It says in SA YD Siman 87 Sif 6 that if one "smokes" together a meat and milk or if they "cook" it in the hot springs of Teveria then they are patur (however this is still ossur to do and of course eat.)
In the Sefer Chedrei De'ah (See here Sif 6) he writes that the process of smoking that one would be patur if done is only when the smoke is NOT yad soledes bo. However if it would be yad soledes he writes that this is "c'risichas aish gomer" (like a boiling fire.)
How is this understood? The point of the Shulchan Aruch to tell us that if you "smoke" meat and milk together that you are patur is to say that it's not "derech bishul" (the normal way of cooking.) But would seem simple that the smoke must be yad soledes in order to do something to this mixture. If the smoke is cold (or at least not yad soledes) then one could say that this case would be perhaps muter completely or at least not ossur b'dieved.
Are there any additional sources that speak about this (that either go with the opinion of the Chedrei De'ah or argue.)