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, ...

Owned by Kristof Smits
Last updated: Oct 25, 2022
3 min read

Example

We explain the calculations based on following heat exchanger, in which :

  • the secondary side is a fully domestic cold and hot water circuit

  • the primary side is a fully central heating circuit

  • both circuits are connected with a heat exchanger

Calculations

 

step1 - step 2

First we calculate the diversity flow and the diversity factor, as explained in Step 2 : calculation of the diversity factor and the diversity flow

step 3

Calculate the secondary power, based on the formula (in which m is the massflow, T1 the cold water temperature, T2 the hot water temperature, and c the heat capacity for water)

step 4

Calculate the primary return temperature.

In case of a heat exchanger, both variables are determined by the UA value of the heat exchanger, and 3 equations that apply (see further).

In case of other regime changing base circuits like a storage tank, the primary return temperature can be a design parameter chosen by the user).

The primary supply temperature is always a design parameter chosen by the user.

Now that we know the primary return temperature, we can calculate the primary flow.

In the example above, the primary return temperature is calculated and results in 28°C. The primary flow is also calculated and results in 0.58 m3/h

step 5

The primary power and primary diversity factor are not impacted by regime changing base circuits, this is actually obvious :

  • first law of thermodynamics : the power at primary and secondary side is always the same (assuming no thermal losses)

  • the diversity factor is a “probability of simultaneous tappings“ which is obviously not impacted by components in the network

It is correct to state that the diversity power = diversity factor * total power

It is NOT correct to state that the diversity flow = diversity factor * total flow
This is only correct at secondary side, but not at primary side.
At primary side the diversity flow is determined by the heat exchanger, not by the diversity!

Heat exchanger

In the example above (in which the UA value of the heat exchanger is known) a system of three equations determines the primary flow and primary return temperature. In all equations, the power P is equal.

  1. system equation at secondary side

  2. system equation at primary side

  3. heat exchanger equation in between :

 

 

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