Maintaining a Hot-Water Boiler

Hot-water controls typically include a combination gauge, often called an altitude gauge, that lets you keep an eye on both water temperature and pressure. It also lets you know when the boiler needs water or is malfunctioning. With some systems, a pressure-reducing valve takes care of the water problem automatically. Hot-water systems depend on an expansion tank that must be properly charged with air to prevent the water from boiling. With newer installations you’ll find this tank hung from the ceiling near the boiler, as in the drawing at right. In older homes it may be located in the attic. Newer expansion tanks include a purge valve that simultaneously releases water and lets in air. Older versions have a gauge glass, like the one on a steam boiler.

If your system is a forced-water (hydronic) system, look for one or more motor-driven pumps—called circulators—on return lines near the boiler. Some circulator motors are lubricated permanently and do not need maintenance. Others require a few drops of light oil annually. Read the instruction plate attached to the motor, however, because overboiling also causes problems.

Systems that have more than one circulator may be zoned to independently control the temperatures in different areas of your house. Zoned systems have low-voltage, motor-driven zone valves on the supply lines. Each valve obeys orders from its own thermostat. These require no regular maintenance, but they can fail occasionally. If the pilot light won’t stay lit, for relighting it and for replacing a thermocouple.

Flushing the System - Flush a steam boiler if the water in the gauge glass is rusty and flushing through the blow-off valve doesn’t clear it. Flush a hot-water boiler if you need to work on a radiator or a pipe. To do this, shut off the power and automatic feed, if you have one. Open vents or bleeder valves in the highest radiators. Attach hoses to the boiler drain and the return drain, open them, and let the water run out. Shut the drains, refill the boiler, and drain it again. Repeat the process until the water in the gauge is clear. For a hot-water system, you’ll need to bleed all the radiators.

Combination gauge. A combination gauge on the boiler has three indicators. The moving pointer shows actual pressure; the fixed pointer, the minimum pressure. If the moving pointer drops below the minimum, the system needs water. The lower temperature gauge shows water temperature. Maximum boiler water temperature is set by moving a pointer along the sliding scale of an aquastat (shown above right). Don’t tamper with an aquastat setting.

Pressure-reducing valve. This safety device automatically maintains the correct water pressure. To be sure it’s doing its job, check the combination gauge and call for repairs, if necessary. If there’s no pressure-reducing valve, you can manually feed the boiler by opening the feed water valve and closing it again when pressure reaches 12 pounds per square inch (psi). High water consumption means there’s a leak in the supply or return piping or in the boiler itself.

Troubleshooting a Hot-Water Boiler

Despite their complexity, hot-water boilers provide trouble-free service for years. When a problem does develop, it is usually with the expansion tank or a circulator, not the boiler itself. Get to know your boiler and all the controls and valves. Especially in an older home, these may be complicated. If necessary call in a heating expert to explain it, and label all the valves.

Water spurting from a pressure-relief valve means that there’s not enough air in the tank. The tank has filled with water, which expands as it heats up and trips the safety. Check this by touching the tank when the system is heating. Normally the bottom half will feel hotter than the top; if the top seems hot also, it means the tank has filled with water and must be bled. With most tanks, you’ll need to first let the system cool. Then attach a hose to the tank’s purge valve and run off two or three buckets of water. The valve lets in air at the same time. An older tank might have an ordinary valve rather than the purge type. With these, first close a second valve in the line between the tank and boiler, and then completely drain the tank.

After bleeding the tank, return all valves to their normal settings and start the boiler. Let it run for about an hour, then check the system’s pressure on the combination gauge. When a circulator fails, its motor may continue to run. This happens because the motor and pump are connected by a spring-loaded coupling designed to break if the pump jams. However, the system will not heat properly. Usually the broken coupling makes a racket. A leaking circulator means the pump seal must be replaced. Call a service technician for all repairs to a circulator.

Troubleshooting Before Calling for Service




No heat

No power to the boiler; low water level; burner problems

Raise the thermostat. Check the switches, fuses, circuit breakers, and water level. Troubleshoot the burner’s safety controls.

Poor heat

A sudden change usually means too much or too little water; a gradual change results from deposits in the boiler or on the heat exchanger

Check the combination gauge, then the expansion tank. If the problem developed slowly, try flushing the boiler, then call a professional for a tune-up.


The circulator; pressure-relief valve; piping; or, more rarely, boiler tank

Is water coming from the pressure-reducing valve, underside of a circulator, or supply or return pipes? Water may travel quite a distance from a leak, but always in a downward direction. Consider repairing pipes yourself, or call a plumber for service.

Only some radiators heat up

Suspect trapped air, especially in units far from the boiler. If an entire zone is cold, the problem lies with a zone valve or its circulator.

Bleed air from the cool units. Check the circulator. If a zone valve is stuck, you’ll feel heat in the pipe up to the valve but not beyond.

Clanging pipes

A sudden racket usually means a circulator has gone bad. Chronic banging noises may be the result of improperly pitched return lines.

Check the circulator. For banging, use a level to check the slope of all return lines.

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