The boiler's water line is surging.
Dirty water, a too-high pH, overfiring and improper near-boiler piping can all cause the boiler's water line to surge up and down. If there's an automatic water feeder on the boiler, it will open and close each time the water line rocks. Before long, you'll have a flooded boiler.
Don't blame the automatic water feeder; solve the surging problem by cleaning the boiler, adjusting the firing rate or correcting the improper near-boiler piping.
The boiler has a leaky tankless coil.
Even a tiny pinhole in a tankless coil will flood a boiler because the pressure inside the coil is so much greater than the pressure in the boiler. Isolate the coil and watch the gauge glass for an hour or so. If the coil is the culprit, change it.
The gravity-return system has motorized valves.
To get the water back into the boiler, a gravity-return system has to balance itself like a scale. The "leftover" steam pressure at the end of the main combines with the static weight of the condensate that stacks in the vertical space between the boiler water line and the lowest, horizontal steam-carrying pipe (the "A" or "B" Dimension - see The Lost Art of Steam Heating). These two forces overcome the pressure inside the boiler and allow the condensate to enter.
When motorized valves close, you lose that "leftover" pressure at the ends of the mains. The static weight of the returning condensate alone usually isn't enough to overcome the pressure in the boiler, so the water backs out of the boiler. If there's an automatic water feeder, it will open and add water to the boiler. The next time the motorized valve opens, the condensate will flow into the boiler and flood it.
If you use motorized zone valves with a gravity-return system you also must use a boiler-feed pump and end-of-main steam traps.
The boiler has very narrow sections.
This is a problem you'll sometimes run into with residential steam boilers. If the sections are too narrow, the rising steam bubbles will lift the boiler water level to a point higher than the level in the gauge glass. This happens because there isn't any steam in the water in the gauge glass. The two columns of water (in the boiler and in the gauge glass) ride at different levels until the burner shuts off on high pressure.
When the steam bubbles condense, the water in the boiler will fall to a point lower than the water in the gauge glass. The water in the gauge glass offsets this by falling into the boiler. If you have an automatic water feeder, it will open and eventually flood the boiler.
This type of problem is built into the boiler design, and it's tremendously aggravated by dirt. You can sometimes cure it by underfiring the boiler. Underfiring produces fewer steam bubbles, giving each more room. But don't underfire to a point where you'll only be simmering the water. Also, make sure the boiler is as clean as possible.
The water's pH is too high.
It should be between seven and nine. A pH of 11 or higher will make the boiler water foam. Foaming water will leave the boiler with the steam, and that will lower the boiler's water line. The automatic water feeder will respond by adding water to the boiler. When the condensate returns from the system, the boiler will flood.
Correct the pH with chemicals. Ideally, it should range between seven (neutral) and nine (mildly alkaline).
The automatic water feeder isn't closing tightly.
All it takes is a buildup of sediment on the feeder's seat to keep the feeder from closing tightly. Since the pressure in the city water main is so much higher than the pressure in the boiler, water will continue to flow into the boiler and flood it if the feeder isn't tightly closed.
Check the feeder by doing a broken-union test. Bring the boiler water level to the "feeder closed" point. Open a union on the outlet side of the feeder, below the boiler water line. There should be a gate valve or a ball valve after the union to keep water from flowing from the boiler. If the feeder is tightly closed, no water should flow from the union.
If you supply the feeder with hot water, lime scale will accumulate on the feeder's seat and eventually cause the feeder to fail. Feeder manufacturers recommend you feed only with cold water if you want to avoid this problem.
The line between the feeder and the boiler is clogged.
Some automatic water feeders have mechanical, float-operated valve. If there's backpressure in the feeder's discharge line, the feeder might not close tightly. Backpressure comes from a buildup of lime scale in the feed line. The feeder stays open and allows water to enter the boiler.
You can check this by doing a broken-union test. Bring the boiler water level to the "feeder closed" point. Open a union on the outlet side of the feeder. Now open the gate valve or ball valve between the union and the boiler. If the line is clear, you should get a steady flow of water from the boiler. If all you get is a trickle, change the line.
The feeder bypass line is leaking.
Check the valve in the feeder bypass line. If it's not holding tightly, water will enter the boiler and flood it. Use a broken-union test to test the valve. Install a union after the bypass gate valve or ball valve, and open it. If your gate valve or ball valve holds, no water should flow from the union. If water does flow through, repair or replace the valve.
There's a check valve in the gravity-return line.
Equalizers make check valves unnecessary, but you'll still find them on many steam systems. If a check valve gets clogged with dirt, it will slow the rate at which condensate returns to the boiler. If there's an automatic water feeder on the boiler, it will add water. When enough condensate backs up behind the clogged check valve, the valve might open and allow the water back into the boiler. Since the automatic water
Clean the check valve, or remove its flapper. If the gravity-return system has an equalizer, you don't need a check valve.
The boiler is overfired.
If you overfire a boiler the water will surge violently and some water will carry over into the pipes. An automatic water feeder will replace that water, and the boiler will flood when at the end of the steaming cycle. You should fire to the connected load of the boiler (piping and radiation).
This is the boiler's D.O.E. Heating Capacity load. Don't oversize replacement boilers.
The near-boiler piping doesn't meet the manufacturer's specs..
Manufacturers know that if the steam leaves the boiler too quickly it will carry water with it. They also know that high-velocity steam can tilt the boiler's water line, and cause a low-water cutoff to operate (or not operate!). That tilting water line can also open an automatic water feeder and flood the boiler. This is why manufacturers tap modern steam boilers with multiple outlets.
The steam boilers of yesteryear were different from the ones we're using today. One outlet may have been enough in the old days because the boiler was much larger, had wider sections and a more generous steam chest. A boiler of the same rating nowadays, however, may need two or three outlets to dry the steam and balance the water line.
This is why it's so important to read the manufacturer's installation instructions and follow them. If you pipe the new boiler the same was as the Dead Man piped the old boiler, you're probably going to get into trouble. Read the boiler manufacturer's instructions thoroughly, and follow them to the letter.
The automatic water feeder is set at the wrong level.
A feeder's job is to maintain a safe, minimum water line, not an operating water level. Don't think of a feeder as a convenience item; it's a backup safety device for the low-water cutoff. If you raise the feed level to the center of the gauge glass the feeder will let water into the boiler before the condensate has a chance to return from the system. When the condensate finally does return from the system, the boiler will flood.
Check the manufacturer's installation diagram and, if necessary, make the correction.
Someone is overfeeding the boiler.
Ask around. There may be an enthusiastic superintendent or home owner who likes to feed the boiler while it's steaming. If they add water by hand before the condensate returns, the boiler will flood when the condensate finally does return.
Educate the building owner on the right way to feed a steam boiler. Don't add water by hand unless the boiler is off and the condensate has returned from the system.
Want more troubleshooting tips? Check out A Pocketful of Steam Problems (With Solutions!).