Hybrid heating: Is a heat pump with a gas condensing boiler sensible in old buildings?

Unfortunately, the current owner cannot tell me exactly what the heating load is. I can only determine it based on the data.

She currently still has this old radiator heating. In January and in February the heating was set to 3 and it was already quite warm. The heating is a Bruderus Logana G 105 from 1988, energy class D.

So, in summary, you could say it’s best to consult an energy advisor, insulate the facade and roof, install triple-glazed windows, underfloor heating upstairs and downstairs, and then a heat pump definitely makes sense, right?

Are you doing the flooring anyway? Then I would install underfloor heating. If not, it’s quite a lot of effort. As you describe it, that is the absolute ideal condition.

As long as it is only the healthy doubt of laypeople without any knowledge, everything is fine. There are entire institutes that deal exclusively with that, they must know.

There are entire institutes for many things that have dealt with them full-time and did not see the crash coming. Mortgage-backed securities and the financial crisis triggered by them were such an illusion of knowledge and control. Even in mid-February 2020, no expert seriously predicted what Corona would mean, and no one had the supply chains destroyed by it on their radar either. The central banks—recognized experts in inflation—are currently realizing that inflation is not as temporary as claimed for almost two years. And obviously, a month ago no one foresaw that because of the Ukraine war, nails, tiles, and roof shingles would become scarce. The vast majority of "experts" were completely clueless in all these cases. I'm sorry, but experts are also just clueless people with a lot of expertise when it comes to the future. A look at past predictions unfortunately confirms this far too often. And on top of that, many experts are like weather vanes. The statements of the Leopoldina over the past two years are a great example of this. There was nothing to be seen of independent expertise and critical thinking there. And yes, is right, it is important to get started. But just as important is to choose appropriate solutions instead of, like Procrustes, simply imposing one solution on all problems. Because only solutions that are moderate, sensible, targeted, and thus also economical will find lasting acceptance. At least that is my liberal illusion of society ;) So, that shall be it from my side on the topic now. I rather do not want to usurp the thread with this.

It even makes sense with less extensive measures - as you describe it, it would be perfect. (Maybe also photovoltaic panels on the roof if it is going to be covered anyway.)

Yes, photovoltaic panels are still planned, but the roof faces east, although it has a 40-degree pitch.

Are you doing the flooring anyway? Then I would do underfloor heating. If not, it is quite a lot of effort. As you describe it, it is the absolute ideal condition.

Yes, there is still carpet laid and that should of course be replaced. Floating screed. And then underfloor heating upstairs and downstairs.

However, if I want to have windows subsidized, I have to install triple-glazed windows. And if I do that, I also have to insulate the facade.
Or does a heat pump “pay off” with the existing facade and only double glazing?

3-layer windows are not a problem, even with uninsulated facades. Mold is then 99% a ventilation and heating problem. If you are installing underfloor heating anyway, then eliminate thermal bridges consistently. Increase the insulation on the upper floor ceiling to 20cm. Then the heat pump will work.

You don't necessarily have to do the facade and windows together, do you? What is the condition of the facade and windows? Age? And are they insulated at all?

The facade and windows don't necessarily have to be done together, right? What is the condition of the facade and windows? Age? And insulated at all?

The facade actually still looks good, no cracks, all white. The roof overhang is about 1m.
Year of construction is 1973, but well maintained.

The U-value of the facade is 0.97 W/m2k, it consists of:
Calcium silicate brick (115 mm)
Styrofoam (20 mm)
Calcium silicate brick (175 mm)

And additionally partially slate.
However, it should be noted that there are very large window areas; the entire south side is almost only window area, top and bottom.

The windows are made of wood and insulating glass, so they should definitely be replaced.

The top floor ceiling still has 10 cm of rock wool. Otherwise, the ceilings are made of reinforced concrete.