Do you get a Zehnder? If yes, make sure that the HB uses the original comfoflow boxes. They can be easily connected for expansion. Filter strips, silencers, etc. And if possible, round pipes with a large diameter.
Helios DLV 125, we made them ourselves. They are only inserted into the pipe with a rubber seal, simply pull out and push in. However, there has been no calibration of the system so far, I will probably have to rent such a device to get the valve settings somewhat acceptable.
@t-im_norden At the moment Vallox is planned, it would certainly still be changeable. I’m not exactly sure if expansion is practically possible, or worth it...
Take a look at the comfowell system and then you'll know what I mean by expandable. I am currently retrofitting an activated carbon filter because of all the wood stove heaters around here. I just have to remove the back panel from the distributor and then attach the filter box there. Whether Vallox has something like that, I don't know.