Radiant hot water heat under front walk to melt snow in Rye, NY. Does not seem to work?

2 loops filled with anti-freeze connected as a separate hot water heating zone to boiler that heats the house. Connected to a timer switch, has a separate circulatory pump.

Tested in winter of 2013-2014 could not get snow to melt.

How many BTU’s to get the walk to above freezing when temperature is 20? The walk length is about 20 feet.

Can’t see it in pictures there is a foil layer to reflect heat up. The tubes are under flagstone about 1 inch thick.

How hot? What is the maximum temperature of inlet water from boiler? If normal boiler output is 140 to 160 if separate dedicated boiler were installed, that put out VERY HOT water at over 200 degrees… would this work?