Inside the house 10° warmer than outside - how does that happen?

because what use is exchanging air with the outside if it pushes the air back in almost as warm as it let it out?

If the technology were that simple, you would have freezing cold air currents in winter.

Could that be caused purely by the sunlight through the windows?

Yep. Already spilled the beans here in the forum recently with the first rays of sun

How do you other "new build owners" solve the problem? Shading?

For example, yes. Or enjoy the warmth ;) In this weather, you hardly notice what's going on inside :p

but we have roller shutters, we could close them if necessary. But would that help?

... try it out?!

I put it in the box from the summer cassette and that's it. It doesn't look so delicate that it would just break like that

We have a thermometer in the room. We have the LWZ 604 air. Up to the underfloor heating connection, it's relatively similar in construction. From there, I have no idea.

Find out where the system takes the 20 degrees from and whether the zero point was set. Our system always assumed 4 degrees more. Wow, it was cold in winter until the error was found

I'm not really an expert, but in your picture from the start post, it can be seen that "Sollwert HK2" is set to 29.5 degrees. The target room temperature for HK2 is also set to 25 degrees. Is that intentional? Is the heating system perhaps actually continuing to heat this heating circuit?

Okay, then we probably have to get used to it. I'm just surprised that it's so bad today. It has been pouring all day and outside it's a maximum of 20 degrees.

Well, thanks to good insulation it just takes time.

I have the option to turn on "Bypass". Should I do that?
Currently it is set to "Exhaust". Otherwise, I could choose: "Off, Supply, Bypass" or "Summer cassette" (but we don't have that).

You should check the manual if it works automatically.

I don't understand that, what does it mean?

That means that the heating circuit has taken the ground temperature and the heat pump shows you this temperature. You can measure the ground temperature at your place for comparison.

“Our unit only has an outdoor sensor and consistently maintains 24 °C indoors for 20 °C, so that does not correspond.”

I explained that before, the 20 degrees that the heat pump shows as indoor temperature is not a measured value but only theoretical for calculating the heating curve.

Clarify where the system takes the 20 degrees from and whether the zero point was set. With us, the system always assumed 4 degrees more. Wow, it was so chilly in winter until the error was found.

That's a good hint, I can ask the heating engineer. It's always a bit cumbersome to convert that (so you have to know that the 20°C the heat pump shows is actually 24 and always calculate from there). But we didn't really have problems with the temperature because of that, we just kept turning it down. At the beginning it was just way too warm because we had set the temperature on the heat pump to what we wanted. Then the heating expert explained it to us, it was also T_im_Norden who said that the value is not the actual "real" value but rather serves as a reference. So we slowly reduced it until the temperature matched.

I'm not really an expert, but in your picture from the first post it can be seen that the "Setpoint HK2" is at 29.5 degrees. Also, the set room temperature for HK2 is at 25 degrees. Is that intentional? Does the heating maybe really heat this heating circuit further?

Everything that is shown/related to HK2 can be ignored. We only have one heating circuit.

You have to look through the manual whether it operates automatically.

The manual says absolutely nothing about this whole passive cooling/bypass stuff. I can only see it in the web interface, maybe the menu item doesn't even exist on the heat pump, I would have to check. But I can't find anything about it in the manual. Also not in the installation manual.

I already explained that, the 20 degrees the heat pump shows as indoor temperature is not a measured value but only theoretical for calculating the heating curve.

Yes I know, it still irritates me that it deviates so much from the actual "real" value, or that you cannot align it. I could use a thermometer, measure the room temperature and teach the system "currently it is X degrees", then you wouldn't have to convert all the time.

[Tecalor] is, as far as I know, the no-name brand of [Stiebel Eltron]. That’s why the menu navigation is pretty much the same as ours.

Regarding the bypass, the question is whether one is installed or not. Since you also have a compact unit, possibly not.

You can put the cassette in yourself, it’s no problem. Why should the heat exchanger be damaged if it’s stored in a box instead of the heating system? As long as no pets or children tamper with it ;)

Regarding summer operation, I would also check the settings. At the moment, it’s too warm inside. But when you reach a tolerable temperature level inside, it could otherwise happen that the heating turns on on a cold night, even if it’s 30 degrees again during the day.

Otherwise possibly interesting for you:
https://www.hausbau-forum.de/threads/Zehnder-q350-optimale-sommer-einstellungen.39421/