Forced hot water
Quite often my clients comment on the small size of the boiler for the forced-hot-water system. They apparently are used to seeing a boiler originally designed for use in a gravity system or one that was converted from coal-fired to oil-or gas-fired, both of which are considerably larger-two to six times as large, depending on the manufacturer. Regardless of the size, you can easily recognize a forced-hot-water system by the presence of a circulating pump in the distribution return pipe just before the connection to the boiler. (See FIG. 14-7.)
Boilers The boilers used in these systems will be made of either cast iron or steel. Cast-iron boilers are more resistant to corrosion than steel boilers and thus have a longer projected life. The projected life span of a modern cast-iron boiler is about twenty-five to thirty years. However, many of the boiler manufac-turers will provide only a twenty-year warranty. The older gravity-type boilers were probably made with a heavier-gauge metal. I have inspected many that were over fifty years old and still going strong. (See FIG. 14-8.) Steel boilers, being vulnerable to corrosion, have a shorter projected life, usually about twenty years. I have seen steel boilers that required replacement after fifteen years.
Other than for reasons of efficiency and economy, the only time a boiler must be replaced is when a leak has developed that cannot be effectively patched. Sometimes, just after firing a boiler that has not been operational for a day or more, you might see a slight amount of water dripping into the firebox (assuming that the firebox is accessible for visual inspection). This is often the result of condensation caused by cold water circulating into the boiler or some slight joint movement. As the boiler heats up, the various sections tend to move slightly. In some cases, this results in a slightly open joint that allows water to drip out. However, as the sections continue to heat up, they expand, compressing the joint and sealing the leak. Although this condition is usually not a problem, if you see water dripping into the firebox, it would be best to have the condition checked by a professional.
On occasion, I have found a regular tank-type domestic water heater (of the type described in chapter 16) being used in the heating system in place of a boiler. From a safety point of view, this is acceptable because there is a high-temperature and pressure control along with a thermocouple control for the gas valve. However, water heaters have a projected life of seven to ten years and are often guaranteed by the manufacturer for only five years. In addition, with the exception of those homes located in the sunbelt, the Btu (heat) output of these units per hour is less than that needed to heat most homes adequately. This type of setup may be effective for heating an addition to a house where the existing heating system cannot be extended. The only drawback is the short projected life of the water heater.