After tracking down an unexplained heat build up in my reactor planner's designs I found out that it due the way exchangers handle really small levels of heat in the surrounding components they end up adding heat to the reactor.
Due to rounding errors the exchanger would give out more heat than it actualyl had, leading to an overall rise in heat.
Eg: 60k Coolant Cell (with 150 heat) -> Heat Exchanger (with 15 heat) -> Heat Vent (with no heat) and the reactor core has 23 heat.
I watched as the exchanger gave 12 heat to the coolant cell, 2 heat to the vent and 4 heat to the core. A total of 18 heat given away but the exchanger only had 15 to begin with.
I know it's only a minor thing as it only effects exchangers handling tiny amouts of heat, but I though I'd mension it anyway.