Principles
To simplify the situations are reduced to bivalent systems, with a source more virtuous (geothermal/hydrothermal) and a aerotermica less virtuous. In general, however, this does not is a  restrictive pattern, because of done it can insert several source systems who work together. The control acts considering a hysteresis on the source side to determine the limit below which triggered and the source more virtuous.

Since the virtuous source is, in this case, a source for the definition "benefit not guaranteed", the exchange capacity tends to decrease until it meets a too low value to enable its operation. Typically this value is a few degrees above the water freezing with not glycol. In special cases, such as where it is expected glycol (use of solar collectors) point SWAP can be lowered.

Design
►The source less virtuous but more available should be have an exchange capacity equal to the heat pump.
►The most convenient source can have any capacity keeping in mind that if it is higher it may be convenient to think of a reservoir. If it is much lower than the SWAP it will still be limited by the control.

Application
►Heat recovery from industrial waste
►Water with flow rates not guaranteed
►Solar collectors
►Polyvalent systems