It seems to us that there are two issues here. One is how rapidly a given tank heats water. The other concerns energy efficiency and longevity. By far, the latter question is the more easily answered.

Longevity and efficiency

If you've read very much of our site, you'll likely know that we like to see people buy a water heater that has an exposed, hex-head, magnesium anode installed in it. That offers at least the chance of removing it for inspection without totally destroying it. It also leaves room in the hot port for what is known as an outlet, or combination, anode rod comprising an anode, hot-water outlet and dielectric plastic-lined steel nipple.

If you were to buy a 12-year-warranty heater, likely the only difference between it and a 6-year-warranty one would be extra anode: either a larger-diameter hex anode, or a normal one along with a half-length outlet rod in the hot port. We'd rather see you put a full-length rod there -- and save the premium you'd pay for the longer-warranty tank. And installing it just involves screwing it into the hot port.

We'd also want you to install a flush kit comprising a curved dip tube and ball valve drain assembly if you live in a hard-water area. This is a trifle more complicated than the anode, but anybody can do it on a new tank. Oh, and by the way: if you install the flush kit, don't forget to use it once a year! Put it in your scheduler, if you use one. The time slips by faster than you think!

Finally, we'd like to see everybody buy tanks with R-16 insulation rating or better. Unfortunately, things have recently become more complicated. Some manufacturers have stopped showing the insulation R factor on the label, and don't even want to talk about it anymore. They've begun stressing EF, or energy factor, which they say is more exact. Our feeling is that it's also more ambiguous. EF comprises insulation, tank design, burner design and other factors. It's also true that an electric heater will have a much higher EF than a gas one, yet in some parts of the country, an electric tank will cost you a lot more to operate than a gas one because of utility rate structures. But some spec sheets still show R factor or tell how many inches of insulation there are.

You can still use EF to compare heaters, but we think you should also measure the insulation thickness by removing the combustion chamber hatch on gas heaters and measuring the jacket thickness. Roughly, one inch is R-8, two inches R-16 and so on. With electrics, it's more trouble. We don't think too many stores are going to let you open an electric element port for this. If the temperature/pressure relief valve is mounted on the side, you may be able to measure there. But we believe electrics already have at least R-14 anyway.

There has been much talk and a certain amount of e-mail to us regarding instantaneous water heaters, also known as tankless or point-of-use. If you install a well-insulated tank-type water heater, you can equal the efficiency of these latter for a great deal less cost and trouble. You'd think the instantaneous heaters would be more efficient, but it's not necessarily true. And they're expensive and finicky compared to the more common tank heaters. If you'd like to know more about our thinking on this, go to Tankless.

There are specific circumstances where this type of heater is the preferable choice, but in general, we suggest going with the simpler, easier-to-live-with, tank-type heater.

Needs and comparisons

Who needs what? That is the question! There are ways to rate one water heater against another. But what does that mean relative to you? With fuel bills going up, any logical person simply wants "enough."

If you just bought a house or condominium and are trying to figure these things out, don't forget that nearly all the manufacturers have sizing programs that can help you figure out what matches your needs.

If you're replacing a water heater, the greater question is, "Did the old one work well enough?" Now if the answer is "no," a good many folks would think, "I have a 30-gallon, so I ought to buy a 40-gallon" or "I have a 40-gallon, so I ought to buy a 50-gallon." Don't think only in terms of gallonage. Think in terms of Btu input or first-hour recovery.

These terms involve how rapidly a tank can heat water. A boiler that can heat a whole apartment complex or skyscraper sometimes has no storage, only a series of copper tubes running through a very hot fire. The water is heated instantaneously. Some very powerful commercial water heaters have very little capacity. For instance, a 75,000 Btu, 100-gallon light commercial water heater is a good deal less potent than a 250,000 Btu, 65-gallon tank.

First-hour recovery essentially combines the hot water a tank can hold with what it can heat in one hour. Second hour would be less as you would not be starting with a tank full of hot water. Used in conjunction with the yellow federal energy sticker, it provides a means of comparing different tanks, along with the other things we've mentioned.

So which brand is the best? It's not so simple as that. All makers make pretty good tanks with certain caveats. The process of applying the glass lining is imperfect. One tank will be better-made, another worse-made -- and all the action takes place where we can't see it!

The best you can do is buy a tank where you know you have a magnesium anode, add a second, outlet rod, and keep an eye on them. How do you tell? As far as we know, Rheem/Ruud/Richmond/General Electric always uses mag rods, even though there's no bump on the hex head. Anybody else, a mag rod will have that bump. The picture at right shows you what to look for.

Aluminum hex heads will be flat. If you can't see a hex nut, it means either that a hex-head rod is hidden under the cover, or that there is an outlet rod in the tank, and there is no easy way to tell (unless you own the tank and don't mind testing the anode for bendability) whether it is magnesium or aluminum.

As to other brands, as far as we know, American/Whirlpool always uses aluminum anodes. Bradford White always uses magnesium, but many of their tanks have one combo rod in the hot port. State/Reliance/Kenmore offers both combo rod tanks and tanks with a hex anode, which can be either aluminum or magnesium. As of February 2008, only the Premier line has magnesium. All the others use aluminum anodes. A.O. Smith makes both hex and combo rod tanks and uses both aluminum and magnesium.

If you contemplate adding an anode to a new tank, make sure both rods are of the same metal. Otherwise, the magnesium rod will be consumed more rapidly in the presence of an aluminum one and you won't get as long a life. On the other hand, if the tank is five years old or more, having dissimilar metals is less of an issue, since the original one will likely be largely consumed.

Also, if you are buying an aluminum/zinc anode for odor problems, don't put a second, combo anode in. That's contrary to all our counsel concerning other water heaters, but we've learned that having two anodes in the tank, even aluminum/zinc, may worsen the odor.