Most of the time, your water heater just sits where it's been placed, quietly doing its job. Or maybe not so quietly if you've got a lot of sediment buildup. But unlike Fido, if it's unhappy, it's not going to walk to the door and bark and scratch. More likely, it will just suddenly poison you, roast you or simply blow you up.

What? You didn't know your lowly water heater could be so lethal? Well, time to wake up and smell the roses. Or maybe carbon monoxide. Oops. You don't actually smell that. You just breathe it until it sickens you or you die.

"So," you might ask, "how can this thing be?"

It can be if a tank has been improperly installed, or if hazards have been allowed to develop with time. We'll go down the list. And we'll make a little disclaimer: We're based in California, so we know California codes. We don't know every code in the United States. But these are basics that should apply most everywhere. We all deal with the same laws of physics.

First off, There are the instructions that come with the water heater. Follow those. Ignore them at your peril. As for the rest, following our little picture, we'll start at the top and work down.

Vents

There is much that could be said about proper venting. Most of it is pretty technical. So here are basics that the rest of us can understand. First, the vent should be the same diameter as the draft diverter of the tank. The vent should go generally up and out -- never up, down, up and out. Where it passes through walls or roofs, it should be double-walled. And the single-wall vent sections, which are crimped and shove together, should be screwed with three screws per section, especially in earthquake country. But even elsewhere, badly connected vents can fall apart. Then that silent killer, carbon monoxide, can go on the rampage.

Beyond that, poor draft can cause backdrafting. That means that instead of going outside, the fumes come back into the room. That can eat up vent pipes with acid condensation; cause enough condensation on cold piping to drop down onto and destroy the tank; or cause soot problems. To say nothing of seeping into living spaces. But more backdrafting issues later.

Dielectric connectors

These connect the water heater, which is steel, to the plumbing, which is usually copper. Put two metals together in water and you get electrolysis, which is where one metal corrodes away to protect the other. That's usually detrimental in plumbing, although such a reaction also protects the innards of the tank. Anyway, whether you have dielectric unions with solid copper plumbing or copper flex lines, check for leaks. Water heaters have considerable internal rust protection, but nothing much outside.

Temperature/pressure relief valve (T&P)

This valve is designed to prevent a tank from exploding if temperature or pressure exceeds safe limits, by opening and venting. Unfortunately, residential valves are somewhat prone to failure. They should be checked once a year by pulling up on the handle. Water should flow freely out and stop when you let go of the handle. If it does nothing, runs or drips, then the valve should be replaced.Banging on the handle with something hard, like pliers, sometimes will stop drips or even runs. If not, replace the valve. Hooking up the drain line with a union or flex connector makes T&P replacement MUCH easier.

People don't like to test their T&Ps. But then, we don't think it's so much fun to wake up in the hospital, or to patch a big hole in the roof, either. When water heaters explode, it's catastrophic. People are injured or die; buildings are severely damaged. Test your T&Ps! And one more thing: T&P drain lines should go down and out. Never up. If the valve opens, water will pool there and corrode it shut. Or freeze in the line in colder climes. We've seen lines plumbed uphill so many times we've lost count. But there SHOULD be a drain line, usually to within about six inches of the floor, or plumbed outside. That's code around here. It's to prevent you from being scalded if the valve should open while you're standing next to it.

Finally, if water is running out of your T&P line, look for the cause. It might just be a bad T&P. But it could also signal high-pressure problems or a dangerously defective control. Don't ignore it!

Earthquake straps

OK, you in Kansas can laugh at those of us in California, Oregon, Washington and Alaska. On the other hand, those of you in Missouri or Kentucky had better hold your tongues. It is said that the Mississippi River flowed backward in the New Madrid, MO, quake in the 1800s, the strongest ever recorded on the North American continent. And in Daniel Boone's time, if we remember our history, Caintuck was known as The Shakes. And trust us, they weren't talking about chocolate or strawberry.

