ᐅ Inside the house is 10°C warmer than outside – how is that possible?

Hello!
We are beginners at living in a newly built house, and this first spring is raising some questions.
Our heating system (Tecalor THZ 504) has already decided, based on the outside temperature—which is currently around 20°C (68°F)—that it is in summer mode. In other words, the heating is turned off.
The system information still shows the flow temperature at just under 30°C (86°F), but the flow rate has dropped to zero.

The ventilation is still running, and I suspect our heat recovery unit is causing the issue—because what’s the point of exchanging indoor air with outside air if it blows the air back inside at almost the same warm temperature as it took out?
We only have an outdoor temperature sensor. The device doesn’t handle the indoor temperature very well. It claims the current indoor temperature is 20°C (68°F), but it seems to detect (through measuring) that the exhaust air temperature is about 28°C (82°F). This matches what our various thermometers around the house show.

The question remains: why do we have 28°C (82°F) inside at all? Where is the heat coming from?
Could it be caused solely by sunlight coming through the windows?

We have air conditioning, but I’d rather not run it yet when it’s only about 20°C (68°F) outside and raining.
How do you other new build owners handle this issue? Do you use shading? Something else? Or do you not have this problem at all and maybe there is something wrong with our building services?

The heating system still produces hot water, so it won’t be inactive for half a year.

That’s the sun. How are you handling shading on your property?

Is there (or isn’t there) a summer bypass for your ventilation system?

i_b_n_a_n schrieb:

Is there a summer bypass for your ventilation system?

There is a "Bypass" setting on the ventilation system, yes. However, I couldn’t find any explanation of what it means in the user manual. In the ventilation settings, there is a value called "Passive Cooling," and you can choose from: "Off, Exhaust, Supply, Bypass, and Summer cassette." I already looked up the summer cassette online. It seems you would need to replace a component, which is too risky for us (actually not allowed for amateurs). Removing or storing it could damage the heat exchanger. I’m also unsure how this would affect our warranty if we try to do it ourselves. Unfortunately, the user manual doesn’t provide any information about any of this.

guckuck2 schrieb:

The heating system still produces hot water, so it won’t be inactive for half a year.

That’s the sun. How is the shading situation at your place?

Wow, we hadn’t really thought much about shading – but we do have roller shutters that we could close if needed. Would that help at all?
I’m just amazed that it’s noticeably warmer inside the house than outside. We never had this problem (of course) in the 1992 house.

That makes a huge difference, and without it, you will either overheat or have to countercool.