Almost any fluid can be used in a heating system, if it is designed for it. If you are looking for home heating systems, you will only find ones using cost-effective fluids, for obvious reasons. Glycols are not cost effective to use at 100% concentration because: 1. They are more viscous, and that makes them more difficult to pump, requiring a larger pump and piping. 2. They don't transfer heat as well as water, requiring a larger boiler and radiators. 3. They are expensive, and since they achieve the lowest freezing point in a solution of 40%-60%, you are defeating their purpose and wasting your money to use them at 100%. While ethylene glycol is still in use, it is very toxic to humans and animals, and the environment. Most systems now are using the nontoxic propylene glycol (it is also used in many food products). This is true for automobiles also. It is best to use a glycol for a hydronic system that contains the proper additives to prevent corrosion. Never use automotive antifreeze, as the silicon will damage the system. The two best hydronic glycol products on the market are DowTherm (ethylene) and DowFrost (propylene). They are also more expensive than the generic glycols sold for heating systems.
