Consumer model selection
Decision aid: simple heat exchanger vs. detailed transfer station - comparison of the models, use cases and prerequisites
Overview
For the transfer of heat to the buildings, two consumer models are available: the simple heat exchanger and the transfer station. This page compares the two and gives decision criteria for practical use.
Comparison
| Aspect | Simple heat exchanger | Transfer station |
|---|---|---|
| Model approach | heat flux = building demand as boundary condition, no transfer physics | counter-flow heat exchanger with UA value and secondary-side balance |
| Transfer capacity | unlimited (optionally capped by heating-curve return) | physically limited by UA value and temperature gradient |
| Network return temperature | follows the controlled spread (assumption) | results from the transfer physics and the secondary side |
| Behavior at too low a supply temperature | continues to extract the demand (or switches off hard with heating-curve limitation) | power decreases continuously and for physical reasons |
| Heating curve of the building | optional (only for the temperature limitation) | required (defines the secondary setpoint and the spread) |
| Parameterization effort | low | medium (log. temperature difference, minimum modulation, heating curves per building) |
| Computation effort / robustness | very well-behaved | somewhat higher, more sensitive to unsuitable heating curves |
| Sized variant | yes | yes |
Recommendation
Simple heat exchanger – the standard for network sizing:
- The network is planned so that the supply temperature is always sufficient; of interest are hydraulics, heat losses and generator loads.
- All buildings are considered with a uniform design spread.
- Early planning phases in which the heating curves of the buildings are not yet known.
Transfer station – the model for reliable statements on temperature behavior:
- Undersupply scenarios: What happens with a generator failure, a lowered supply temperature or load peaks? The deficit outputs show which buildings are undersupplied, when and how severely.
- Networks with different spreads: buildings with underfloor heating (e.g. 35/28 °C) and old buildings with radiators (e.g. 70/55 °C) in the same network – each station works against its own heating curve, the network return temperature results realistically from the mixture.
- Temperature reduction: investigations of how far the network supply temperature can be lowered before individual buildings are no longer supplied.
- Return temperature analyses: realistic return temperatures as a basis for generator efficiency (condensing use, heat pump COP).
Prerequisites for the transfer station
The significance of the detailed model depends on the quality of the input data:
- Set the heating curve per building correctly – it must match the heating system of the building and match the network supply temperature: the secondary supply setpoint must lie sufficiently below the network supply temperature even on the coldest day, otherwise a permanent, parameterization-induced deficit arises.
- Choose the logarithmic temperature difference realistically (station datasheet; typically 5–10 K).
- After the simulation, check the deficit outputs to distinguish parameterization errors from real network bottlenecks – see Transfer station.
Both models can be mixed in the same network, e.g. the transfer station only for critical buildings at the network end.
In practice
You do not have to decide globally: compute the network with the robust simple heat exchanger and deploy the detailed transfer station in a targeted way only at the critical buildings – typically the most distant ones or those with the highest required supply temperature. This way the model stays well-behaved and delivers reliable temperature statements where it matters.