I'm making plans for my server room, and to keep it as dust free as possible, I want to make it a closed system. For cooling, what I'd like to do is have air from the ceiling behind the server rack sucked in through a duct, then pass through an air chamber which will have a really good filter, then pass through a radiator, then come back into the room in front of the server rack. I'd probably use a 4" pipe with an inline fan. They are quiet, and move a decent amount of air (100cfm or so). The filter would be large as to give more surface area. The radiator would probably be a car radiator, and I'd somehow hook up pex lines to it. I'd have to improvise unless the sizes are standard and I can get fittings maybe? Not too worried given this wont be under pressure. I'll basically fill it once, and that's it. This pex pipe would be laid all around the perimeter of the crawlspace, maybe one or two passes, then fed back into the system. The crawlspace gets around 10-15c or so in winter. It is only heated from the ductwork and ambient house heat, which mostly rises.
I don't expect this to bring the server room anywhere near 10c, but I figure if I can at least keep it around 25c, I will be happy. Think this idea would work? As for summer, I would probably hookup my AC unit in there, this idea is only for winter. I figure there is no point in using AC in there when I'm trying to heat the rest of the house. May as well spread the heat to the rest of the house instead.