Insulation in the 70s compared to today, modern insulation, heating costs

Ok, that sounds plausible. At least somewhat, although the answer does not fully satisfy my thirst for knowledge, since the comparison may be flawed. In an engine, friction-intensive physical processes occur during cold start, which obviously require more energy than in a warm engine. But where are those in a condensing boiler? Gas burns. Period. It does so whether the burner is cold or warm. There is no friction-intensive movement taking place that would absorb additional energy in the cold state. So where is the additional energy consumption when igniting a condensing boiler to be found?

For example, that besides the water, quite a bit of "drum-rum" is also heated? A gas burner also has its own operating temperature (these days the devices aren’t quite so simple anymore), which must first be reached for full efficiency. As a result, many starts and very short burning times are very inefficient. It’s like grilling, imagine lighting a small lump of coal for each sausage instead of lighting a big batch for all at once.

Ok, that makes sense. At least for gas. In the case of a modulating heat pump, for example, it should look different.

It is not so self-evident with cars either – when heated, the components expand, so why is there more friction in the cold state? Isn't there actually more space between the piston and the cylinder then? And synthetic oils also have low viscosity in the cold state nowadays, don't they?

In the heat pump, a compressor operates, which can now be quite well compared to a car engine (cylinder/piston - or depending on the design, rotary engine?), and these things also get warm..

It's not so obvious with a car either - when heated, the components expand, so why is there more friction when cold? Isn't there even more space between the piston and cylinder then? And synthetic oils nowadays also have low viscosity even when cold?

Think about it again. Do you really think that only the cylinder expands when heated and not the piston?