"Primary - No Recirculation" is used for sizing a primary plant only.
A "Swing Tank" design is a proven technique to use the primary heat pumps to support the temperature maintenance load, while keeping the heat pump equipment isolated from the warm water returning from the recirculation loop. This design strategy is best suited for buildings with low temperature maintenance loop losses (< 60W/apt) and relies on increased storage volume (with tanks piped in series) to ensure storage stratification. Swing tank systems have an electric resistance element in the temperature maintenance tank as a back-up safety factor. Sizing a swing tank system also means increasing the heating capacity and storage volume of the primary system. The temperature maintenance storage volume for the swing tank can be small.
Single-pass heat pump water heaters are most efficient when heating cool city water to hot storage temperatures, whereas multi-pass equipment can still operate efficiently when incoming water temperatures are around 120F. A parallel loop configuration is one strategy used to isolate the temperature maintenance task from the task of heating the primary storage. A "Parallel Loop Tank" is an electric resistance element or a multi-pass heat pump that is piped in parallel with the primary system, specifically to handle the temperature maintenance load.