater in a garage, basement, or separate outside closet. Place the heater close to the taps that get the most frequent use (usually the ones in the kitchen) to reduce heat loss from long plumbing runs.

Position the heater to facilitate maintenance. Provide easy access to the anode and sufficient overhead clearance for checking and replacing it. Make sure electric elements can be pulled out and thermostats can be adjusted. Be sure the relief valve and the drain are easy to reach. Place the heater near a sturdy wall if earthquake strapping is needed.

Water Heater Maintenance Combats Tank Failure
We've maintained tanks for more than ten years; fewer than half of one percent have leaked. Proper maintenance eliminates most causes of tank failure, so theoretically, tanks could go on indefinitely. Here are the primary maintenance tasks.

Check and Replace the Sacrificial Anode as Needed
As long as a tank has a functioning anode, it will not rust. Anodes are screwed into the tank from the top. A torque multiplier is a useful tool (it trades speed for strength) for removing the stubborn 1 1/16-inch hex plug type anode found in most residential heaters. Some heaters may have a different type of anode-one attached to the hot outlet nipple.
Have a new magnesium anode at hand when you check the existing one. (Get a segmented rod if overhead clearance is limited.) If you find 6 inches or more of the steel core wire exposed, or if the rod has passivated (formed a hard, adherent calcium carbonate coating, which prevents further corrosion of the sacrificial metal), replace the rod. Anodes should be checked every three or four years if water is normal and every one or two years if water is acidic or artificially softened.

Get Rid of Sediment
Sediment causes the bottom head of gas and oil heaters to overheat, speeding dissolution of the glass lining and weakening the steel. In electric tanks, the lower element gets buried, overheats, and burns out.
The best way to keep sediment out of a tank is to flush the heater routinely every six months. To do this, replace the existing drain (usually plastic, often finicky) with a full-port brass ball valve. Next, replace the straight cold water inlet (dip tube) with a curved replacement tube. Then, when the drain is opened, the curved end delivers a stream of water that stirs up the sediment, moves it toward the drain, and flushes it out. This method works well with good water pressure (50-80 PSI) and unobstructed piping. If the buildup is very heavy, a tool we developed called the Muck-Vac can vacuum out the sediment (see "Taking on Sludge," EA&R, Nov/Dec '87, p. 37). An option with electric tanks is to remove the lower element and use a 3/4-inch pipe attached to the hose of a wet/dry shop vacuum to pull out the sediment.

Prevent Corrosion
Rusting is common inside steel plumbing, and is hastened at points where different metals are joined together, such as the pipe nipples on top of a tank. Because steel nipples commonly clog up with rust and restrict flow, brass nipples are often used at tank tops. But this just shifts the corrosion problem to the steel tank itself, where the nipple enters. Instead of brass, use steel nipples lined with plastic. The plastic interior does not corrode, and the steel exterior does not shift problems to other locations. Use plastic-lined nipples anywhere in the plumbing where dissimilar metals join.
Promote Safety
Use local codes, installation instructions, and good sense to provide for adequate clearance of combustibles around gas heaters. Check combustion air supply and venting. Operate the temperature and pressure relief valve every six months to be sure it's functional. If it fails to open or if it leaks, replace it.

Installing/Upgrading a Water Heater

When putting in a new heater or rejuvenating an older tank, follow all the codes, rules, and instructions. Then take these additional steps to make an even better job of it.

Add a Second Anode

Most heaters have one sacrificial anode, often identifiable by the hex head visible on top of the heater. (Some tanks do not provide access to the anode; the hex head is hidden under a sheet-metal top. Avoid these.) A second type of anode is combined with the hot outlet port. Tanks with a hex head anode can be equipped with a second anode by adding a combination rod in the hot port (as many 10-year warranty tanks have).

Control Sediment

Excessive sediment buildup can cause premature tank failure and element burnout, as well as noise and odor. Sediment can often be controlled by flushing if the proper parts are installed (see "Water Heater Maintenance Combats Tank Failure").

Upgrade the Relief Valve Drain Line

Because the temperature and pressure relief valve needs to be checked regularly, attach its drain line with a union or a flex connector to simplify valve replacement when it is needed. Drain lines must always lead away downhill, so water cannot be trapped where it might freeze or collect at the valve and corrode it closed.

If the heater is indoors, put it in a drain pan so that if there is ever a leak, the house won't be subject to water damage. You may wish to put a water alarm in the pan, especially if you cannot run a drain line from it.

Add a Blanket

If you are purchasing a new heater, get one insulated to R-16 or more and avoid the disadvantages of a wrapped heater. These include problems with access and the potential for hiding leaks, accelerating rust damage, restricting combustion air, causing electric components to overheat, and obstructing venting. Also, if safety stickers are hidden under a blanket, you may be liable if someone is burned or scalded.

However, older heaters (and even some new ones) have insulation rated R-7 and below. Consider installing a heavy blanket on these for a combined internal and external insulation value of R-16. Make sure all fittings are dry and in good shape before installing the wrap. Put a Gas Appliance Manufacturers Association safety sticker (available from GAMA or a plumber) on the blanket. Leave the anode, relief valve, and controlsexposed for routine maintenance. If your heater's R-value is not on the label, try this old rule of thumb: put your hand on the heater; if it feels warm, add a blanket.

Timers

A time clock turns the water heater off during periods when the household does not typically use hot water. Good insulation greatly reduces the value of a timer on electric heaters (and insulation has a lot less to go wrong with it than a clock). If the utility offers time-of-day metering, however, consider installing a time clock. Some utilities offer additional methods and controls for saving peak power.

