Heat pumps are not suitable for all climates so check with a reputable builder before you do so to see if it is a viable option where you live. I have a heat pump on my spa and pool in N. Fl. (we get the occasional hard freeze here but right now it's in the low 70's and I was in the pool this AM--heated to 85) and it heats my spa to temperature in about 15-40 minutes, depending on the time of year, initial water temperature, and ambient temperature. My heat pump is reverse cycle (heats and cools). However, I do not use it to cool the pool or spa. The reverse cycle feature allows to heat pump to work in ambient temperatures that approach freezing. Normally, heat pumps are ineffective in temperatures below about 50 degrees and a supplemental form of heat such as propane is then needed.