If you are really serious about heating your driveway / walkways... I have done / designed several now... There are some great products on the market that make it easy to do and take a lot of the engineering out of it. I know PEX makes an actual manifold system for it with the balancing vales built right in, Taco makes the thermostat set up that runs the injection pump and several companies are making the small brazed plate heat exchangers to do it with pretty cheap. Last one I did was for a large commercial customer, we did over 750 liner feet of side walk and a 1000 sq ft entrance area. We used a small natural gas boiler, small brazed plate exchanger and lots of pex w/ the pre engineered manifold. Worked really nice... runs about 38*F with a max of 40*F surface temp... then you don't need to worry about damaging the concrete or asphalt.