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EST. 2002
Home Air Products™
My Location: Ashburn, VA [Change]
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The Amtrol THERM-X-TROL ST-5 has been discontinued.
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An expansion tank is a required component for all hot water boiler systems, and is designed to control system pressure and help reduce energy consumption of heating and circulating operations. These Amtrol expansion tanks are factory pre-charged and 100% tested to keep the diaphragm flush against the tank when the system is first filled with cold water. As the system water temperature increases, the expanded water is absorbed by the tank. As the system water temperature reaches its maximum, the diaphragm flexes against the air cushion to allow for the increased water expansion. These expansion tanks feature a Butyl/EPDM diaphragm for superior air retention, nine times better than natural rubber.

Thermal expansion occurs as domestic water is heated. Left unchecked, the resulting pressure increase can cause relief valve discharge and other potentially unsafe conditions. Therm-X-Trol® expansion tanks absorb this expanded water, keeping water pressure at a safe level.

Standard Features

  • Easy to install - maintenance free
  • Molded, polypropylene liner for maximum corrosion resistance
  • NSF 61 approved
  • 5 year warranty

Water heater manufacturers and plumbing codes require the installation of an expansion control device if a backflow preventor, pressure reducing valve or check valve is installed in the domestic supply line.

Expansion Tank Type

There are two basic expansion tank types: Plain steel and bladder-type. Both are intended to control the high end pressure of your boiler system and keep the relief valve from opening due to increased system pressure caused by the expansion of water. Plain steel tanks have no moving parts, and require a separate pressure reducing valve to maintain initial fill pressure of 12 psi (or higher depending upon the requirements of your system). This type of tank looses its air cushion over the years and will need to be drained periodically. Bladder-type tanks contain a moving bladder, and requires either a separate pressure reducing valve (as mentioned above) or a fill-trol valve to maintain initial fill pressure of 12 psi (or higher depending upon the requirements of your system). This tank will not lose its air cushion unless the bladder becomes detached, at which point the tank would need to be replaced.

Expansion Tank Type
Max Temp
200 °F
Tank Volume
2.0 Gallons
Max Accept Volume
.9 gallons
Max Water Pressure
150 PSI
Product Weight
5 lbs
8 Inches
12 5/8 Inches

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