New heat pump for old buildings without renovation

I have a KFW55 house with solar hot water and gas (8600kwh gas/year heating+hot water) and underfloor heating. The new 6.8KW split air conditioner with 4 indoor units currently serves as heating. The issue of peak load coverage is one thing, but I will also turn off the air conditioner at -10 degrees and rather fire up the gas. It is optimal: The gas heating then runs nicely in the optimal condensing range. The heat pump of the air conditioner runs efficiently at moderate temperatures. I can calculate when what is worthwhile with the help of my electricity and gas tariffs.

I just want to note here that your gas heating should run in the optimal condensing range at any time during heating operation. If not, there would be something seriously wrong.

1. You build yourself the classic (cheap) property developer/GU variant with gas + solar -> Cheap? It’s a proper Viessmann system with underfloor heating.

I said "cheap," not bad. Gas has always been cheaper to purchase than a heat pump.

From the start, you should have gone for air conditioning as heating, domestic hot water heat pump, or central instantaneous water heater for hot water and photovoltaics. -> I built in 2013. Back then I still had gas costs around 28-40€ per month, so manageable. Air-to-air systems were also not as efficient as they are today.

I know, hindsight is always 20/20 and easy to be a smart aleck. However, air conditioning was already efficient back then and installed millions of times. Today, such a system could be perfectly planned into a new building. Install indoor units in the walls at suitable places. Save on underfloor heating. Etc.

As a consultant, I always advise against newly installed solar thermal systems. The effort-to-benefit ratio is no longer worthwhile nowadays. Existing systems can of course remain, as that is better than nothing. For new projects, I clearly recommend (manufacturer-independent) a photovoltaic system with a heat pump. In general, these variants always share the same common disadvantage already mentioned here: the initial investment is very high. For solar thermal systems, one should not forget the ongoing electricity costs and the sometimes very high efficiency losses. The ROI should be under 10 years for a financially profitable implementation (excluding the green mindset and hobby aspects). For implementation, a project-specific decision must be made as to which heat pump makes sense, whether a hybrid solution is more sensible, and whether it will be only a heat pump. The heating demand as well as the hot water demand are decisive factors. In general, I like that all these discussions here are already going in the right direction. Between air-to-air and water-to-water heat pumps, the comfort factor still needs to be considered. For example, a warm floor is a different kind of heat than warm air from a blower. Therefore, there is no universally right or wrong solution here. P.S.: I consult as a graduate engineer on a part-time basis, 100% manufacturer independent, with my focus purely on efficiency and financing. I don’t have to sell anything, as it is a hobby and service for a good cause for me.