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.