But for everybody, if your water heater falls over and severs the gas line, for any reason, you might find yourself singing "Great Balls of Fire." Strapping ain't such a bad idea, even in seismically stable areas. (Note that the tank in the photo shows only one strap. Top and bottom straps are required in California.)

Shell

We just threw this one in to remind you of something. What you see is NOT the water heater. It's just the sheetmetal shell that surrounds the tank. How many times have people told us, "It looked just fine, then it started leaking." On the other hand, if the shell looks like total hell, don't expect the tank inside it to look much better. And on the other, "other hand," be aware that water on the floor is not necessarily the end of your water heater. It can come from other places: T&Ps, drain valves, rain down vent pipes, even broken water mains. We've seen all these.

Control

Or temperature control. On commercial tanks, they're set in degrees, but on residential tanks, expect warm, hot, very hot, or something like that. There is so much variation on what those settings mean, heater to heater, that all we can tell you is that the right temperature is at least 130 degrees at the tap, which you can test with a meat or candy thermometer.

Why 130? Well, at 120, legionella bacteria that cause Legionnaire's disease can grow. Then when you take a shower, you catch it by inhaling the mist. Anything above 130 vastly increases scalding risk, energy use and sediment buildup. Truth is, 130 is a compromise temperature. You can be scalded there, too, if you spend long enough under the water, but it is sufficiently hot that you'll probably draw back, but not so hot that you'll JUMP back. Most bathing injuries are not from burns, but falls.

Combustion chamber hatch

This is not a hazard, but a place to look for hazards. Getting the outer hatch off is usually no big deal. But be careful if you want to remove the inner one. If the tank has been firing, it can be quite hot. We know!

But getting that cover off is worth it. You can read the future there.

Oops, sorry. You won't see the winning lottery numbers in the flames, or discover three spells to snare the mate of your dreams written in fiery glyphs on the back wall. But you WILL be able to determine whether your appliance is humbly serving you or getting ready to poison you or roast you. What's the lottery compared to that?

One thing you want to check is the flame. But there is a certain risk in words. What we do is based on years of experience. We weigh this and that and this other and determine a result. We can show you pictures, and will, but even that can be somewhat subjective.

Anyway, now that we've weaseled, where we're heading is this: Water heaters breathe (at least gas or oil heaters). They suck air from underneath, combust with it, exhale it. If there's anything under the tank -- like dust -- they might breathe that, too, fouling the burner and then causing the tank to soot up and backdraft in a vicious circle. Eventually that can result in a fire hazard where flames billow out the bottom.

Take a good look at the photos at right. The top one shows a little blue flame at the bottom, but way too much yellow higher up. The middle one shows a lot of black in the combustion chamber. Yeah, we know, this is a multi-flue commercial heater. Unless your house is very big, you won't have one like that.

But the principle is the same. If the roof of the chamber is black, it's bad news. It means drafting problems, combustion problems, fume problems, fire hazards. Not nice. By the way, the top and middle photos are of the same combustion chamber.

As to the bottom one, that was a goner. If your combustion chamber roof looks pretty much like THAT -- with much rust and water marking -- time to call a plumber.

However, if all you see is gray metal, maybe some white condensation marks, then your tank is probably OK. But be sure and vacuum underneath it, to keep the nightmare scenarios at bay.

Pedestal

For quite awhile, across the United States, it's been code that tanks in garages, like our demo tank, be mounted on a pedestal to keep the burner at least 18 inches off the floor. That's to keep it from igniting fumes that tend to hug the floor. A new standard recently went into effect that mandates a baffled combustion chamber that is supposed to resolve that problem.

Those of you with electric tanks are likely to say, "Hey, wait a minute. What about us? Well, we haven't forgotten you. Everything above applies to you except for the part about vents and combustion chambers and such, because naturally, you don't have those.

What you DO have is thermostats and elements and high limits and reset buttons. And insulation. That might seem like a minor point, but it's not.

Open an element port and the first thing that meets your eye is insulation. It's there for a reason. If you remove it and fail to put it back, your heater might not function properly, since the thermostats have to sense the tank temperature and not that of the outside air.