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; Varem laboratories have shown that diaphragm vessels has a heat loss 60% higher compared to the bladder vessels