Yes the hotter it is outside the better they work. But think of it this way, how does an airconditioner make the inside air colder than the outside air? The answer is the same for a heat pump, which is an airconditioner that works in reverse. Normally you run an airconditioner to extract heat from your room and dump it outside. Your pool heatpump works by extracting heat from the outside (making it cooler) and dumps heat into your pool (inside).