Geo-Thermal Question

Just starting to get into Geo-Thermal work. My fusing kit gives you a time frame from this time to this time to heat the pipe and fitting using the fusion tool. Example: 1-1/4 Geothermal pipe and same size fitting. My fusion kit says to heat pipe and fitting for 12 to 17 seconds using the fusion tool before fusing pipe and fitting together. Do you guys who do it find it's better to heat the pipe and fitting on the low side of the timetable, the high side of the timetable, or somewhere in the middle. Which is best for the best, most complete/accurate fusion of the pipe and fittings to ensure that you have a good, completely leak-proof fusion joint. Could you please explain why in your explanation, thank you

You're supposed to fuse a fitting into pipe? How would you then rotate the fitting? I thought fusion was to put two pieces of PE pipe together without a coupler.