What Is a Gravity Heating System?

Gravity heating as an option

A gravity heating system makes use of the difference between the densities of water at different temperatures. These heating systems are able to move heated water through a structure because at colder temperatures, water moves downward toward the boiler; after being heated, water moves upward through the system to provide steam heat or hot water to the various outlets throughout the structure.

History

Gravity hot water heating began in northeastern United States between 1875 and 1885. At the time, it was billed as a safer alternative to steam heat, which had developed a negative image due to the possibility of pressurized boilers exploding. Gravity systems were also easier to maintain than alternatives of the time, and quickly became a popular system for heating homes and buildings constructed around the turn of the century. It is still quite possible to find homes built before 1920 with functioning gravity heating systems today.
Between 1910 and 1920, new forced air heating systems began to show up in industrial buildings. The newer technology only heated the air by blowing air over direct resistance heating coils and pumping that warm air through heating ducts. Even though the system separated the heating of air and water into separate systems, the efficiency of the heating unit still offered substantial savings over a gravity heating system. By 1920, forced air heating systems had effectively won the lion's share of the heating market.

Function

Since the hot water circulators hadn't been invented yet, the engineers who designed gravity heating systems had to rely on the basic laws of physics to move the water or air through the system; that is, hot water rises and cold water sinks. So a boiler was installed in the basement to heat water that was fed down from the "down-feed." Once heated, the water expanded and became lighter than the colder water being fed down into the system. That hot water went "up-feed," throughout the system until it reached the expansion tank (also called an overflow tank) in the attic. At this point the water would have cooled enough to regain density and weight and begin traveling back "down-feed" toward the boiler again, forcing warmer water through the system ahead of it.

Benefits

In addition to the increased safety and ease of maintenance compared to pressurized systems of the time, gravity heat systems were incredibly durable, since the only moving part within the entire system was water. In addition, the long-term operational costs were relatively low. However, modern heating technology can deliver far greater benefit at a much lower cost.

Cost

After 1910, gravity heat systems had to compete with a new market entrant, forced air heating systems. Much like the systems we use today, these systems heat the air, then use a fan to blow warm air throughout the system. In a 1914 issue of "The Heating and Ventilating Magazine," a published efficiency study of the two technologies had already proven that a gravity heating system was 220 percent more expensive than a comparable forced air system of the time. Though more modern gravity systems are available, they have not kept pace with their forced-air counterparts. Also, there are a few significant drawbacks to implementation of even a modern gravity heating system.

Considerations

When deciding if the installation of a gravity heat system is an option, there is a critical component your building must have: a basement. Since the system relies on the vertical circulation of water or air (there are systems that can heat without steam), the heating source must be located below the area being heated. Furthermore, a system balance of temperature must be maintained for proper function. Only a professional would be qualified to make the necessary adjustments which might prove costly. Finally, since these systems have been almost entirely supplanted by forced air systems, it may be difficult to find a local professional who's able to install these systems and provide maintenance.

References

• Gravity heating FAQ
• "The Heating and Ventilating Magazine"