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Understanding the Benefits of Expansion Tanks

Why would I need and what do I need to know about Expansion tanks?

What are expansion tanks?
Expansion tanks are small, sealed vessels within a closed water system, such as hot water heat, or a domestic hot water supply. They serve to mitigate the effects of pressure changes within the system which occur with changes in temperature. The air absorbs the change in pressure which reduces the knocking and banging in pipes. This unpleasant noise is called water hammer, and occurs when a fluid in motion is caused to stop or make abrupt turns. It is not only unpleasant, but can lead to pipe failure as the noise is simply a detectible symptom of a pressure wave traveling though the system.

Newer versions of expansion tanks are smaller than previous styles and include a diaphragm to separate the air from the liquid under pressure. The dry side, containing air, also has a valve similar to the kind on a tire where air can be added and the pressure checked. Older expansion tanks had no diaphragm, and air would dissolve into the liquid and then rise to the highest point in the closed system where it would come out of solution.

Old systems expansion tanks were one single chamber with unseparated air and water. The free air in the pipes went into solution in the water, and would then come out with temperature changes. These loops had to be drained periodically and refilled. The air had to be bled out of valves located at the highest elevation. Older systems where the air was allowed to circulate also were more subject to corrosion due to oxidation inside the pipes and fittings. Old systems were sometimes not closed. In an open system there is no checkvalve, or backflow preventer. When water from the mains is heated in a 50-gallon home hot water heater it can expand to as much as 52 gallons. In an open system the expanded water was allowed to flow back into the water main, possibly causing contamination. Closed systems are now the standard.

Water heated from 50° F, an average mains temperature, to 120° F will expand by 2%, causing a rapid increase in pressure in a closed plumbing system. Without an expansion tank a homeowner is simply asking for broken pipes. New tanks with a diaphragm are vastly superior. The diaphragm keeps the air separate from the water which eliminates the need for draining and bleeding. By eliminating air from the pipes, corrosion is reduced or eliminated. Expansion tanks are part of the installation code in many locations, but they are highly recommended even where they are not required. They add very little cost if put in during original construction. Systems without a tank will have a relief valve and water may leak out onto the floor when the pressure rises.

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