A steam boiler boils water to make steam, and the steam condenses in the radiators and runs back. Water is lost along the way, through vents and small leaks, and the level in the boiler falls. If it falls too far, the burner heats an empty boiler, and that can crack the cast iron. The controls that prevent it need a little attention from the homeowner, and it takes a minute a week during the heating season.
Reading the sight glass
On the side of the boiler is a vertical glass tube between two valves. The water should be roughly halfway up the tube, and often there is a mark. Look at it with the boiler off or idle; when it is steaming, the level bounces.
- Low: add water, see below.
- Empty: turn the boiler off and do not run it until it is filled. If it has been running dry, call a plumber before firing it again.
- Too high, near the top or above: the automatic feeder is overfilling, or someone has added too much. Drain some out, and if it keeps rising, the feeder needs attention.
- Dirty, brown water bouncing violently: the boiler needs skimming or flushing, which is part of the annual service.
Adding water
Most steam boilers have an automatic water feeder that adds water when the level drops. Older or simpler installations have a manual fill valve on the cold water pipe to the boiler. To fill by hand: with the boiler cool or idle, open the valve slowly and watch the glass, and close it at the halfway mark. Do not add cold water to a boiler that is hot and empty; the shock can crack it.
The low-water cut-off
This is the safety control that shuts the burner off if the water drops too low. There are two kinds:
- A float type, a cast iron chamber on the side of the boiler with a small drain valve at the bottom. Sludge collects in it and can jam the float. Flush it weekly during the heating season: with the boiler running, put a bucket under the drain valve, open it until the water runs clear, and the burner should shut off as the level drops. Close the valve, and the burner should restart once the feeder or you refill it. If the burner does not shut off during the test, the cut-off has failed and needs replacing before the boiler is used again.
- A probe type, an electrode in the boiler with no chamber. It needs no weekly flushing, but the probe should be removed and cleaned at the annual service.
If you are not sure which you have, or where the drain valve is, have the plumber show you at the next service. It is the single most useful thing to learn about a steam boiler.
Other things to keep an eye on
- The pressure gauge. A residential steam boiler runs at a very low pressure, typically under 2 psi. A gauge reading well above that means the pressure control needs adjusting.
- Leaks. Water on the floor around the boiler, or a level that drops fast, means a leak somewhere in the system, sometimes in a buried return pipe.
- The return lines. Wet returns near the floor rust out. Rusty water in the glass and a constantly falling level point to that.
The annual service
Before each heating season, a plumber or heating technician should clean and test the burner, check the safety controls, flush or clean the low-water cut-off, skim the boiler if the water is dirty, and check the vents. Keep a carbon monoxide alarm on every floor.
When to call a plumber
- The low-water cut-off does not shut the burner off in the test
- The level drops quickly, or the boiler has run dry
- Constantly dirty water, or violent bouncing in the glass
- Pressure well above normal
- The annual service
Find a licensed plumber near your town, and ask whether they work on steam.