Product Description

There are two methods of combating the pressure fluctuations in a system caused by thermal and pressure changes – atmospheric and pressurised.

The OET utilises the atmospheric method, the oldest method of expansion accommodation in closed-loop systems.


Before the advent of closed expansion vessels, it was via a header tank (commonly referred to as an open expansion tank) installed at the highest point in the system.

These were often large cylindrical tanks made from copper or stainless steel, with a ball float installed within to allow water infill as necessary from an external source (mostly from town water supply). The tank would have an overflow and drain and a safe tray.

Masterflow has designed a standard range to suit most applications, and offers custom-design tanks for space-limiting situations. These tanks are made from 304 grade Stainless Steel and come complete with a safety tray and lid, as well as an approved float valve for mains supply into the tank.

Options include:

  • Non-metallic supports for positioning between the tank and the tray (as detailed in AS 3500
  • minimum 12mm thick)
  • Wall brackets

Standard tank capacities:

  • 35 litres
  • 60 litres
  • 100 litres
  • 200 litres
  • 300 litres
  • 500 litres
  • 650 litres

Frequently asked Questions

  • What options are available for dealing with expansion in closed-loop systems?

    An open expansion tank is the simplest for small volumes, where the tank can be easily placed high enough above the highest point. A closed expansion tank with an RPZ valve can be used anywhere that is convenient, provided it’s connected to the return line of the system and the mains pressure is high enough to pressurise and fill that expansion tank. The downside here is that the RPZ valve does require regular testing. That same expansion tank could be supplied with a refill unit, which includes a break tank. This tank will protect the water supply frokm contamination, as per the open feed tankl.

  • Do I need an expansion tank on an open-loop system?

    No, not normally. Open-loop means there is a break to atmosphere, as is the case with condenser water circuits with cooling towers.

  • What are the disadvantages of insdtalling an old-style feed & expansion atmospheric type system?

    They must be installed a metre higher than the highest point of the system,so OHS challenges must be navigated for initial installation and ongoing maintenance checks. Structural hardware such as a platform and access ladder are required. Pressurised tanks are a lot more economically priced and have lower installation costs.

Relevant Resources for Open Feed & Expansion Tank (OET)


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