Introduction

To avoid pipe oversizing, Hysopt will aggregate and weigh the different heating applications in the network. On each pipe, the supply for

1. domestic hot water (DHW)

2. central heating prioritised by DHW (e.g. when HIU’s are applied)

3. central heating not prioritised by DHW (e.g. when electric heaters are applied)

are aggregated based on the chosen aggregation method under model settings.

Aggregation methods

Diversified design flow rates for heating prioritised by DHW, heating not prioritised by DHW, and DHW in the common pipe sections are calculated separately. To calculate the installation components (pipe sections, primary pump, storage tank, boiler, ...) it is however necessary to have one design flow rate and one thermal power. Therefore Hysopt has developed three methods to aggregate the flow rates and thermal power in the common pipes as explained below.

It is up to the user to choose any of the three aggregation methods that fits the user’s design preferences.

To select your aggregation method, go to the model settings :

Next, in the tab “Domestic Hot Water“, you can select one of the following three “primary flow rate aggregation methods”.

1. Maximum of diversified central heating and diversified DHW flow

2. Weighted average of diversified central heating and diversified DHW flow

3. Sum of diversified central heating and diversified DHW flow

Maximum of central heating and diversified DHW flow

In the graph below, the design flow rate of DHW and central heating (CH) is shown as a function of the number of apartments. The CH flow rate, logically increases linearly as the number of apartments increases (blue solid line). The DHW flow rate increases non-linearly because of the diversity we apply, as a consequence, the DHW flow rate at the upper apartments will be determining the flow rate, as the number of apartments increases the CH flow rate takes over. In this method, the maximum of both will be used for component calculation (pipe sections, primary pump, storage tank, boiler, ...). The design flow rates, thermal power and temperature regimes (CH and DHW) are shown on the labels below.

This method is always used to aggregate DHW and CH on the primary side of an HIU within one dwelling. Why? Either the HIU is in DHW mode, either the HIU is in CH mode.

Weighted average of central heating and total domestic hot water flow

In the second method the weighted average of central heating (CH) and total domestic hot water (DHW) flow is taken into account. In contrast to the first method this method takes into account that some of the HIU’s are in DHW mode and all the others are in CH mode. Simply taking the maximum of CH and DHW flow rate would result in some cases with a flow rate which is too low. Hysopt uses the diversity factor f = DHW,S / DHW,T and then compensate for the central heating volume flow rate and thermal power on units not in DHW mode, by computing the combined volume flow (V_dot_CH,DHW). Below you can see an example which is used to explain how the combined volume flow rate is calculated. On the basis of the known supply temperature, flow rate and thermal power, the return temperature is calculated.

In the graph below, the design flow rate of DHW and CH is shown as a function of the number of apartments. The CH flow rate, logically increases linearly as the number of apartments increases (blue solid line). In the case of DHW the total flow rate DHW (green dots line) is much higher than the DHW flow rate with diversity (green solid line), as more apartments are added the difference increases. As explained above, because not all apartments uses DHW (simultaneity) the HIU’s that are left uses CH therefore the combined flow rate is calculated (black dots line).

Sum of diversified CH and diversified DHW flow

In the third method, there is no distinguishment between “central heating prioritised by DHW” and “Central heating not prioritised by DHW”. The calculations thus starts from the following two separate flows:

• Diversified DHW flow

• Diversified CH flow (= central heating prioritised by DHW + Central heating not prioritised by DHW)

Unlike the second method, the third method simply combines the diversified DHW flow and the diversified CH flow to determine the combined flow rate. This approach is based on the CIBSE CP1 2020 guidelines. Essentially, the DHW and CH flows are diversified independently of one another. After calculating both flows separately, they are aggregated through straightforward addition.

The example below provides a detailed overview of how the diversified flows are calculated at the location marked: :

Domestic Hot Water

• DHW heat flow if there would be no diversity: 121.1 kW

• Diversity factor for DHW (based on Based on DIN 1988-300 for apartments): 65.3%

• Diversified DHW heat flow: 79 kW

• Diversified DHW flow (▲T_dhw≈28 °C ): 0.69 l/s

Central heating

• CH heat flow if there would be no diversity: 25 kW

• Diversity factor for CH (based on CIBSE CP1 2020): 69.6%

• Diversified CH heat flow: 17.4 kW

• Diversified CH flow (▲T_CH = 40°C): 0.11 l/s

Combined flow

• Combined heat flow = diversified CH heat flow + diversified DHW heat flow =17.4 kW + 79 kW= 96.4 kW

• Combined volume flow = diversified CH flow + diversified DHW flow = 0.69 l/s+0.11 l/s = 0.80 l/s