Hi,,Just joined the forum.I have lurked in the past and always enjoyed the members contributions and the great advice.My 25+yr old oil fired hotwater furnace took a poop and now I am looking at the trend in geo thermal.At the present I am doing my heat -loss calculations.I have downloaded a program to determine the size of the system that I need,so my question is..I am using70F as my indoor design temp,but what do I input here in South central Pennsylvania (Hershey)for my outdoor design temp?Is 13F good or too high?