How Long From Cold to Warm
The graph above shows how many hours it would take each of 4 pool heaters with various heat outputs to raise the temperature of a given pool volume (gallons) by 10°F.
When you first open your pool or when it has lost all of its heat, the water will typically be about the same as the average daily air temperature for UNCOVERED pools and about 10 °F warmer for COVERED pools.
If you assume that a cover is in place for the initial heat-up then heat losses can be basically eliminated from consideration with very little error. Even with no cover there is a relatively small added heat loss to make up as the water gets warmer, so the times shown by this graph are fairly close and still useful.
CONCLUSIONS: Even a big gas heater takes a long time to heat up all but the smallest pools. A properly sized HEAT SIPHON should take no more than 2 to 3 days to initially heat up your pool. MOST IMPORTANT THOUGH IS THE FACT THAT HEAT SIPHON MAKES 24/7 - 85°F AFFORDABLE!!