Part of the National Energy Policy Act of 1992 governs household water fixtures, including toilets, shower heads and bath and kitchen faucets. All have to have water-saving designs.

There is no need to remove existing fixtures, but if you plan to remodel or add new devices you have to meet the law. The act also requires that fixtures be manufactured to comply. But ask questions and read labels to avoid buying out-of-date merchandise.Faucet aerators are the least expensive water-saving devices and probably the easiest to install. The law requires them to restrict flow to a maximum of 21/2 gallons a minute. Older aerators often have flow rates as high as 8 gallons.

Aerators are screens that attach to the ends of faucets. They allow air to mix with the water, resulting in finer droplets that wet more surface area than a solid stream.

Most water-saving aerators also contain a disk that restricts the water from the faucet. Others have a variable-control device, often an o-ring that flattens under water pressure to reduce flow. Under low pressure, the ring keeps its normal size, providing a compensating higher flow.

When selecting an aerator, be sure that its threads will fit the faucet. Remove an existing aerator by unscrewing it with channel-locking pliers. Wrap the pliers' jaws with tape before installing the new aerator to avoid scratches.

Like aerators, water-saving shower heads are generally inexpensive and easy to install. They also have to restrict flow to less than 21/2 gallons a minute. Although their basic mechanism is usually a water-restricting disk or a flow-control o-ring, many have added features to improve or vary the spray.

Older shower heads often produced weak, lukewarm sprays. According to reports from owners, many new models deliver satisfactory results equal to or even greater than the heads of yore.

Install most heads as with aerators. When unscrewing the old head, grip the shower arm tightly to keep it from turning, which can cause leaks. To prevent leaking, wrap two turns of plumber's thread-sealing tape clockwise around the shower arm's threads before attaching the head.

Some shower arms are fitted with unremovable balls. If you have one of those, you may be able to buy an adapter for the new head. An easier solution is to buy a new shower arm. To replace a shower arm, shut off the water to the shower and use a wrench to unscrew the arm from its fitting inside the wall. Wrap thread-sealing tape clockwise around the end of the new arm and screw it into place.

Water-saving faucets are also available. Many have an aerator, but some have flow regulators. One of those is a modified solid stream called laminar flow, which increases wetting like an aerated stream but is less noisy and produces less splash.

Some new water-saving faucets contain temperature-regulating features that prevent scalding. In a home with small children or the elderly, they are a worthwhile safety investment. Installing a water-saving faucet is no more difficult than with a conventional one. The most difficult part usually is removing the old faucet, which may be corroded.

Water-saving toilets are the most expensive and difficult fixtures to install. Municipalities sometimes offer rebates. When combined with savings on water bills, they may cover the cost of the toilet and installation in a reasonable time.

Toilets manufactured before the '80s typically consume 5.5 to more than 7 gallons of water a flush. Water-saving toilets developed during the '80s consumed 3.5 gallons; the new Energy Policy Act requires them to consume no more than 1.6 gallons.

So-called low-flow toilets, which consume 1.6 gallons, have been available since the 1980s. Design problems, notably the frequent need for double flushing to clear the bowl, made many early models unpopular. But recent modifications have improved them.

There are two basic designs. Gravity-flush models operate much like conventional toilets. Differences are a taller, narrower tank, which creates a more-powerful column of water during flushing, and a steeper bowl, which washes clean more readily. Gravity-flush, low-flow toilets cost far less than $100. Their main disadvantage is a continuing need for occasional double flushing.

Pressure-assisted toilets are the other type. They have a plastic air cylinder in the tank. Water entering the tank compresses air in the cylinder. When the toilet is flushed, by pressing a bulb on top of the tank, the compressed air forces the water out of the tank at high velocity.

Prices for pressure-assisted toilets start around $150. They make somewhat more noise than gravity-flush toilets, but double flushing is reportedly no more necessary than with old-style 3.5- or 5.5-gallon models. Installing toilets is fairly simple, but unless you are experienced, hire a plumber.