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Propylene Glycol Heat Transfer Fluid - Page 5

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Posted by Gary on May 27, 2005, 11:03 pm
DJ wrote:

The Home Power article on closed loop systems says:
"Pressure Relief Valve:
Every hydronic heating system must allow for protection
against excessively high pressures due to high
temperatures. A pressure relief valve of 50 psi is
typically adequate to protect closed loop plumbing from
excessive pressures. "...

Lots of people have less than 50 psi potable water pressure.

My well pump does not kick in until the storage tank pressure drops to
around 25 psi, which is plenty for my house.

In addition, pressure relief valves can (and do) fail closed.

If you want to say that you can insure that you will NEVER put Glycol
based antifreeze in the solar hot water system, thats fine.  But,
saying that even if you do use Glycol, you are protected because leaks
in the heat exchanger will flow from potable to collector loop is
simply not true in anywhere near all cases, and is unsafe.



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Posted by DJ on May 28, 2005, 1:23 am

Gary wrote:

Agreed. Standard, as I remember, is 45. I was talking about the
pressure relief on the glycol side, however. As I remember,
Thermo-dynamics is set at, as I remember, 5.

It has been my professional experience that they more often fail open,
though. But agreed, it could happen. And wheels and wing parts do fall
off aircraft flying overhead, too, though. Life is risk.

Gary, I'm a licenced Journeyman Millwright. That means my whole trade
revolves around the fact that Man has never made a machine that won't
fail at one time or another ;-). You're preaching to the choir, amigo.

My point is:
A) The deck is stacked towards any leak bleeding DHW into the glycol,
and not the other way around. But more importantly...
B) Only a total and compete idiot would put an amazingly toxic poison
anywhere near his drinking water when a perfectly safe and similarly
priced equivalent exists, so leaks are pretty much moot regarding the
safety of any member of the gene pool worth saving ;-).

Posted by Gary on May 29, 2005, 1:17 am
 DJ wrote:

The excerpt from the HP article IS talking about the pressure relief
on the glycol side.  The glycol side is normally pressurized to about
25 psi when filled.
EarthSun (a large manufacturer of solar DHW systems) recommends that
the glycol side be charged to a pressure of 20psi + (vertical ht from
collector to tank)/2.31 -- this usually comes out around 25 psi.
The glycol side pressure will increase on a sunny day as the glycol
expands and pushes fluid into the expansion tank.  They recommend that
the glycol side pressure not exceed 45 psi on a sunny day, but their
pressure relief valve does not activate until the glycol side pressure
reaches 150 psi.

This is from their manual:
"Pressure Relief Valve: Will open and purge the
glycol loop HTF at 150 PSI. If this valve opens and
HTF fluid is expelled contact your contractor immediately.
This valve also may be opened to drain the
HTF from the charged glycol loop for replacement."

One can envision many circumstances in which the potable water
pressure falls much lower than the glycol side -- for example: 1)
normal operation of a well system will not turn the pump on until the
pressure tank gets down to about 25 psi, 2) If I am leaving the house
for a few days, and the weather is cold, I turn my potable water off,
and open a few faucets to reduce the chance of freezing -- so the
potable water pressure is zero, 2) any kind of sag in the mains water
pressure (this happens routinely in some areas), 4)  electric power
failure if you are on a well will allow the house water pressure to
drop to zero as you use water from the pressure tank, ...
I would think that almost any home will have potable side pressure
lower than the glycol side pressure from time to time, and some will
have this situation quite often.


We disagree on this,  I can easily see the 2nd, 3rd, or 4th owner of a
  solar DHW system having a leak in the system, and refilling it with
the wrong kind of antifreeze just because he/she is not familiar with
the different flavors of glycol.  I agree that the manuals have
warnings about this, but even engineers and Millwrights don't always
read the manual :-)
I think illness or death is a pretty high price to pay for not reading
the manual.


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Posted by DJ on May 29, 2005, 3:46 am

Gary wrote:

5 on a TD system at startup.

A boil relief, then, realistically.

Actually, quite a few of the SDHW systems I install are on municipal
mains, so it would almost never happen... But, you're right, it could.

Two things wrong with this. One, a service I offer is to repair
existing SDHW systems. I've seen twenty and thirty year old systems
chugging away with their first owner; I've never seen a second, third,
or fourth owner.
Second, if one was looking for glycol for a plumbed in SDHW system,
given a choice, would you go to a plumbing store, or an automotive
store? Yeah ;-).

Gawd, we almost never HAVE the manual ;-). But when we have it, we do.
They beat that into us during our apprenticeship ;-).

Yes, it is indeed. My point exactly. Automotive glycol shouldn't be
anywhere NEAR a domestic hot water system. Even double wall exchangers
can fail. They can leak onto the floor, where any toddler or pet could
lick the tasty sweet fluid.

And people that wouldn't know to read the manual would have to go to
full-service gas stations to make sure they didn't put diesel in their
car ;-). They should also not be allowed to have both propane and
natural gas in their houses, because they might confuse themselves.
These same people that display that degree of ignorance should also not
have renewable energy systems in their house because they shouldn't be
trusted with high amp wiring, and shouldn't have SDHW systems on their


When something goes wrong, they should pull out the yellow pages, turn
to "Solar..." and pick a professional to fix their stuff, preferrably,
the guy who installed it in the first place.

You should not trust a layer of copper or stainless steel to protect
you from death if you can at all help it. Chlorinated water eats at
stainless steel, especially the cheap stuff they often make HEs with
(wanna see something cool? Put a stainless steel utensil in chlorox).
Automotive antifreeze corrodes copper. A leak from a doublewall can
also kill.

Nope, my answer: keep the automotive antifreeze out of your SDHW
system, no matter how many stages and walls the heat exchanger has.


Posted by daestrom on May 29, 2005, 12:55 pm

Absolutely.  In some industries it's called 'defense in depth'.  Why have
only one barrier to safety when you can have two.  Sure, an accident *could*
still happen (heck, a meteor can strike me dead before I finish typing
this), but it is much less likely.

P.S.  apparently the meteor called in sick..... ;-)

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