You're misinterpreting how hU works completely. If you have 32 EU/t from a Batbox going into a recycler using 1 EU/t you don't lose 31 EU/t, hU works exactly the same. The fluid heat exchanger simply doesn't use hot coolant for the time in that the fermenter doesn't use hU up because it's got some in it's internal buffer.
I know that EU will just go in and overflow the buffer but stay there untill it's needed and used, I just didn't know how the hU transfer worked. I supposed a real-world like system would be used, where excess heat just goes to waste (vents off somehow) or speeds the process up. Either way thx for letting me know. Still, a reactor used just for biogas seems like overkill to me, but whatever.