Well, honey comes to my mind as a "food grade liquid that will not freeze."
   
  I would check with a local heating/cooling contractor, especially one that does commercial installations.  We replaced the steam boiler and radiators at church with a hot water boiler and forced-air blowers.  We had a choice of about seventeen different fluids, from water (by far the cheapest) to petroleum products to propylene glycols and alcohols.  Costs varied wildly and widely.  Obviously you won't need too much.
   
  We went with water as it won't be in a freezing environment, and we also had the option to add a switch that would automatically keep the water level filled from our normal plumbing sources which eliminated our constant need to keep an eye on things.  Obviously your need won't require such diligence.  But we were also warned that water would also cause corrosion and we were given about ten different products to add to our boiler to reduce corrosion.  No doubt that some of these products would alter the freezing point.
   
  Grant
  Jackson, MO   http://www.25hives.homestead.com
   
  
 

       
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