Vestaxx window heating - experiences?

what happens if the heating in the window breaks. Does that mean a new window is needed?

When neighbors of ours moved in, there was a leak in the underfloor heating shortly after, and the entire ground floor became uninhabitable. They were accommodated in a hotel for several weeks and everything was redone. I’d rather replace a broken window. ;) Without innovations, there is no progress. Why shouldn’t there be anything new in this area? I personally find the system very interesting at first and am basically open to it. Whether it makes sense to heat an entire house with it remains to be seen.

Disadvantage: For hot water you need an additional device, ideally a domestic hot water heat pump.

Exactly, in this case the wooden house builder combines the heating with a domestic hot water heat pump. That is added on top. The company follows the strategy of investing the saved costs into a large photovoltaic system so that the heating and heat pump can be powered as much as possible by self-generated electricity. I still need to study the topic in more detail. But at first glance I found it very exciting if it really works as advertised. And whether it is then used throughout the whole house or only in some rooms remains to be seen. Today we even walked through the office building looking for an office where the heating was on. At least all the rooms were nicely warm.

With neighbors of ours, shortly after moving in, there was a leak in the underfloor heating and the entire ground floor was uninhabitable.

Well, of course you can do everything wrong. Otherwise, a leak is usually repaired within a few days (most of that is drying times, so no working time) without having to relocate the residents.

The idea of heating through windows offers a combination of radiant heat source with architectural and design freedom. It is certainly also great for retrofitting in insulated existing buildings without water-based infrastructure. The heating costs are probably not optimized to be inexpensive, I estimate the durability to be long due to the simple low-movement technology, the handling should be super simple, and it is certainly low-maintenance. Neither different nor new is initially "better" nor "worse," but it sometimes seems to be perceived as a threat and often provokes reflexive rejection.

The heating costs should not be optimally cheap,

A purely electric heating system cheap? How so?

What does the energy certificate for the KFW say about the heating energy demand?

If I read mine correctly, with my 55 standard building I need 11,000 kWh per year. With a COP of 4, that is between 3,500-4,000 kWh of electricity. This here is a direct electric heating system. You really have to calculate very carefully for the next decades. The risk must also be taken into account that the concept will not be accepted in the market and that there will be no spare parts in 15 years. For underfloor heating there is always a heat source to connect to.

Solar really has to be very large-scale. Yesterday, from my East-West 10 kWp system I only got 1,412 kWh. That doesn’t even cover my standby consumption in the house without heating operation.

A pure electric heater cheap? How come?

Exactly!

This is a pure electric direct heater!

It therefore has the same heating effect as an electric heater in the room (thermoelectric heat equivalent).
Electric direct heaters have by principle almost 100% heating efficiency because 100% "waste heat".
Possibly somewhat worse at the glass, because – as mentioned – about 8% of the heat is lost outside.

The disadvantage is (in principle) the quadratically decreasing heating effect in radiant heating:
at double the distance, the heating effect is only 1/4.
Unless the radiation hits solid bodies that warm up and heat the air by convection.

Such heating myths regarding electric savings have been spread by interested parties for 40 years.