ᐅ Air-to-water heat pump for underfloor heating and separate heat pump for domestic hot water

The separation tank is exactly what you call a "serial" buffer.
KfW certification or energy saving regulation proof is not relevant to me, as I am from Austria.
The separation tank is used in my case because of the "smart grid" function of the heat pump combined with photovoltaic panels, to maximize the self-consumption of electricity.
Without photovoltaic panels, I agree with you that any kind of storage in the heating circuit makes little sense.

merlin667 schrieb:
The storage tank you refer to as a "serial" buffer is exactly a stratified buffer tank....

Sorry, from a technical perspective, there is a world of difference between a stratified storage tank and a serial buffer tank. Initially, not mentioned "photovoltaic influences" significantly affect sizing parameters. Perhaps you should discuss this in detail with your HVAC engineer. It is much more effective to clarify these basics beforehand than to attempt corrective measures later on. The commonly promoted "smart grid" function of heat pumps requires that the instantaneous photovoltaic output is made available for immediate use. Depending on the photovoltaic system’s design, significant limitations may arise. During the photovoltaic-rich summer months, only the power demand (not the energy demand) for hot water production can be considered, as heating is not needed. In "winter operation," the "smart grid" function is unlikely to be very helpful. Usually, the photovoltaic power available for direct use is far from sufficient. The transitional period, which is strongly influenced by location and user behavior, is critical. It is very helpful to overlay the immediately usable photovoltaic power with the actual power demand/requirement for heating and/or hot water to illustrate the real benefit of the "smart grid." The overall concept must therefore be consistent. For buildings with identical external dimensions but different insulation standards and locations, the economic conditions are likely to differ completely.

Best regards.

€uro, thank you for your input. We developed this concept together directly with the heat pump manufacturer, our energy consultant, and the planning/execution company for the electrical installation.

It is clear that the "smart grid" is not the solution to all problems (surplus/shortage of self-generated electricity).

Our local climate is somewhat particular. Although I am located significantly further south than you (area Austria Slovenia / Italy), it has a positive impact on photovoltaic yield.

However, temperatures of -22°C (−8°F) in winter and +37°C (99°F) in summer are not unusual (these temperatures occur roughly every other year).

merlin667 schrieb:
...It is clear that the "smart grid" is not the solution to all problems (oversupply / lack of self-generated electricity).

Absolutely correct, such "ultimate solutions" basically do not exist, even if some sellers claim otherwise!
The development teams of manufacturers are not fundamentally incompetent; it would be unfair to give colleagues a negative assessment here, which would certainly not stand from a technical perspective.
However, what sales departments ultimately make out of scientific and technical standards is a completely different level/league. The engineers of the original value creation have absolutely no influence on this!

merlin667 schrieb:
...our local climate is "somewhat" special. Although I am located much further south than you (area Austria Slovenia / Italy), it does have a positive impact on photovoltaic yield.
However, -22°C (−8°F) in winter and +37°C (99°F) in summer are not unusual (such temperatures occur about every other year)

Consistent conditions are technically much easier to manage than extreme boundary conditions.
Short-term hourly extreme values are much less problematic than prolonged, sustained drops below the design temperature.
It is no coincidence that, in my planning in Germany, I do not rely exclusively and fundamentally on the HLB design temperature, but on individual, site-specific outdoor temperature profiles from the last 10 years. It is sometimes surprising what particularities emerge in the final result here.

Best regards