“Free-flowing hot spring water” (gensen kakenagashi) means the source water is used as-is, with fresh water constantly overflowing the tub. “Dilution” means adding cold water to adjust the temperature, “reheating” means warming the water, and “recirculation” means filtering and reusing it. These terms tell you what kind of water you're actually getting.
Being able to read the water description on a booking page makes for a sharper ryokan choice.
See Hakone private-onsen ryokan →Key terms
- Free-flowing (gensen kakenagashi): source water used without recirculating or diluting it, with fresh water constantly overflowing the tub
- Dilution: adding cold water to hot source water to adjust the temperature
- Reheating: warming the source water to a comfortable temperature
- Recirculation (filtering): filtering and reusing the water, for hygiene and to maintain water volume
Choosing a private open-air bath
A “private open-air bath” doesn't guarantee real hot spring water versus reheated water — that depends on the ryokan (see the difference explained). Aitayo tracks ryokan with free-flowing hot spring water at /tag/kakenagashi.
FAQ
What's the difference between free-flowing and recirculated water?
Free-flowing water keeps overflowing with fresh source water; recirculated water is filtered and reused.
Is dilution or reheating a bad sign?
No — they're common methods for adjusting temperature and ensuring safety, not necessarily a downside.
Last updated: 2026-07-17 ・ Reservations are on Rakuten Travel (Japanese site).