Thermal Stores vs Dual Coil

Thermal stores are cylinders in which hot water is delivered via a heat exchange coil mounted in the top part of the cylinder. When used with solar, there would be an additional coil mounted at the bottom of the cylinder and the solar circuit would be indirect and filled with antifreeze. The water in the cylinder itself is not at mains water pressure and would have a header tank either integral to the unit or mounted in the loft.
If you consider what happens when you draw hot water form a tap, the top DHW coil has cold water introduced to it which absorbs heat as it flows. The water surrounding the coil will necessarily cool and tend then to fall to the bottom part of the cylinder. As this occurs, the water will be stirred resulting in less stratification of the cylinder. I.E. The water in the cylinder will be of a uniform temperature.

Standard "Dual Coil" cylinder - well stratified cylinderThermal Store - reduced stratification

There some advantages to the thermal store design despite its shortcomings. One of which is that it offers mains pressure hot water without the inherent safety issues associated with un-vented cylinders and as such the requirement for specialist training and certification can be avoided. In addition it is theoretically possible for the solar input to make a contribution to space heating. This carries a huge note of caution and will be dealt with by a separate blog post. If you have any questions feel free to contact us.