Heat pump for KfW55 house 148 sqm

The standard outdoor temperature for our postal code is -12°C

The Vaillant aroTHERMplus is a monoblock, if I understood correctly.
Monoblock certainly doesn't work, because the heating coils, refrigerant lines, and screed are already in the house.
Everything is prepared for a split system, can you still switch to a monoblock there?

I am confused about the term "bivalent."
Some describe bivalent as the ability to use two heat generators, that is, besides the pump also solar or an already existing boiler.
Then I read that the bivalence point is the temperature at which the heat pump just barely has the specified performance to heat the house to the desired temperature. If the outside temperature is lower, the electric heater switches on additionally.
Is that correct?

What does it mean to design bivalent?

And one more question about the heating load: This is calculated for us at -12°C. Does that mean: at -12°C outside we need 5,some crushed kW to heat our house to 20/24°C???? Does it mean at -3°C less kW are needed???? Even less at 5°C????

So, I admit to being a heat pump dummy
but that's what you are here for ;-)

Then I read that the bivalence point is the temperature at which the heat pump just barely has the specified capacity to heat the house to the desired temperature. If the outside temperature is lower, the electric heater kicks in additionally.
Is that correct?

Yes, exactly. And since reserves are factored in, solar gains are neglected (when it’s -10 degrees, the sun often shines during the day), the point is often lower in reality.
Furthermore, this case rarely occurs because your house does not immediately cool down if such temperatures last only three hours at night. You make up for that during the day. The case where this temperature is undershot for several days is very rare.
And when it happens, the electric heater helps. But since this almost never occurs, it’s not a problem.

And one more question about heating load: This is calculated for us at -12°C outside. Does that mean: at -12°C outside we need about 5.whatever crushed kW to heat our house to 20/24°C???? Does it mean at -3°C less kW is needed???? Even less at 5°C???

Exactly! That’s why a bigger pump is doubly bad. At 5 degrees you might only need 2 kW. But the pump can’t go that low and then cycles.
That’s why you size for 90% of the time. Not for the 10%.

Edit: Bivalence then means in this case 1. heat pump 2. electric heater (integrated in the heat pump)

Monovalent would mean covering everything only with the heat pump.

Monovalent would just cover everything with a heat pump.

Which is sufficient in 95% of cases, however.

What is sufficient in 95% of cases, however.

Correct - if the heating load is calculated correctly. But as already mentioned, if the suspenders are included along with the belt, it is sufficient on paper, but like here, not with the smaller one. Then just tell the heating engineer to design it as bivalent.

Regarding the thermal protection verification of the KfW55, only the simplified building heating load is determined and not a sum of the room heating loads according to DIN EN 12831, which can deviate by up to 30%. That is TGA and not thermal protection. However, it should also be considered whether the KFW55 was still calculated according to the old DIN or the current DIN 18599 and whether the thermal bridge allowance was also verified or simply assumed. The simplified procedure does not include any interior spaces but only the building envelope and standard temperature.

Monovalent heat pumps are often far superior to split systems, but German manufacturers have missed this,... important here is the stratified storage tank in interaction. The systems, especially of the one manufacturer mentioned, have monovalent bivalence points from -15° to -20°. The heating element is only used in case the heat pump fails due to a defect. Our German-speaking neighbors, with well-known winter sports areas, offer corresponding studies and evaluations of systems such as stratified storage tanks.

A well-designed heat pump and underfloor heating do not need a stratified storage tank. From the heat pump directly into the underfloor heating. The high volumetric flows of a heat pump are not always conducive to the trouble-free operation of a stratified storage tank. ERR must of course be dismantled then.