Heat loss is much greater in diaphragm vessels

Tests carried out in the Varem laboratories have shown an aspect that is often neglected when designing and producing central heating systems, namely heat loss in expansion vessels.

As these are vessels that, compensating for expansion of the water, accumulate a substantial amount of heated water inside, they are inclined to transfer part of the heat to the room in which they are installed.

However, there are some considerable differences, depending of the design principles used to produce these vessels; in fact, Varem has found that this loss is much greater in expansion vessels with a diaphragm membrane than in vessels with a bladder membrane of its own manufacture.

The heat loss in expansion vessels with a diaphragm membrane is 60% greater compared to balloon membrane vessels.

The reason for this better insulation lies in the fact that the pre-charged air cushion inside a bladder membrane vessel completely surrounds the membrane acting as thermal insulation, while in diaphragm membrane vessels the heated water is in direct contact with the metal plate for almost half of the vessels surface, unavoidably transferring much more heat to the metal wall and, consequently, to the environment outside the expansion vessel.

Details of the tests carried out are given below:

  • water temperature in the boiler: 60C
  • water temperature in the connection area of the vessel: 55C
  • ambient temperature (in the room in which the vessel is installed): 15C

The images below show the thermal values taken on the surface of the expansion vessel in the two cases examined, i.e. with a diaphragm membrane and a balloon membrane. 35C 50C 29C 33C 50C


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