into pool surface 13 through waterfall 14 caused by a low point in spa wall 23 in order to maximize the heat transfer of the pool water falling through waterfall 14 movable surface 11 has been positioned and substantially fixed in position underneath waterfall 14 movable surface 11 may move on the surface of the water but should not float away therefore instead of the recirculated pool water entering the pool immediately after dropping from the spa onto pool surface 13 it continues to be exposed to the ambient air because it falls upon movable surface 11 and is forced to run over its top surface which is exposed to ambient air in a preferred embodiment of the invention movable surface 11 has slightly raised side walls 17 which help channel the water from the end of moveable surface 11 situated underneath waterfall 14 to the opposite end of movable surface 11 this channelling of the water along the entire length of movable surface 11 to the discharge end 18 of movable surface 11 maximizes the time the water is on the movable surface 11 which also maximizes the heat exchange between the water and the ambient air this channelling of the recirculated pool water along the entire length of movable surface 11 could also be accomplished by having movable surface 11 slightly slanted downward away from the end of movable surface 11 under waterfall 14 and toward discharge end 18 another method of assuring that water runs over the entire length of movable surface 11 could be accomplished by having channels or indentations on the upper surface of movable surface 11 channels 15 near waterfall 14 could be designed to channel the water uniformity across the top.