Maintaining a Steam Boiler

A steam boiler works like a giant tea kettle. A gas or oil burner heats water to the boiling point, sending steam to radiators. Some boilers have an automatic feed, usually combined with the low-water cutoff, that supplies fresh “makeup” water when needed. With others, you must manually open and close an ordinary valve. Preventive maintenance includes checking controls monthly and occasionally flushing the boiler. To prolong boiler life and maximize efficiency, schedule an annual checkup in the spring just before you shut down the heating system. Rust thrives when the unit is idle. Ask about chemically treating the boiler water to reduce its oxygen content to prevent rusting. If a gas pilot light won’t stay lit, for relighting it and replacing a thermocouple.

How it works. Modern-day boilers are complex. The water doesn’t simply rest inside a big kettle—that would take hours to heat up. Rather, water circulates around the heat source through a series of passages or tubes. Boilers also require controls to monitor and regulate their operation. These include a pressure gauge and regulator that shuts down the heat source when steam reaches a preset level, a pressure-relief valve that releases steam if the regulator fails, and a low-water cutoff that shuts down the system if the water level gets too low.

Water level. Check the glass water-level gauge every 10 to 14 days during the heating season. If you have an automatic feed, inspect it monthly. Check when the boiler cycle is not firing. If the level is low, open the supply valve (not shown) until you reach the correct level.

Relief valve. This is a safety device that protects against overheating. Test it once a year. With the boiler running, lift the lever for less than a second. It should release steam, then reseat tightly when you let go. If not, have it replaced.

Blow-off valve. Open the blow-off valve (also called the low-water cutoff) once a month during heating season— more often if your water has lots of sediment. Turn down the thermostat, open the valve, and allow water to run until it’s clear of sediment and debris. Be careful because the water will be hot.

Troubleshooting a Steam Boiler

If a boiler goes cold, check to be sure its main switch hasn’t been inadvertently turned off, then look for a blown fuse or tripped circuit breaker at the electrical circuit panel. Also try raising the thermostat setting and see that the thermostat is working. If the boiler’s burner unit is getting power, check the gauge glass to see if the boiler has enough water. When the water level drops below a certain point, the low-water cutoff turns off the burner or the electric heating elements; otherwise heat would “cook” the tank.

You can easily add water to a boiler—but first examine the return lines for leakage. Most steam systems gradually lose water through evaporation, but a big return leak will trip the low-water cutoff after just a couple of heating cycles. (Because supply lines carry only steam under relatively low pressure, they rarely develop leaks.) If you have a leak in a supply or return, call a plumber, not a heating contractor. One of the techniques might allow you to run the system until help arrives.

When you do add water to a boiler, be certain that you don’t overfill it. Steam systems depend on an airspace above the water line, called a chest, where steam builds up a head. If you flood the chest, water could back up the return lines or trip the relief valve—both messy situations. If your boiler has an automatic water-feeding device, you won’t notice any problems with the water level; the automatic feed will make up any shortage with each heating cycle. This means that a leak could go unnoticed for quite some time. Also constantly introducing fresh, cold water to the system will add to fuel and water bills. This is why you should shut off the feed every so often—most systems have valves or bypass piping for that purpose—and keep an eye on the water level for a few days. Remember to flush the automatic feed at the intervals recommended by the manufacturer. A feed that gets stuck open could flood the boiler.

Troubleshooting Before Calling for Service




No heat

No power to the unit; no water; burner problems

Check the thermostat, switches, fuses or breakers, and the water level. Boiler burners differ little from those on furnaces.

Poor heat

Rust and scale in a boiler, constricting passages and reducing efficiency; buildup on heating surfaces of soot from combustion

Flush the boiler. Cleaning the heating surfaces is a job for a professional.

Chronically low water level

Leaking return lines or, more serious, a leak within the boiler itself

For return-line leaks, see the main text above. Boiler leaks require major repairs or may mean you need to buy a new unit.

Clouded gauge glass

Usually the boiler needs flushing, but sometimes the glass itself needs cleaning

Flush boiler. To clean gauge glass, turn off boiler, close valves, loosen nuts above and below glass, lift up the glass, pull it out, and clean it with a bottle brush.

Noisy pipes

Probably water trapped in return lines or in the return main

Check the pitch of all returns—they must slope back toward the boiler. Adjust the slant, if necessary, with new pipe hangers.

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