Sale of Primary Residence

I bought my primary residence in 2014.  I lived in it for 3 years and then moved temporarily so I rented it to a friend for a year and a half.  From all that I have read I clearly qualify to not pay taxes on the sale.  I made about 20k on the sale.  When I answer the questions in Turbotax it is making me pay taxes.  The questions are very hard to understand and I've even gone through and answered differently.  No matter how I answer I can't get the system to not take out tax.  Please help.