Open, that's called 'thermosiphon' effect... same as a chimney, except for being a closed loop.
For me, it's always been about the sudden blast of noise that wakes me up, and the rather high battery drain. I don't mind sleeping cold, but I resent the very devil out of having to crawl out of that warm bed to turn the furnace on in the AM. I finally moved the thermostat to within easy reach of the head of the overcab bunk. I think my problem's solved, but I await next winter to find out.
If that doesn't work out for some reason, I've toyed with the idea of a hydronic system based on the existing water heater in my 19' Tiger CX. Just a thought experiment for now, but you can buy individual components made for water-cooling desktop computer CPU's (used by gamers & such). Small 12V pumps and fans and heat exchangers in various sizes aren't that expensive.
I want to find out what temperature drop I can expect to see over the various sizes of exchangers before I go much further. I might build a small unit for 'background' heat, just for the heck of it.
Jim, "Some days the best thing about my job is that the chair spins."
