You said.. I can't understand the "Why".
For the challenge of it, maybe. Or perhaps for more even more quiet than low RPM fans can provide.
I considered trying to build a passive radiator but decided just to go with what has worked for others. I'm using two 170mm fans @900 RPM on an '89 Camaro heatercore for push/pull. Not silent but close enough. Plenty of heat dissipation with low noise. My thanks to those who travelled the watercooling road before me and posted their knowledge online. I'm an imitator but a damn good one!
As for passive rads used to heat a house, could you recommend a specific type? All the ones I've seen are ancient, huge, and incredibly heavy compared to a PC. That's one way to get a lot of surface area I must admit... not something you want to bring to a LAN if you do that sort of thing.