The Case of the Telltale Tank

If you have been reading this column even occasionally, you know that I receive a lot of phone calls and e-mails from friends with questions about their mechanical systems. That said, the following will probably not surprise you: I was in a basement checking out a boiler when my phone vibrated. I usually don’t bother to leave my phone on when I’m in a basement, and on the rare occasion that I do, I usually don’t pick up.

But when I saw that my friend Victoria was on the line, I decided to answer. Victoria is no heating expert, but she knows a lot more than most people, and she is frequently at loggerheads with the insufferable know-it-alls on her cooperative’s board – especially when they are about to waste a lot of the co-op’s money.

“V, what’s up? I’m in a basement and can’t really talk right now,” I told her.

“Mmghrk,” she replied.

“If you can hear me, e-mail me what’s going on and I’ll call you back tonight,” I said, probably too loudly.

She must have heard me, because when I got home there was an agitated e-mail from her in my inbox. The gist was that there appeared to be a significant leak in her boiler, a contractor had taken a look and recommended a new boiler, heating season was starting in a month, and the know-it-alls were about to vote to go ahead with the replacement. Could I take a look ASAP?

My recollection was that her building had a hydronic, or forced hot water, heating system with multiple atmospheric gas-fired cast-iron boilers. It hadn’t been installed that long ago, but before Victoria had been on the board. Which is too bad, because if she had known, she would have told me and I might have been able to successfully intervene. (Would it surprise you to learn I don’t like those systems very much?)

This was something of a clandestine operation, as Victoria did not want her board opponents to know I was going to be on-site. You know the drill: they find out someone’s coming, they take umbrage, they insist on being there, and they flap their gums incessantly while you’re trying to assess the problem. No thanks.

Fortunately, Victoria has a good relationship with the super, and he agreed to let me in one afternoon while the know-it-alls were at work. As I entered the boiler room, I immediately noticed the evidence of the “leak” – a rusty, white-powder-encrusted inkblot on the floor (see photo, above). Then I noticed something else: the location of the inkblot. It was directly beneath the discharge pipe of the pop safety valve and nowhere else.

This almost certainly meant that there was no “leak” per se, at least not the kind that required replacement of the boiler. The two likeliest possibilities were a) a bit of grit had lodged itself on the valve seat, allowing boiler water to spurt out, or b) the system expansion tank was too small.

I laid out the possibilities to Victoria. “Expansion tank? What’s that?” she asked. “Well,” I said, “when the boiler water heats up, it expands. Since water is essentially incompressible, and the volume of the distribution pipes is fixed, if there isn’t a way to absorb that expansion, water will shoot out of the path of least resistance in the system.” In most cases, that’s the pop safety valve.

We poked around a bit before finding the tank. It was tiny, not much bigger than what one might find in a single-family house, but this was a 20-unit building. I was pretty sure we had uncovered the culprit. Someone had decided to save a very small amount of money by installing an undersized tank (see p. 46). Now what?

I left it to Victoria to solve the political puzzle. Once she did, I was able to advise her about the proper fix. Technically, I should have performed a more elaborate calculation of the volume of the piping system, which of course is what the original designer and contractor should have done. Fortunately, there’s no penalty for oversizing an expansion tank, and they are not expensive, so I estimated the volume based on experience. Victoria told me later that a new tank (at right) had been installed, and the “leak” had disappeared.

One thing still bugged me, though: why had this issue only now been noticed? The tank was undersized from the get-go. Turns out, it was only after a few years of discharge that the rusty inkblot had grown large enough in the dark boiler room to be obvious. And, of course, there was no water meter on the boiler makeup line; if there had been, and it had been logged regularly, the problem would have been caught long ago.