A friend of mine who happens to be a contractor went out to look at a hot water boiler that was causing its owner a problem a few years back. It seems the flame had chewed through the back of this steel boiler and was licking up the side of the jacket.

There was no water in that boiler and yet the burner just kept running because there was nothing on that boiler that could shut it off. That boiler didn’t have a low-water cutoff. No one thought a low-water cutoff was needed because the system had an automatic water feeder. They figured that if the system developed a leak, the feeder would feed and that would be that.

The trouble was this was an old radiant heating system. The pipes were made of metal and some of them had sprung leaks. The original installer hadn’t thought to use a vapor barrier below the concrete slab, so when the pipes leaked, the water went down, not up, and no one noticed it.

So the feed valve did what it was supposed to do. It fed a lot of water into the system in a hopeless attempt to bring the system pressure back up to 12 psi. That feeder didn’t have a prayer, though, because the pipes were leaking as fast as the feeder was feeding. And did I mention that this job was in a part of the country where the water was as hard as an anvil?

Now, here’s the thing about hard water. Calcium and magnesium, which are the elements in water that give it its "hardness," have this weird property of being inversely soluble. That means that when the boiler heats the water, these elements come out of solution and turn into a rock-hard solid. In other words, calcium and magnesium do the opposite of what sugar and salt do when they hit hot water. Calcium and magnesium form what we commonly call "lime." It’s the stuff you find in the bottom of a water heater after a few years.

On this particular job, however, the calcium and magnesium kept building up inside the guy’s boiler and that became a problem for him. Before anyone noticed, the lime had sealed the insides of the boiler. Less and less water flowed into the boiler; it was all going out to the pipes buried beneath the concrete floor. The thermostat kept calling for heat and the circulator kept spinning in space, but the boiler was dry. The automatic water feeder kept trying to keep up with the loss in pressure. And the burner kept running. And after a while, that burner chewed through the steel sides of that boiler and nearly burned down the building.