When we built out house we plumbed in an extra circuit for apron/sidewalk heating.  Have not got it going yet since still working on outside part of our house - just got deck done a couple years ago.

The plumbing for the outside circuit is heated by a water to water heat exchanger on it's own pump system.  This way the antifreeze ratio is different compared to the circuit that heats the garage or the basement floor (we have air source/propane ducted system for rest of house.)

The liquid has to remain flowable enough on days like yesterday when it was 28 below and the apron heater is not being used - but could be.

Will I ever use it?  Not sure.  I do plan to put the piping in the concrete when that gets poured.  There would be a controller set up to not allow heat to other areas and only to outside apron for a set period of time for the melting process. 

That it was an interesting idea but may not get used a lot.