Most expansion tanks come pressurized to 12 – 40 psi, but before you install the tank you have to match the system pressure. Start by removing the protective cover from the air valve on the tank. Use a tire gauge to check the pressure.

• Expansion Tank Installation Diagram
• Expansion Tank Installation For Chilled Water System
• Water Heater Expansion Tanks Residential

Before we get into the common issues associated with plain steel expansion tanks and old Air ControlSystems, let me first describe the expansion tank, which is an integral part of any Air Control or Air Elimination strategy.

An expansion tank is a pressure vessel designed to accept the additional fluid volume created as a result of heating the water in the system.

In other words, as water is heated, it expands. And, as water is virtually incompressible, that additional volume must be directed somewhere, else you will quickly find your safety valves relieving the excess pressures or you might discover the weakest point in your piping system!

Most expansion tanks used today are bladder, or diaphragm, type. Essentially, they incorporate a rubber balloon that fills with the expanded volume and keep the water permanently separate from the air in the tank acting as a cushion.

Older style expansion tanks were commonly called “plain steel” and had no bladder – they were just an empty tank. They included an air fill valve, a drain valve, and often a gauge glass (or “sight glass”) in order to visually see the level of water on the inside. The idea was to charge the tank with enough air pressure so that, at ambient temperature, the tank volume was half air and half water, as indicated by the sight glass. It should also be installed at the highest point possible. During normal operation, expanded system water volume enters the tank, compressing the air; while the air separator would vent the air up to the tank to ensure that air cushion was always present.

Four Reasons Owners and Design Engineers Have Moved Away from Plain Steel Expansion Tanks

Although we still sell a few each year, most owners and design engineers have moved away from plain steel expansion tanks for the following reasons:

1. Air elimination vs. air control: Air stays in direct contact with system water in a plain steel tank. Therefore, air can never beeliminated from the system.eliminated from the system.
   
2. Leaks: The sight glass, or gauge glass assembly, is a weak point. Seals within the valves on these assemblies tend to dry out and contract, allowing the needed air cushion to escape. You may see pressure relief valves popping off in the system.
   
3. Installation limitations: Plain steel tanks must be installed horizontally at the highest possible point in the system to ensure air bubbles flow UP into them. So, they are hung high in the ceiling – difficult to install and difficult to service. This is much less flexible than bladder or diaphragm tanks that may be installed vertically or horizontally, in the air or on the floor.
   
4. The “out-of-sight” glass: In most existing installations, you cannot see the sight glass. It is either up high with the tank where it isn’t visible, or the glass has clouded internally. How do you know if the tank still has air if you can't see the gauge?

There are likely other reason folks have moved away from plain steel tanks, but these four are the most common. So, if this is old technology, why write this blog?

1. Some designers still 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.
   
2. There are a lot of old systems with plain steel tanks. Many owners and their maintenance personnel may not be aware of what they have. And, if they encounter pressure issues (relief valves popping) or air problems (flow issues do to air lock, corrosion, etc.), it could very well be related to the old plain steel expansion tank.

It may be time to convert your old air control system to an air elimination system. For help sizing and selecting equipment for a new hydronic system, contact an engineer at your local manufacturer's rep for an expert consultation.

Blog images courtesyTaco® Comfort Solutions™

How to size and select a proper Expansion Tank

When water is heated, it expands and its volume increases. Since water is considered non-compressible, in a closed-loop system, this thermal expansion puts an incredible amount of pressure on the piping and components of the entire system. If unresolved, this water pressure could cause leaks and even complete ruptures in the piping. Expansion tanks provide extra space for the expanded water and relieve the system’s pressure. Other than that, not having an expansion tank will allow the pressure to open your relief valve resulting in wasted energy, shortened system life, and an overall potential safety hazard.

Expansion Tank Installation Diagram

The expansion tank itself is just a container of air separated from water by a diaphragm.

When the heating system is filled with cold water, the expansion tank’s pre-charge pressure is equal to its fill pressure which causes the expansion tank’s diaphragm to extend fully against the tank. As the water temperature of the system increases, the expanded water is absorbed by the expansion tank. After the water temperature has reached its maximum and begins to decrease, the diaphragm returns the cooled water back to the system.

Types of Expansion Tanks

There are several types of expansion tanks available depending on your system’s needs. Domestic hot water plumbing systems use Amtrol’s Therm-X-Trol expansion tanks. For radiant and hydronic heating systems, Amtrol’s Extrol, Radiant Extrol, or a Fill-Trol expansion tank should be used.

Choosing the right size for your Expansion Tank

Before purchasing an expansion tank, you should find out how large of an expansion tank your system will require. The decision will be based on two factors. The first is your hot water heater’s or boiler’s capacity in gallons. This information will be on your water heater’s or boiler’s factory label. The second is the water pressure in your piping system. For plumbing applications, this information can be acquired by either attaching a small gauge to any faucet or by calling your local water supply company. For hydronic and radiant heating applications, the system’s pressure usually never exceeds 30 psi.

If the expansion tank you purchase is smaller than what your system requires, the excess pressure from the expanding hot water will cause relief valve discharge. On the other hand, there is no problem with having an expansion tank that is larger than what your system requires. In fact, if unsure that your expansion tank will be able to safely accommodate your system, it is common practice to choose an expansion tank that is one size larger than the required size. As a general rule of thumb with expansion tanks, it is better to oversize than to undersize.

Plumbing Applications

Therm-X-Trol

Therm-X-Trol expansion tanks are made with non-ferrous materials, which makes them suitable for domestic hot water systems.

• Secures the water heater and fixtures

• Eliminates water and energy waste

• Assures that relief valves will not open

• Reduces risky pressure build up

Sizing Charts

Commercial Applications (Up to 180*F)

Hydronic Heating Applications

Extrol and Fill-Trol

Extrol expansion tanks are used far more frequently than Fill-Trol expansion tanks. In fact, the only difference between the two is that Fill-Trol expansion tanks include a specially designed pressure-reducing fill valve. Extrol expansion tanks require a separate fill valve.

Extrol and Fill-Trol tanks are sized with an extra factor to consider. Your boiler will, more than likely, have its BTU/Hr production rate information printed on it. Extrol and Fill-Trol models are sized the same way and are of comparable size.

• Provides permanent separation of system water from air
• Controls and normalizes the water pressure of the system
• Uses a Butyl/EPDM diaphragm for far better air retention than natural rubber
• Simple to install

Characteristics of Fill-Trol Expansion Tank

• Doesn’t require a separate automatic fill valve
• Same features and model as regular Extrol expansion tank

Sizing Chart

Radiant Heating Applications

Radiant Extrol

Radiant Extrol expansion tanks are designed to work in both open and closed loop radiant heating systems and are also suitable for snow melting applications.

Expansion Tank Installation For Chilled Water System

With Radiant Extrol expansion tanks, another factor to consider in sizing them is the size and length of the tubing you’re using.

• Specifically designed for high-efficiency radiant systems
• Appropriate for barrier and non-barrier PEX systems
• Befitting for use with glycol
• Non-ferrous connection that resists corrosion.

Sizing Chart

Water Heater Expansion Tanks Residential