During my visit to Asan, I also visited this lovely little sauna just up the street from the Hot Spring/ Tourist Hotel. There is no jjimjilbang here, and the sauna is limited to 5 baths and one dry sauna room. The baths are cool, hot, hotter, event, and massage. The event bath was unfortunately not filled when I visited, and I was disappointed by the temperature - I like hot water, but this place is just too hot to spend any amount of time in. I guess I should have expected it, the Onyang Oncheon area is well-known to be the hottest natural hot spring in Korea!
On the other hand, the massage bath was great. It's a waterfall style area with a shallow wading pool and small stools to perch on while your muscles are being pounded by the downpour. I waited in a short queue to get my time in this tub, and it was worth it. Also, the whole place smells AMAZING!
Chungcheongnam-do, Asan, Oncheon-1-dong 221-2
Open daily 5AM - 12AM (midnight)
Enter from 5am-8pm (use from 5am til 12midnight that day)
Sauna: Adults 5,500 Students 8-18 years old 4,000 Children 1-7 years old 2,500
Wikimapia of Shinchungtang, Asan (if the building's not tagged, it's the one at the center cross-hairs.)
Shinchuntang website (I've been having trouble loading this page in the past~ good luck!)