Internal heat pump in the basement not possible due to groundwater?

@

if you don’t understand the relevance of refrigerants (boiling point), you also cannot understand that outdoor versus indoor heat pumps are two different heat pump concepts. The term split does not only mean that the condenser is usually located indoors, but that a heating element/electric, wood, oil, or gas heating is required as a second heat source, because the split heat pump cannot achieve high temperature ranges.
Which at the same time stands for bivalent operation.
You also cannot simply replace the refrigerants, as there are technical differences as well as physical differences (e.g., bivalence point, limit temperature) due to the heat pump concepts.

Usually, split heat pumps are operated bivalently with a second heat source, which is often an integrated heating element that is indirectly also needed for hot water supply.
Except if a split heat pump is operated, for example, as a hybrid with, for instance, a tankless water heater and is sufficient for space heating. Then it is only operated “monovalently” in the same way as the tankless water heater is operated monovalently. There are two separate heat sources.
Note, the annual performance factor always includes all heat sources, which also applies to the heating element; it is often found that only the heat amount for space heating is measured.

This also explains the efficiency difference, because outdoor units are usually only operated monovalently and do not require a second heat source.
Important for all water-bearing heat pumps are modern stratified storage tanks, of which there are unfortunately just a handful.
Test results can be found publicly at the Swiss SPF Institute for Solar Technology.

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Reference is always only to residential buildings without cascades and no air/air heat pumps

Outdoor versus indoor heat pumps are two different heat pump concepts. The term Split not only indicates that usually the condenser is located indoors,

Yes

but that a heating element/electric, wood, oil, or gas heating is needed as a second heat source, since the split heat pump cannot achieve the high temperature ranges.

No

A heating element/electric, wood, oil, or gas heating is needed as a second heat source, (...) which simultaneously stands for the bivalent operation.

Yes

Of course, split heat pumps can be designed and operated monovalently. Depending on NAT or the preference for the legionella program, however, this is not always advisable.

@
Laying out is only possible if there is a hybrid operation, i.e., an external 2nd heat generator. (See DIN 18599)
What someone operates inefficiently or in a health-damaging way is their business. As long as the germs do not enter the public network.

In private residential construction, you will hardly find a split system that is operated monovalently,
because economic efficiency is also considered, also because no one knows where electricity prices are headed.
Especially due to the newly emerging dynamic electricity tariffs. Because the periods of electricity consumption are rarely freely selectable.

Ultimately, it is about the ratio of electricity consumption in kWh to the amount of heat generated in kWh.

I cannot follow your argumentation and strongly disagree. Of course, one can design and operate monovalently regardless of what any DIN standard requires.

And I found 2 split systems in our development area that are operated monovalently (up to NAT).

Okay, I can partially understand the reasons that speak in favor of an interior installation.

Have you made any progress with the manufacturer regarding the question of a wall breakthrough in the presence of pressing water?

What does your general contractor say about the conflict of interest?

If there really is no permissible option for sealing and your general contractor objects, you have to accept that for now. Because the groundwater level does get pretty high. What will it be like in the next 80 years? Weather phenomena tend to become more intense. I would also play it safe there.

On our 300 sqm property, there is also a rather suboptimally placed outdoor unit, but that was still the "lesser evil" for us.

Tell me if there are any new insights?! Maybe you’ll get some good advice if you show a bird’s-eye view of your floor plan of the basement and property?! We also received great tips this way that we hadn’t thought of ourselves.

@OWLer
As soon as germs enter the public network, you are committing an offense.
See the prohibitions regarding toilets with popodusche. Nowadays, germs can be traced back to an apartment.

Apart from the fact that operating a split system like this is inefficient.
Germany has simply missed out on modern heat pump technology here; our neighboring countries have been ahead for years,
even though electricity costs are much cheaper there.
However, the BEG subsidy has now responded to this. If only natural refrigerants are allowed in the future, the issue of split heat pumps
and thus indoor heat pumps in new buildings will be history.