glycol and drainback systems [RE-wrenches]

Fri Sep 23 04:37:00 PDT 2005

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Travis,

Like you said, 25 years with the same glycol is truly impressive. Reduced
maintenance and fewer components are also big advantages of the drainback
philosophy. 
And how does the glycol drainback achieve this reduced maintenance? By
being able to _effectively_ high limit the glycol fluid _not_ just the
storage tank.

The downside is a bigger pump to overcome static head pressure _every_ time
the system starts and correspondingly higher electrical consumption for the
life of the system. The situation of relatively cold fluid being pumped up
to an already hot collector is a sourse of thermal shock in the drainback
scenario. 

Tell me, Travis, what happens when a pump controller fails or a thermistor
fails in the drainback? If there is a proper glycol mixture is the system,
it would be hard to have freeze damage.

Lastly, I would hope that you are using proplene glycol rather than
ethylene glycol in your systems. The difference? The  first is _non_ toxic,
the second _is_ toxic.

Thanks for asking, I'm sure there are folks out there that also did not
understand and also did not ask.

Bill Loesch
Solar 1 - Saint Louis Solar

Message text written by INTERNET:RE-wrenches at topica.com
> 
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

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