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Choosing Between Plain Steel and Bladder-Type HVAC Expansion Tanks

To better understand the issues associated with plain steel expansion tanks and old Air Control Systems, let me first describe the expansion tank, which is an integral part of any air control or air elimination strategy.

Expansion tanks are pressure vessels designed to handle water's increased volume after it has been heated.

Water expands as it is heated. Since water is virtually incompressible, all that extra volume must be directed somewhere, otherwise you might find your safety valves relieving the excess pressures or your piping system might find its weakest point.

The majority of expansion tanks used today are bladder-type or diaphragm-type. Water is permanently separated from air in the tank by using a rubber balloon that fills with the expanded volume. Earlier expansion tanks were commonly called 'plain steel' and did not have a bladder - they were just empty tanks. In addition to having fill and drain valves, they often had gauge glasses (or "sight glasses") to show how much water was inside. At ambient temperature, the sight glass should show that the tank volume is half air, half water when charged with enough air pressure. It should also be installed at the highest point possible. When the system is operating normally, the expanded system water volume enters the tank, compressing the air; while the air separator vents the air over the tank so that the air cushion is always maintained.

Why Owners and Engineers are Moving Away from Plain Steel Expansion Tanks

In spite of the fact that we still sell a few plain steel expansion tanks each year, most owners and design engineers have moved away from them for the following reasons:

  1. Air control vs. air elimination: In a plain steel tank, air is in direct contact with the system water. As a result, air cannot be eliminated from a system.

  2. Sight glass leaks: Sight glass or gauge glass assemblies have a tendency to leak. The air cushion is allowed to escape when the valve seals on these assemblies dry out and contract. The system may pop off pressure relief valves.

  3. Tanks made of plain steel must be installed horizontally at the highest point in the system for air bubbles to flow UP into them. This makes them difficult to install and service since they are hung high in the ceiling. There is less flexibility in this type of tank than in diaphragm or bladder tanks, which can be mounted vertically or horizontally.

  4. In most existing installations, the sight glass is out of sight. There are two possibilities: either the glass is up high with the tank, and so cannot be seen, or the inside of the glass has become cloudy. Unless the gauge is visible, how can you tell if the tank is still filled with air?

In addition to these four reasons, there are likely other reasons why people have moved away from plain steel tanks. If this technology is old, why write a blog about it?

There are still some designers who use plain steel tanks. Perhaps, they had a bad experience with a bladder tank, or maybe they just have not thought about the pros and cons of air control with plain steel tanks versus air elimination systems with bladder tanks.

Additionally, there are many old systems with plain steel tanks. In many cases, owners and maintenance personnel don't know what they have. The old plain steel expansion tank could very well be causing pressure (relief valves popping) or air problems (air lock, corrosion, etc.). Your old air control system may need to be replaced with an air elimination system. Get expert advice on equipment selection from your local manufacturer's rep if you need help sizing a new hydronic system.

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