Insulated old building from 1921: Conversion from gas heating

If it’s not too late yet, I would leave it. Mixed operation is not recommended. You need a buffer tank AND you have to keep it at the high temperatures suitable for the radiators.

They also had all that calculated and in the end it’s only about 3 radiators, since the bathroom was already renovated and will not be touched. I think that will be how it stays.

and in the end it's only about 3 radiators

As long as they don't only increase the flow temperature, that's fine.

One idea might be to install an air conditioning unit in the upper floor. It helps to keep a cool head up there in summer and can provide a bit of heating in winter if the radiators don’t quite manage. Then you could get by with a single, low flow temperature.

The flow temperature seems to be set to 75°. The flow temperature is set to 75 degrees, as is the case with us (we also have a Vaillant eco boiler). However, this is a gas condensing boiler. The modulation or control is handled by the heating system itself, as I assume it is an outdoor temperature-controlled heating system. The 75 degrees on the controller don’t mean anything! We have similar conditions but are energetically even worse positioned (built in 1920, double glazing, but no wall insulation). The flow temperature at 0 degrees outside is about 46 degrees in our case. This is not ideal for a (future) heat pump either. But with consistent energy renovation and the 33-type radiators combined with photovoltaic, it’s feasible.

The flow temperature is set to 75 degrees, as it is with us (we also have a Vaillant eco boiler). But this is a gas condensing boiler. The modulation or control is handled by the heater itself, as I assume it is an outdoor temperature-controlled heating system. The 75 degrees on the controller do not mean anything! We have similar conditions but are even worse off energetically (built in 1920, double glazing, but no wall insulation). Flow temperature at 0 degrees outside is about 46 degrees for us. This is also not ideal for (a future) heat pump. But with consistent energetic renovation and the 33 radiators in combination with photovoltaics, it is feasible.

That would be an explanation. The 75° is only shown on the display when you turn the controller – afterwards, the display jumps back to the displayed value, see above.

I have now first made contact with our heating engineer, who is a master craftsman and can/is allowed to perform heating load calculations. The company has always been very competent and is more the type that doesn’t try to sell you anything you don’t need. He will call me back next week, ask for rough data, and then come over and take a closer look at everything. Whether I then have a heating load calculation done by an engineer, I will have to see. I also have two people lined up for that.

Yesterday I also read some studies and watched videos on YouTube, etc., and I think you are right that a heat pump + photovoltaics is the best choice. Our house also meets the requirements. One would just have to install underfloor heating on the ground floor, which we could then have done at the same time as insulating the floor towards the basement. Whether underfloor heating also needs to be installed upstairs remains to be seen. I would definitely not want to rip open the bathrooms (from 2017) for that. Upstairs there would then only be 2 children's rooms and a small hallway left. In the attic, it also doesn’t make sense since we hardly need to heat there at all and after the children move out, it will barely be used because we will then move down to the upper floor.

I am still completely undecided about the type of heat pump.