Dear Solar Guy,

        I have a solar water heater and have heard that the anti-freeze
should be replaced periodically, but I'm concerned about the toxicity of
anti-freeze if it leaks into my water system.  Please advise me.  

 B. R. Taster, Sulfur Springs, TX.

-----------------------------------

Dear B.R.,

        You are correct.  Just like it is necessary to change coolant
(anti-freeze) in your automobile, since it breaks down over time and becomes
somewhat acidic, anti-freeze in solar collectors should be changed
approximately every 3 or 4 years.  Systems that use anti-freeze require a
double walled heat exchanger so as to negate the possibility of
contamination of the potable water should a leak occur.  Also, when
replacing the anti-freeze, one should use propylene glycol (which is
non-toxic), rather than ethylene glycol which is toxic.    

The Solar Guy.