As I mentioned in my Pails of Sap post sap has a sugar concentration of about 2% and before it can be syrup the concentration should be 66%. That is achieved by removing the water and you remove the water by boiling the sap! It takes approximately 40 gallons of sap to make one gallon of water. It definitely is a time intensive process that takes a lot of patience!
Early in the season I often boil in the house just a couple words of caution