solar thermal question - anyone use a 20% methanol & 80% water mixture?

Methanol should never be used on any potable water system due to toxicity issues, not to mention the corrosive effects it has on many commonly used plumbing materials. (Ethanol is far safer on all counts.)

I will have a double-wall heat exchanger separating the methanol mixture and the potable water.  The plan is to have it heating up a geothermal closed loop system.

Please don't let the lower up front cost of methanol distract you from the down-sides. Ethylene, Propylene or Corn-base Propylene Glycol are better alternatives to methanol. Corn Glycol products have the lowest corrosive results I have seen, allowing higher temperatures and longer replacement intervals and the mixed use of stainless steel. (It is more viscous, however.) Methanol would require acidity and corrosion inhibitors/monitoring; corrosion problems are accelerated by the use of tap water instead of deionized or boiler/distilled water! Both Propylene and Corn-based Propylene Glycol are safe for pets and ground water if leaks occur, unlike Ethylene Glycol. see also: http://www.oee.nrcan.gc.ca/publications/infosource/pub/ici/caddet/english/r413.cfm?attr=20 http://en.wikipedia.org/wiki/Geothermal_exchange_heat_pump http://www.geoecs.com/14.html and don't forget to Google methanol corrosion