Diversity and Aggregation


Domestic hot water flow rates have always been challenging to calculate, because we need to take into account the parallel usage of hot water tapping points. Not all tapping points (like showers) will be use at the same time, so we need to take into account in each part of the system, the maximum probability, or the maximum DHW flow of parallel tapping points.

These are defined in diversity standards. Most of these methods only account for parallel flow rates, and not for parallel power, because domestic hot water networks are mostly operated as single pipe / fixed temperature systems.

When carrying over these standards to the central heating system (HIU’s, heat exchangers, storage tanks, ….), the propagation and aggregation of diversity becomes very important!
This effect is amplified by the fact that central heating often operates at lower power but higher flow rates / smaller temperature delta and domestic hot water heat exchangers operate at higher power but lower flow rates.
We have extended the calculation to cope with diversified central heating and domestic hot water usage, and with combination of power needed in mixed systems.

Hereby we take some principles into account :

  • diversity is a kind of “statistic“ calculation, defining the maximum percentage of simultaneous DHW tappings

  • regime changing components introduce a difference in primary and secondary volume flow, and a difference in primary and secondary temperatures,
    but they will not introduce a difference in primary and secondary diversity or primary and secondary power


Hysopt incorporates lots of diversity standards for DHW and CH.
On request we are happy to add new or additional diversity calculation methods in the Hysopt software.




To explain diversity and aggregation, we use following example model of 2 HIU’s.

First of all, we make a distinction between the calculations within one dwelling, and the calculations over multiple dwellings.


Within a single dwelling

Over multiple dwellings