1. Since I don’t think the additional investment is worth it, I am currently of the opinion not to pursue KfW financing, but maybe I will be proven wrong now :)
2. From what temperature do I heat entirely with electricity when using an air-to-water heat pump?
3. Would a combination of controlled residential ventilation and an air-to-water heat pump (Viessmann offers something like this) be a good choice, and power the air-to-water heat pump with a photovoltaic system?
4. Or is it more economical to rely on a condensing gas boiler and possibly expand it with a small air-to-water heat pump powered by photovoltaic? (My heating engineer advised me on this setup)
Good morning!
Regarding 1.: Your finance specialist/financier needs to calculate or compare this for you. Generally, one cannot say this directly as it depends on many factors. A tip here would not be really professional.
Regarding 2.: You probably mean from which point the air-to-water heat pump requires the electric (auxiliary) heating element! This depends on the system itself (manufacturer, type), environmental conditions (weather), the heating and hot water demand, and the house itself. A small example: At -10 degrees outside temperature, the heating forward temperature of 32 degrees may still be produced, but hot water at 57 degrees is then too much. The next day, the air dew point is such that the outdoor unit threatens to ice up, and here the heating element also comes into play. My boss has an (8-year-old) air-to-water heat pump in his house. From about -3 degrees, that old unit then runs on the heating element. Today he regrets having chosen that system back then because very high electricity costs occur. His house is quite similarly insulated as mine but he has almost double the monthly heating costs compared to me (gas condensing boiler). He cannot get more than 42 degrees hot water in winter without the heating element turning on/the unit (outdoor part) icing up. However, there have been considerable technical improvements since then; air-to-water heat pumps have become significantly better! Over the year, with a good system, you now have a COP > 3, rather 3.3 - 3.5. Thus, operating costs are roughly on par with a gas boiler at today’s gas prices, even if the auxiliary heater sometimes needs to turn on for a few days (-5 to -9 degrees for heating and higher temperatures of about -5 degrees for hot water are, I think, a very rough *guideline*).
Regarding 3.: Opinions will differ here again. a) About the photovoltaic system, in my view: Yes, it definitely makes sense in combination with an air-to-water heat pump (summer, autumn, spring). Whether it delivers significant contributions in winter depends on location/weather, snowfall... Now, I am actually a fan of photovoltaic systems with a high self-consumption rate (I will install one myself), so my judgment/advice here is not entirely objective. And concerning b): The combination of air-to-water heat pump with controlled residential ventilation is often offered. It is quite appealing as it saves space. Personally, I do not consider such a combined device optimal and would use separate devices myself (if I had an air-to-water heat pump), but that’s a matter of taste. However, I would never want to do without controlled residential ventilation, although this is also a matter of personal preference.
Regarding 4.: Seen this way, it is certainly a clean and nice solution that pleases the technician’s heart. With one small downside: high investment! Taking a gas connection (+2k), buying a burner (+3k), installing both systems and integrating them (not to be underestimated!). Provision costs for the gas meter will also come on top of the second electricity meter (180-200/year, high basic costs with probably little gas consumption, for that money alone you can buy quite a bit of electricity). The heating engineer is pleased, the electrician too, with the several thousand euro additional turnover. In the short term, you will very likely not recover these extra costs, especially as a modern air-to-water heat pump has a COP of over 3; the annual average costs interest you, not a few days of hardship in Bavarian snowy weather.
My personal advice to you in summary would be:
- Good air-to-water heat pump with high COP from a brand manufacturer
- Photovoltaic system (possibly small, just for the air-to-water heat pump), and if you like and have fun, a larger one at the same time, with a small battery buffer as the icing on the cake (better spend the money from point 4 on something like this)
- Controlled residential ventilation with heat recovery and air-to-water heat pump as separate systems
Best regards, have a good start to the weekend
Thorsten