How to Save a Boiler's Life
It's a given these days that all boilers have low-water cutoffs and that makes me very happy because I've seen hot-water boilers explode.
One involved a sand hole in a cast-iron section. This house is on Long Island, New York. The water leaked into the combustion chamber, turned to steam and spewed from the chimney. The feed valve never opened to replace the missing water because unwanted steam was also forming inside the boiler. The feed valve didn't sense a drop in pressure so the water stayed in the piping.
The homeowner felt chilly one afternoon. He moved the thermostat. The circulator started and the relatively cold water hit the very hot boiler, turning instantly to steam. The rapid expansion of the steam turned the boiler's back section into shrapnel. The rest of the boiler tore itself loose from its pipes, stormed across the basement, hit the opposite wall, and nearly knocked the house off its foundation.
Another time, also on Long Island, and also hot-water. The house, built in 1950, was on a slab, with copper-in-concrete radiant. The buried pipes leaked in the '70s and copper-fintube baseboard stepped up to replace it. The baseboard ran from corner to corner, stopping only to duck into the concrete by the front- and back doors.
The tubing that ran into the slab by the front door sprung a leak, but no one noticed it because this boiler had an automatic fill valve that was constantly replacing the leaking water. The homeowner noticed a rise in his fuel bill, but he just figured that's what fuel bills do. He didn't call anyone to check it out. He also didn't notice that the slab by the front door had gone radiant.
One day, while the couple was at work, guys from the water company shut off the main so they could fix a busted pipe. They were at it all day. So was the leak in the buried pipe. The boiler kept running, trying to satisfy the thermostat, but the feed valve had no water to feed.
Now think about how very hot 600 pounds of cast iron can get over the course of a day when the burner keeps running because this boiler didn't have a low-water cutoff.
The guys from the water company finished their work and turned the water back on. The feed valve fed into that cherry-red hunk of hell, which exploded and took half of the house with it.
So how do you keep this from happening to you? I'm convinced that the only way to avoid an explosion is through really good service, eternal vigilance, and a working low-water cutoff on every boiler. We can't depend solely on feed valves to feed, because sometimes they do that at the wrong time, or not at all.
Low-water cutoffs belong on every boiler, even the older ones. Especially the older ones.
Use them and you'll not only save the boiler's life, you'll probably also save the family.
Steam as the Star of the Show
While steam displacing water can have a nasty effect on a water-based hydronic system, there are plenty of places where steam can be the star of the show, and for both heating and cooling. There really is no better way to move massive amounts of energy through relatively small pipes than by using steam at high-pressure. We reduce that pressure dramatically once it reaches its destination.
New York City's district-steam system, operated by Consolidated Edison, is the largest, and nearly the oldest in the world (Denver's is the oldest). Each winter, ConEd sends out nearly 10 million pounds of steam every hour to their 1,800 customers in Manhattan. One of those customers is the iconic Empire State Building, which reduces the operating pressure to just 1-1/2 psig. Hard to believe, I know, but it's true.
ConEd does this year-round through the use of boilers that are as tall as office buildings, 169 km of steam pipes buried beneath the city's streets, and 3,000 steam manholes (that's where the steam traps are). When properly managed, steam is a noble servant.
ConEd's system may be the biggest, but there are also many college campuses using district-steam systems to both heat and cool their buildings. Steam is also perfect for hospitals, where heating and cooling, as well as sterilizing and cooking, are essential.
So while feral steam inside a runaway hot-water boiler is something to both fear and avoid, steam, when mastered, can be a very efficient workhorse, especially in those larger, commercial/institutional buildings.
Three places to look for trouble
- Check the aquastat setting. You shouldn't need more than 82° C in most buildings. If the water is getting hotter than the aquastat's setting, and the aquastat is in a well, check to see if there's heat-transfer grease in that well. Many boiler manufacturers leave out the grease. Squirt some in there to provide good conduction and watch the aquastat smile.
- If pressure goes critical, the relief valve should pop. Is there a plug in that relief valve (the knucklehead's cure for drips)? Is it piped to the outside where trapped water might freeze inside the relief pipe? Ice is as good as a plug. Don't be afraid to test that relief valve. If it doesn't relieve well and then seat after you pop it, sell them a new one.
- Is there a low-water cutoff on that boiler? Is it working? You'll never know for sure unless you test it.
Things to note
- Before we had low-water cutoffs, and to back-up those early relief valves, boiler manufacturers would position a fusible plug in the side of the boiler, below the waterline, and above the crown-sheet. Should the water level drop below the plug, the excessive heat would melt it and relieve the pressure. Stand back!
- If you're going to have a boiler explosion, you're better off having it with a steam boiler. Hot-water boilers contain more water and that's where the energy gets pent up. This poses a much greater risk than steam at an equal volume. Think back to that water heater the MythBuster guys sent soaring. Yikes!