According to the wiki, the IHDs are supposed to freely exchange heat among the neighboring components, it's self, and the reactor hull so that the temperature is equal.
http://wiki.industrial-craft.n…Integrated_Heat_Dispenser
However, the temperature of all the IHDs remain very slightly above 0 long after my reactor has cooled off. Long after all the neighboring cooling cells have returned to normal. The IHDs should always balance the heat such that they wind up with the empty portion of any remainder.