What is a key advantage of using an indirect hot water storage vessel?

An indirect hot water storage vessel offers increased energy efficiency primarily because it utilizes a separate heat source, such as a boiler, to heat the water indirectly through a heat exchanger. This method allows for better temperature control and can make use of energy sources that are more efficient than direct electrical heating. Additionally, because the water is heated separate from the vessel during peak times, it can maintain a higher average temperature of stored water without consuming excessive energy, leading to lower operating costs in the long run.

The other options, while they may have merits in different contexts, do not accurately represent a primary advantage of indirect hot water storage systems. For instance, fast heating times are generally characteristic of direct heating methods rather than indirect systems, which take longer due to the heat exchange process. Similarly, while installation costs can vary, indirect systems often require more components, such as additional piping and a separate boiler, which may increase costs rather than decrease them. Lastly, compact size is not typically associated with indirect systems, as they usually require a larger space for both the storage vessel and the boiler setup, in contrast to some direct systems that provide hot water as needed.