Which heating? Please provide recommendations

Do a heating load calculation and get advice from someone who knows what they're doing. An [LLWP] is likely only a good choice very rarely in the long term. If a gas condensing boiler without extras can be approved, that would certainly not be a bad choice, but then it is probably worth taking a look at an air-to-air heat pump again...

We have a Nibe750 exhaust air heat pump with decentralized ventilation (+ underfloor heating). I cannot understand why this type of heating system is always spoken of so badly (especially by people who have not installed the system themselves, but only spread hearsay). We have a solid house with 160 sqm, a heating load calculation was carried out. The system was set up on site during a 4-hour training directly by Nibe. Even now in winter at -10, the electric heating element has not turned on once. I cannot confirm the statement of the energy guzzler. There is always fresh air in the house, no odors ever linger, and there is a pleasant, consistent temperature. I do not want to do without that anymore :) We previously lived in a semi-detached house with a gas condensing boiler, and it was immediately clear to us that we did not want that at all for our own home. You can probably ask 10 people and get 10 different opinions :)

We have a Nibe750 exhaust air heat pump with decentralized ventilation (+ underfloor heating). I can't understand why this type of heating system is always spoken of so badly (especially by people who haven't installed the system themselves, but only spread hearsay). We have a solid house with 160 sqm, a heating load calculation was carried out. The system was set up on-site by Nibe itself in a 4-hour training session. Now in winter, even at -10, the electric heater has not turned on once. I cannot confirm the statement about it being an electricity guzzler. There is always fresh air in the house, no odors ever linger, and there is a pleasant, even temperature. I wouldn't want to miss that anymore :) Before, we lived in a semi-detached house with a gas condensing boiler and for us it was immediately clear that we didn't want that at all for our own home. You can probably ask 10 people and get 10 different opinions :)

What insulation standard do you have? Approximately where do you live? (The last winters here, in NRW, have been very mild). So you have an exhaust air heat pump that heats the underfloor heating with the energy from the exhaust air? So not an air-to-air heat pump, but an air-to-water heat pump? Decentralized ventilation means in your case that cold air flows in the amount that the exhaust air pump sucks out? How high was your electricity demand exactly for the last 365 days?

So, the 750 is definitely an awesome device, no question. And a lot of nonsense is written about it, especially in forums, particularly by people who have no experience with it. The main problem we have in Germany is our energy saving regulation. Thanks to many options and adjustment screws, you have the possibility to turn a slow horse into a racehorse. Comparisons have been made many times between the individual heat pump systems. But in Germany, no equivalent houses were compared, it was just a stamp eff xyz and then the comparison. If I now really compare equivalent houses, then I come to a difference, for example, between 750 and 1225 (as a geothermal heat pump) of about 8 kWh/m²/a. For 140 sqm about 1100, I don't really feel like calculating, 250 bucks per year.

Now let's look at the investment costs in contrast. The 750 is about a thousand more expensive than the geothermal heat pump. But added to that now comes: heat source, then a controlled residential ventilation (admittedly, that's the catch) but the difference is about 10 grand.

Now you can calculate for yourself when the extra effort pays off...

And that the heat pump salespeople are nuts about their prices, that’s not even worth discussing :D

However, one must also mention the many disadvantages of the Nibe / exhaust air heat pumps, there are many negative reports from people who actually have one! Your calculation is so much of a guess... If you have poor insulation and are in a cold region (often and for a long time) with a heating load of 6 kW at -10 degrees, how is an exhaust air heat pump supposed to heat effectively? It has to extract 6 kW of power from the indoor air (extreme air volumes!) and at the same time cold air flows in through the inlets, which must be additionally heated (requiring even more power). This can only work reasonably well if the house is very, very well insulated. One of my neighbors has better insulation than I do, but with the same area much higher costs and is unhappy with the draft. That is an empirical value.
Therefore, when recommending exhaust air heat pumps, it is incredibly important how and where the building is supposed to be constructed. Generally, I would rather advise against it.

This is how it looks... it's simple physics, you don't necessarily need experience with the system... an air heating system is great in Spain... here with us only with additional measures