Unvented Cylinders with Internal expansion are known by the trade as “bubble tops”. This is an apt description as they do indeed trap a bubble of air in the top. As the cylinder is heated up, the expansion of the hot water is taken up by the bubble.
As you probably know water under pressure absorbs air. Some people will be familiar with milky water coming from their taps. The milkyness is the bubbles in the water formed as the air is released.
In some areas of the countr y, local conditions have led to some bubble top unvented cylinders losing their bubble prematurely. This has been disruptive and costly for all concerned. Indeed, some have even been converted to external expansion to solve such local problems.
If the air bubble is lost, it is usually necessary to drain down the unit and refill it. As anyone with a water meter will tell you, refilling it costs money and then the additional inconvenience of having to pay to heat the water again. You would be even more upset if you had to find and pay for a plumber to do this for you.
Contrast this with the rubber diaphragm inside our expansion vessels.
This diaphragm separates the water from the air - PROBLEMSOLVED.
The pressure can easily be checked and topped up with a foot-pump without losing a drop of water.
Add the additional flexibility of remote siting that an expansion vessel brings and the choice for us was clear - External Expansion.