I am currently working on a distillery setup. Using some gates and pipe wires I managed to automatically choose liquids to heat and liquids to cool by priority.
However there seems to be an issue with Dense Oil (hot). As long as I heat up Oil (cool) or Water the setup runs fine but as soon as I try to heat up Dense Oil (hot) the heat exchanger stops working and my server log is flooded with messages like
"[Server thread/WARN] [BuildCraft]: Invalid heat values!".
This is reproducable. As soon as I remove the Dense Oil (hot) from the processing, everything runs fine and the server messages stop.

BuildCraft version: 7.99.22
Forge version: 1.12.2-14.23.5.2806
Link to crash report or log:
Singleplayer or multiplayer: Multiplayer
Steps to reproduce: Heat Exchanger: Put Mixed Light Fuels (hot) to cool and first Oil (cool) then Dense Oil (hot) and then Water to heat. It will stop on Dense Oil and continue as soon as Dense Oil is removed.

I had a look at the sources and found what caused my issue. The heat levels of fluid to cool and fluid to heat are equal which does of course not make much sense if I think about it.
I better rethink my setup.