Hydronic Thermal Expansion Tanks



This article,"Thermal Expansion tanks For Hydronic Heating Systems", will help the average homeowner understand the basic design elements and operational uses for these tanks. From composition to implied uses, this article will cover the necessary information, to understand this equipment.

There are two distinct types of thermal expansion tanks that are used in hot water heating systems. One employs an air precharge and the other relies on atmospheric pressure within an enclosed tank to absorb the expansion.

Thermal expansion. These are the words of the day, when these tanks are being installed. The reason they're needed, is to absorb the physical thermal expansion, that occurs when water is heated. Hydronic heating systems are closed loop systems and they run between 12 and 25 lbs of pressure, within the boiler and radiation, as well as any piping the system contains.

When the boiler fires the water is heated and thermal expansion makes the area within the system, too small to hold the expanded water volume. This results in a climbing pressure and eventually will set off the 30 lb. pressure relief valve, that all hot water systems employ.

Installing a thermal expansion tank is the solution to this problem. Most installations now, will employ a precharged tank, installed into a tee somewhere in the system. Usually this apparatus is located close to the boiler for servicing reasons.

A rubber diaphragm is across the center of the tank. The bottom half is separate from the top and charged with 12 lbs of air. The top half, which is connected to the water system directly, will fill with system water. When heated water begins to expand, the air is easily compressed within the sealed, air charged side.

Water, or any liquid for that matter, is very difficult to compress. air however compresses quite easily and this is the place where the extra volume goes. This allows our hydronic system to maintain a stable and proper pressure range. The lower a system can run and work, will result in the least amount of leaks developing on any components, as well as the boiler sections themselves.

The second type of thermal expansion tank, has no precharge of air, just an empty tank that holds air on one end when opened to the system. These are basically obsolete with the new innovation of the precharged tank. They had to be emptied quite often, as they would slowly flood with water loosing their beneficial expansion absorbing properties.

You can check your own expansion tank for problems by tapping above centerline and then below, the sound should be radically different between the two areas as the top is full of liquid and the bottom full of air. If the sound is the same then your tank probably has a blown diaphragm and needs to be replaced.

Another good indicator that a tank has gone bad will be the sound of running water in the baseboards or piping due to the precharge of air having been dumped into the hydronic heating system. This will necessitate that the system be completely purged of air when the new tank is installed.

So run downstairs now and tap on that tank, if the top sounds the same as the bottom, you may need to give your local serviceman a call. Thermal expansion is a very powerful natural element that can destroy heating equipment. This can result in major repair bills. Thermal expansion tanks are relatively cheap and should be replaced immediately when diagnosed.

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