BTW... It's not necessary to have "elbows" in the exhaust plumbing. For the few people who've insisted on venting, I've told them to route it straight up.
Years ago, I had a customer who couldn't leave well enough alone. He insisted on venting and got some flexible air conditioning duct to vent with. (You know, the big, puffy- looking, shiny stuff that's super- flexible.) He put it on his heater and routed it across the van, moving it through the door when the door was open and back when closed. It was about 8 feet long.
Well, it started sooting up. And about every 2 weeks, he'd bring it by with soot to clean out, until finally, after the third time, I got it through to him that he was actually causing the problem. (He was Korean and didn't speak English very well. So there was a serious language barrier.)
After that, all was well...