glycol and drainback systems [RE-wrenches]

[Travis Creswell, Ozark Solar]

I'm obviously not seeing the entire picture.  Why even use glycol in a drain
back system?  Assuming a properly installed collector loop any component
failure should result in the complete draining of the collector so freezing
is not an issue.  Glycol lowers the heat transfer efficiency and adds a
toxic fluid system the costs more then just plain water.  What am I missing?

25 years with the same glycol is pretty impressive.  I would have never
imagined that it would last that long.

Regards,

Travis Creswell
Ozark Energy Services

-----Original Message-----
From: Bill Loesch, Saint Louis Solar [mailto:bill_loesch at compuserve.com] 
Sent: Thursday, September 22, 2005 4:22 AM
To: RE-wrenches
Subject: glycol and drainback systems [RE-wrenches]

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Fellow RE-wrenches,

Tom Lane asked me to forward these comments about drainback hot water
systems.

Thanks,

Bill Loesch, Solar 1 - Saint Louis Solar

-------------Forwarded Message-----------------

From:	"Tom Lane", INTERNET:tom at ecs-solar.com
	
Date:	9/21/2005 10:18 AM

RE:	glycol and drainback systems

 
The lack of information about drainback systems is common. The use of glycol

with drainback systems has been common for 25 years.  First, a tilt of 23 
degrees or more results in almost all fluid draining back in the reservoir. 
Second, at zero pressure a 50% solution of water/glycol starts vaporizing at

130 degrees F.-a 40% solution of water/glycol starts vaporizing at 120 
degrees F. The vaporizing of any fluid in the collectors well below 160 
degrees F. results in a fluid film vaporizing out of the collector into the 
reservoir. Glycol/water completely boils at 216 degrees. (-30% glycol 
solution) to 222 degrees F.(50% glycol solution).  I have examined (during
reroofing) 20 to 25 year 
old thermal collectors which were using a glycol/water mixture and the 
absorber headers all looked brand new. All pressurized anti-freeze systems 
that old when examined revealed erosion of the copper walls.  
Brandon Levett at Solar Service Inc., has been installing commercial and
residential
 drainback systems in the Chicago area since 1977. Their
website,www.solarserviceinc.com 
provides photographs of large commercial drainback systems that have been
glycol drainback
50% solution for over 25 years.


p.s.I would 
definitely NOT use a anti-scald valve as a diversion valve for any PV driven

system, you are asking for vapor lock of the system. 

Gator Tom