hot well
/ˈhɒt wɛl/noun
1A spring of naturally hot water; = hot spring.
2 Engineering. A reservoir or vessel into which heated water passes after use; specifically one that receives water from the condenser in a condensing steam engine or a steam turbine which is then used to supply the boiler.
Origin
Late Middle English; earliest use found in John Trevisa (c1342–?1402), translator. From hot + well.