Abstract: Water in a spa bath is heated by an air source heat pump installation 500 enclosed within a housing 502 and operable by way of a control system 520. A fan 504 draws ambient air A into the housing 502 through an air inlet 506 and thence through an evaporator 508. The air inlet 506 and the evaporator 508 each extend substantially from side to side and from top to bottom of the housing 502 and are balanced in relation to the flow or air A, being each formed with airways that in aggregate cross-sectional area are substantially the same. This reduces the drop in air pressure and hence the power required to drive the heat pump installation 500.