@chrisintaipei:
anyway, i got a bit worried and checked d2500cc's manual, it specifically says being a fanless board, it requires some kind of case fan or the help of fan from power supplier, and that it should not be totally fanless. so, i'd say it's still better to stick a case fan into the case albeit a very slow one.
Ok, in summer I have to reevaluate.
It's not wrong to use a fan. My intention was to reduce power as much as possible. Therefore I decided to go fanless.
I tried powerd option under pfsense but without impact to the power consumption. So I diabled it again. But you can check if your temperatures feels better with this option enabled.
Somewhere here in the board you can find how to include mbmon into pfsense to see the temperatures on the status page of the administration webpage.