In addition to directing the warm air into and up heat riser housing 44, the top of each upper vent 24 lifts the air coming from an upper vent 24behind it. As this air flows over upper vents 24, the pressure in the interior of housing 12 forms an eddy at inlet 32 of upper vent 24, a horizontal vortex, that increase the draw from the interior of housing 12. Thus, the velocity and mass of the air entering heat pump riser 40 increase.