Installed on my pool about a month before winter was over. When installed water was 52 F. With no weather over 80 F the water was ready for swimming around 70 F in the first or second day of true summer weather. Highly recommend. For this that say it only warms the top layer of water this is true but when the pump circulates the water it of course mixes it all together. Saved me from buying and running an electric pool heater.
Rating: [5 of 5 Stars!]