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

That depends on whether a bypass is installed. You should normally have two manuals for the system. One is "Installation and Operation" on page 32 in this manual, and one is just "Operation," if not, download it from the manufacturer. "But there must also be a way to get rid of the heat recovery of the ventilation in summer. Presumably, we need to buy a summer cassette." Have you read the linked post? It explains why a heat exchanger does not bring heat into the house in summer and why a bypass is useful.

In the discussion, I wonder how less technically savvy people are supposed to figure it out. An acquaintance of ours says: a heating system must work! No 'ifs' and 'buts'. Sure, once a heat pump is properly set up, it may be a fine thing... but until then?

Always been a problem of this (and basically every) technology. Without compulsion, only tech geeks get this and that... the masses stick to what they know.

This morning, though, it was bull hot in the bedroom again. Where does that come from?

Either you are heating by accident or the interior walls have nicely stored the heat that came into the house. That’s actually the point, so you don’t have such high heating costs in winter. With some air conditioning, you cool the air and quickly get comfortable temperatures; the stored heat, however, doesn’t cool down that fast. You’ll figure out how to manage best. With heating and air conditioning, it will work – but with low energy consumption, that’s out of the question. We have foregone all that technology (and thus any subsidies); on some days it just gets hot, and everyone in the household has to deal with it. We don’t have any particularly demanding creatures, although sometimes some whiners. In our townhouse built in 2001, we often hosted exchange students from the USA – Texas A&M and Los Angeles USC. They were so used to air conditioning that they almost died of heat here in summer :D.

Maybe store it in the attic. But in summer there are melting temperatures there.

On my, and probably your, WT it says "max 50°C". So the attic is probably out.

both the setting "summer cassette" and "bypass"

The heater doesn't know what it's like inside you. You have to tell it that ;)

What do you mean by that?

It COULD happen that the heater, with unfavorable settings, decides on a cold night that it is no longer summer and briefly heats because the HK setpoint is significantly above the HK actual. For example: It is 30 degrees during the day, at night it cools down to 10. Summer mode is set to, say, 16 degrees, hysteresis 2K. Now for the heater, from 14 degrees it is "winter". Inside it is 23 degrees, i.e. HK actual ~23 degrees. HK setpoint is, for example, 25 degrees at 10 degrees outside. Then the heater would heat briefly, although you might prefer to have 1 degree less in the house in the morning, as it will heat up again anyway at 30 degrees outside temperature. Now there are three possibilities. - You don't care, usually the best ;) - You check and meticulously set everything so it doesn't happen on automatic. - You go twice a year to the heater and manually set it to hot water or heating mode. I prefer the latter, but everyone has to decide for themselves. But since I realized that you have an air conditioner, this is all an exciting discussion. But you can ignore all that, if it annoys you just turn on the AC and be happy that everything can be so easy :D Don't you even have photovoltaics? Then I wouldn't care at all! If I were you I would: - Bring the house now to a bearable level or desired temperature with all means. Either ventilate fully, or AC, or both. Then: - From now on consistently shade and keep doors and windows closed as much as possible during heat. - Buy a summer cassette, or not. You always have Plan B :cool: Now in the transition period, when heating is no longer used, the advantages clearly outweigh the WT. When it gets really warm it can be different. Then the WT cools the air during the day but heats it at night. You have to suffer one way or another if you don’t have a bypass. - Keep the supply air low during the day either via passive cooling or timer, increase it when it is cooler. - If you want to spare the air conditioner a bit, either bypass or fly screen. With a fly screen you can pull cool air through tilted windows at night via the exhaust air if you want. Without insect risk. (Then a summer cassette would definitely make no sense, your "bypass" would be the windows) That would be a bigger investment in both cases. Whether I would make it in addition to the investment (and luxury) of an air conditioner, I don't think so.