Insulate Pipes

Help keep the heat in the water by insulating all the hot plumbing lines you can get to (and the cold line back 3 ft from the heater). Use a good-quality plastic or rubber foam at least 34 inch thick. Do not cover unions or fittings at the ends of flexlines (these areas need to be clearly visible), and stay clear of the draft diverter on gas heaters.

Install Heat Traps

Heat traps may also be used on the tank (see Figure 1) to help keep the heat where it belongs. Ball check valves work, though flow restriction and a chattering noise can occur at high flow rates. You can make your own heat traps by bending long flex connectors into an inverted U shape. In some circumstances, heat traps can save as much as a blanket.

Figure 1. Schematic of a water heater. Note the heat traps, made by bending the flex connectors into an inverted U shape.

Consider a Flue Damper

Another device that should be on the market quite soon is a nonelectric flue (not vent) damper produced by ACT Metlund Systems. This uses a lightweight poppet to seal off the flue under the draft hood when the burner is not firing and opens immediately upon firing. It should cut standby losses significantly.

Options beyond the Water Heater

Hot Water Distribution

Inadequate hot-water distribution can make even the best water heater's performance unsatisfactory. Distribution is best considered during new construction, but remodeling or repiping are the next best times. People do not want to wait for hot water. They would also prefer not to waste water or energy. Conventional long runs of branched piping manage to do all three. They contain gallons of cool water that must be wasted before hot water reaches the user. If the runs are not insulated, they can easily lose 20^oF between the heater and the point of use, requiring the heater to be set hotter-which is both wasteful and dangerous. Also, water remaining in the pipes quickly cools when use is finished.

Consider the advantages of using a manifold system instead of a main line with branches. With individual direct runs of 3/8-inch tubing, this system contains much less water than a 3/4-inch main with 1/2-inch branches, so less water is wasted and less time is spent waiting for hot water to arrive. (It takes 40 ft of 3/4-inch, 82 ft of 1/2-inch, or 133 ft of 3/8-inch copper pipe to hold 1 gallon of water.) This "new" idea was mentioned in a 1950 book. (Bear in mind that smaller-diameter runs may take longer to fill bathtubs or spas, and they should not be used if water pressure is low.)

Impressive tank-top corrosion is visible on this water heater.

Tempering Tanks

Another idea worth revisiting is the tempering tank (see Figure 2). This is simply an uninsulated tank located in a warm or sunny area like an attic or sunroom and hooked up in series with the present heater. Here the entering cold water can be warmed to room temperature or higher before it enters the heater. Electric utilities used to suggest this measure to help electric heaters provide more hot water.

Solar Heaters

A tempering tank located in a sunny spot is similar to a solar batch or bread box heater. Both active and passive solar systems can preheat water for a conventional heater, and at the right time of year, solar alone can provide all the hot water the house needs.

Solar is capable of delivering much of a household's hot water needs flawlessly for decades if the system is well designed, well built, and capably installed. Unfortunately, many systems have been net energy losers because they were overly complex, poorly installed, and not maintained. We do have the know-how to build efficient, long-lived systems (see "Efficiency and Solar Water Heating: Untapped Potential," HE July/Aug '92, p. 21). With solar, it's particularly important to keep it simple and not install more than the resident can deal with.

This non-electric flue damper from ACT Metlund can cut standby losses significantly and should be on the market soon.

Recirculation Systems

In order to have hot water at the faucet faster, many people install systems to keep hot water circulating in the pipes. If the major modifications to the plumbing system mentioned above cannot be made, a recirculation line can save water and make for happier users. However, don't use a gravity system or simply plug in the circulating pump to run 24 hours-these options waste energy.

Recirculation Controls

Grounded plug-in timers cost about $15. They allow you to keep water in the pipes hot only when it's wanted, usually about 8 hours a day instead of all 24. If there is no one to keep an eye on the timer, however, power outages can throw it off. Also, timers do not work well in apartment buildings where residents keep different hours. Thermostatic control of the recirculation system can provide additional savings. In houses, return temperature can be set as low as 105^oF.

The Metlund System

Probably the most efficient recirculating system is one made by ACT Metlund Systems. It gives the benefit of recirculation without the added plumbing. It's called a demand system, as hot water is only delivered when called for, greatly cutting plumbing heat loss. Using a pump and a motorized valve mounted under a sink, hot and cold lines are joined long enough for cool water to be pumped from the hot line. Once a sensor picks up rising temperature, the pump shuts off and the valve closes. Hot water is then available at the taps. Usually, the system is activated by push buttons, which can be wired in parallel and placed conveniently at any fixture. The system can also be operated by a wireless remote control.

Figure 2. A tempering tank (right) is a smaller tank attached to a water heater where cold water can sit and warm up to ambient air temperature before being heated. If located in a hot attic or a sunny location, the water in the tank can heat up above the house air temperature.

Add a Supplemental Small Heater

Another technique is to put a 2 to 6 gallon electric heater at the end of a long plumbing run. This tank should have at least three times the capacity of the plumbing run. It is hooked up in series with the main heater, providing instant hot water. Before the small heater runs out of hot water, it is replenished by the main unit. A tankless heater may also be appropriate here. In either case, energy is spent to save water. In areas subject to drought and rationing, this can be more than just a convenience.

Whatever water-heating choice you make, safety, simplicity, low cost, and ease of use should be your major goals.

Larry and Suzanne Weingarten operate Elemental Enterprises in Monterey, California, which services conventional and solar water heaters. They also manufacture a sediment removal tool and have written The Water Heater Workbook-A Hands-On Guide To Water Heaters.

-This article is part of a series on energy effcient remodeling, which is finded by the Environmental Protection Agency and The Department of Energy.

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