From when does the water softener start?

Hello,
we have planned an air-to-air heat pump in the new house and now the house manufacturer has recommended a water softening system to us, if only because of the warranty on the heat pump.
After numerous searches on the internet, we did not become any wiser, many opinions for but also many opinions against ....... and now I hope for your experiences ......
Thank you

Hello,
What does an air-to-air heat pump have to do with water softening? I suspect you mean a drinking water heat pump, which is additionally installed for water heating in many air-to-air heat pump systems (?).
From 20 degrees of hardness, it is certainly recommended purely for comfort reasons, below 15 degrees of hardness rather pointless. In between... a matter of feeling.
The hotter the heat exchanger, the more lime deposits; therefore, in my opinion, a heat pump should be rather uncritical in the medium hardness range. If the additional electric heating rod were constantly in use, it could look different, as it gets hotter.

When I was looking for a domestic hot water heat pump (not for heating water) back then, the manual stated up to what ° water hardness it could be used. After an initial shock, I looked more closely and realized that the specification referred to French degrees of hardness. Converted, this heat pump could handle our very, very hard water. So it was more a matter of overall perspective—do you want to constantly wipe off lime scale stains? Less of a technical problem.

Not only do the limescale spots decrease, but showering is also more pleasant. The water also simply tastes better (which apparently is not just imagination, but has also been confirmed by visitors who had no idea we have a water softener). However, the water around here is otherwise really quite hard.

We will install one, the raw water hardness is about 16-20°dH. After softening, we have about 5-6°dH. This is very pleasant, you notice it, for example, in the detergent consumption and when showering (own experiences in the parental home).

Similar to us, from about 21°dH to 6°dH. However, it certainly also depends on the starting situation whether it makes sense.