Is a multisplit air conditioner suitable for heating?

very good air conditioners are available for 500-700€, good ones already from 300.-- You can install them yourself or you know someone who knows someone.....

But they still have to be developed for that price. 300 € :D

I know someone who orders cheaply in Italy and does not come to these prices. Doing it yourself is forbidden in Germany and not exactly trivial. If you have it done, you pay 1000,-€ per kW. Currently probably more.

Roughly speaking then 5 kW = 5000€ including or excluding subsidy?

Let's say the subsidy is taken into account, so a total of 5000€, no heating installer would install a 5 kW bivalent air-to-water heat pump in an existing heating system for me in my life. And presumably, the hydraulics would be even more botched than before.

And if they do, that would barely be enough without adjusting or supplementing the heating surfaces to achieve lower flow temperatures. The option to cool in summer would probably also not be available.

Exclusive funding.
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One more thing about the prices. I had 3 cheap air conditioners for 300-400.-- (Praktiker -25%, even cheaper on ebay) of which the first one broke down after 15 years. I have now replaced it with a converter system for 700.--. The oldest of the cheap ones is now 19 years old. I do the installation myself, commissioning costs me a case of beer.

one more thing about the prices. I had 3 cheap air conditioners for 300-400.-- (Praktiker -25%, even cheaper on ebay) of which the first broke down after 15 years. I have now replaced it with a converter unit for 700.--. The oldest of the cheap ones is now 19 years old. I install it myself, commissioning costs me a case of beer.

You still lose out compared to a good split system.
The operating costs, especially now at >45 cents/kWh, should not be underestimated if you want to keep your house nicely cooled.

You still end up paying more compared to a good split system.

I believe has indeed installed split air conditioners as well. Just cheap ones.
And in terms of efficiency, they are not that far off from the expensive brands. You won't make back the extra cost for a Mitsubishi.

And electricity only costs 45 cents for a few. The average is 36 cents. With photovoltaics it's then 10.