Looks like you have the length set at the data stage to the vanilla game underground lengths, and the length cannot easily be modified by later stages due to the long hidden heat pipe you make to connect the two. Rather than trying to adjust this to a later mod load stage and account for all the mods, could you add startup settings to define the maximum underground length of the various types of underground heat pipes?

The mod is currently not set to handle changes of the length for already placed underground heat pipes

Actually it wasn't that bad, i feared problems when the overall max distance would dip below a certain point as well as some interactions with distances that would now no longer reach yet longer distances still being available.

So long story short, they are configurable now, i didn't really test it tho so i would really appreciate it if you as the requester of this feature does their best trying to break it.

<Puts on QA hat>

:D