Introduction

Domestic hot water flow rates have always been challenging to calculate, because we need to take into account the simultaneous usage of hot water tapping points. Not all tapping points (like showers) will be use at the same time, so a so we need to take into account in each part of the system, the maximum probability, or the maximum DHW flow of simultaneous tapping points. These are defined in diversity standards. Most of these methods only account for simultaneous flow rates, and not for simultaneous 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

Standards

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.

DHW

• Based on DIN 1988-300

• Based on the CIBSE CP1 2020 / Danish Code of Practice DS 439

• Based on the Cibse CP1 2015 / Danish Code of Practice DS 439

• Based on the Swedish regulation DHA F:101

• Based on the French Costic MTA 2016

• Based on the French DTU 11-60

CH

• Based on the CIBSE CP1 2020 / Varme Stabi 7th edition

• Based on a logaritmic function

Overview

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

Step 1 : recalculation of the tapflows

Step 2 : calculation of the diversity factor and it's effect on the DHW flow and DHW power at DHW side

Step 3 : calculation of the DHW flow and DHW power at CH side, influenced by a regime changing component like a heat exchanger, HIU, storage tank, ...

Step 4 : aggregation of the DHW flow/power with the CH flow/power at CH side

Over multiple dwellings

Step 5a : propagate the DHW flow/power

Step 5b : propagate the CH flow/power

Step 6 : aggregation of the DHW flow/power with the CH flow/power

Example (outdated)

Consider a system of appartments, each having a satellite heat exchanger for instantaneous domestic hot water production. To keep the example simple, we assume each appartment to have a DHW heat load of 40kW with a temperature regime of 70°C / 30°C, and a 15kW central heating heat load, with a temperature regime of 70°C / 60°C. This results in following flow rates in relation to n. We use M for total mass flow.

Because all domestic hot water (DHW) units will never be active at the same point in time, we use a simultaneous flow rate for DHW, using the following formula:

m = a * M^b + c

with a = ..., b = ... and c = ...

Hysopt then computes a simulateous factor f = m_DHW/M_DHW. We then compensate for the central heating volume flow and power on units not in DHW mode, by computing the combined mass flow

m = f * M_DHW + (1-f) * M_CH

Simply taking the maximum of M_CH and f * M_DHW  would result in a flow rate which is too low. For component selection and pipe sizing, the maximum of m and M_CH is used.

To give an indication of the regime and power associated with these simulataneous flow rates, we compute

Q = f * Q_DHW + (1-f) * Q_CH and Q sizing = MAX(Q_CH, Q)

Graphs (outdated)

To clarify things further, we include some graphs for the combined flow rates and power. These graphs are for increasing domestic hot water power / flow rates at 70° / 30°C with a fixed power of 50kW heating at 70° / 60°C.