Theoretical model

The building behavior is modeled according to the commonly used RC-equivalent network technique, in this case, the 3R2C model variant is used as illustrated below.

The idea is to provide a rough approximation of the load profile using a minimum number of model parameters. The air capacitance (C_air) is calculated using the internal zone volume, the specific heat, and a default correction factor that takes into account convective airflow movement.

The equivalent thermal resistor (R_total) is calculated using the design load (P) from the prior building study, and the internal and external full load design temperatures (see formula below). 

The concentrated building structure capacitance (C_building) is approximated and related to the full load power by default values (heavy, medium, or light structure). Depending on the location of the insulation, the capacitance can be positioned using a θ factor which defines the location where the equivalent resistor is split to attach the capacitance.

Both the auxiliary heat from the heating system, the solar gains, and the internal occupancy gains are directly connected to the internal air node. The solar gains depend on the orientation and the window area. The external temperature Te and the solar irradiation are retrieved from a weather data file. 

Despite the relative accuracy of this RC equivalent model, it is a quite satisfying approach when studying the controllability of the hydronic heating system. Particularly, the air capacitance is important in the small time scale plant dynamics and can be easily determined. The much slower and less accurate interaction with the building structure is less important when studying hydronic controllability aspects. Also for comparing the partial load energy efficiencies of alternative hydronic designs, the real load model accuracy is often of minor importance as long as the load is representative. 

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Practical usage in Hysopt

As explained above, a zone is under influence of a couple of different parameters which can cause both heat gains and losses. By clicking on a zone these parameters can be altered

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Design load

The load needed to heat the zone from the design winter outdoor climate to the specified winter indoor climate, which can be implemented as relative (compared to the design capacity) and absolute design load.

Volume and building structure

The building structure can be changed between heavy, rather heavy, rather light, and light.

Window area, U-value, and orientation

Specifies all the information on the windows in the zone, calculated through ASHRAE data.

Internal losses/gains

Calculated through a standard table that shows the heat gain of people, devices, lights, and others, which can be changed to the needs of the project.

The values shown in the table are a percentage which is calculated compared to the “Heat losses/gains ratio of design load”. Let’s explain this through an example.

The installed radiators in the zone are 10 kW in total. It is estimated that the heat gains in this zone are 10% of the design load → 1 kW. This 1kW is now compared to the heat losses table, where 1kW equals 100%. Take line 12, at this time of day people account for 250W of heat gain, devices account for 400W and finally, lights account for the remaining 350W for the total of 1kW calculated.

During different times of day, these percentages will change. The example table takes into account that no one is in the zone during the night and hence, has a 0 place.

The graph below shows the heat gains for windows (in green, for 20 m² with UA value = 1W/m²K) and the internal gain (in red, as explained above).

Assign base circuits and pipes to a zone

You can assign and unassign base circuits and pipes to a zone, using the “assign“ and “unassign“ button on top in the zone library.

To assign :

1. press the assign button

2. click the base circuit or pipe

3. click the zone

Now the BC is assigned to the zone. if you hover over the zone, all the base circuits and pipes that are assigned to the zone are highlighted.

To unassign :

1. press the unassign button

2. click the base circuit or pipe

Why assign to a zone?

1. exchanged heat : assigned base circuits and pipes will exchange heat with the zone (if applicable). E.g. a radiator assigned to a zone will deliver heat and make the zone temperature change. But be aware that not all assigned base circuits exchange heat with their environment.

2. validation and generation of BIM objects in the Hysopt BIM Feeder : by assigning a base circuit or a pipe to a zone, we know where the object is located so we can generate (or validate) the corresponding BIM object in the corresponding Revit space.

When to assign to a zone?

Some base circuits are obliged to assigned to a zone, like for example radiators.

Some base circuits are not obliged to assigned to a zone, like for example heat pumps and pipes.

Base circuits
The typical heat exchange behavior of base circuits that are not obliged to assign to a zone is described here : Heat pump | Which ambient temperature is used for ASHP’s?

Pipes
The typical heat exchange behavior of pipes that are not assigned to a zone, they exchanged heat with the environment temperature as defined in the pipe insulation :