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Solar drainback systems (Part 1): besides requiring no antifreeze, some also eliminate the need for heat exchangers between the collector and water in the storage tank--minimizing supply water temperature and maximizing efficiency.

Article Excerpt
After 30 years of designing and installing hydronic systems, I've developed a philosophical statement about antifreeze solutions:

"The only good thing about antifreeze is that it doesn't freeze."

Although these words certainly don't rank up there with other notable quotes, they none-the-less suggest that use of antifreeze in hydronic systems has some "baggage" associated with it. Here's a quick summary:

* Glycol-based antifreezes lower the specific heat of the solution. For example, a 50% (by volume) solution of propylene glycol has a specific heat of about 0.86 Btu/lb/[degrees]F. That's 14% lower than the specific heat of plain water (1.00 Btu/lb/ [degrees]F). Mixing antifreeze with water "dilutes" the ability of the fluid to absorb heat.

* Glycol-based antifreeze solutions have significantly higher viscosity than water. This increases the pumping power necessary to move them through a piping system relative to water. The head loss of a piping circuit operating with 50% propylene glycol increases about 39% relative to the same circuit operating with water. When the flow rate is increased to compensate for the lower specific heat, the pumping power requirement for 50% propylene...

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