SOLAR HEATING SYSTEM
23, 2002: HP article feedback - to stagnate or not?
the Home Power article that I wrote about my system
the streets last week (download it
There has been a
of concern and discussion amongst the industry experts
my stated plan to "stagnate" the system in the summer.
paragraph in which I stated that plan slipped in at the
minute without sufficient time for peer review.
Home Power staff are very conscientious about fact checking
the details of their articles and plan to print corrections,
means basically disconnecting the circulation pumps
leaving the 50% Dow Frost HD glycol antifreeze solution
the collectors as-is. I have been told in no uncertain terms
one expert that this is dumb because the collectors
achieve temperatures exceeding 325F, which is the
at which the glycol begins to degrade and it will eventually
acidic and begin to eat through the copper pipes.
is a Very Bad Thing!
(see more on glycol
additives, stagnation, etc. below)
have been advised to drain the collectors and flush them
water, then drain them, leaving the pumps off.
optionally cover the collectors with tarps or something
prevent them from heating up. I plan to get some
quotes on custom made boat hatch covers soon.
I did a quick stagnation test, as it was an ideal day
bright and clear with temperature about 70F. I turned
the pumps at 1:00pm (1/2 hour past solar noon) and
15 minutes. After re-connecting the pumps the
temperature from the collectors climbed from
operating temp. of 175F to 220F then dropped back
to 175F. From this I can only surmise that my system
2 - 4X8 ft SunEarth
unlikely to achieve 325F in stagnant conditions.
plan to add 2 more collectors before next winter and that
definitely make a difference in the temperature extremes.
2 more collectors I should see the ideal of 200+F
temperatures with a high likelihood of much higher
extremes. I'm considering having custom
covers made for the panels to shade then in the summer.
now the night time temperatures are in the 40's with
dip below freezing a few days ago, so I have time
decide on a course of action.
In recent years the industry has seen an increased use of glycol for
freeze protection in many closed loop applications. Whether closed loop
systems are completely filled with glycol and water or coils are just
treated for protection; problems still occur in these systems.
Three types of glycols have been used for closed loops. The two major
types now used are Ethylene and Propylene. Automotive anti-freeze has
also been used but is not recommended for this type of application.
Ethylene glycols are used for most HVAC applications. These glycols are
used because they offer the most efficient heat exchange media. In
general a 20% ethylene glycol solution will result in a 6% loss of heat
transfer where as a 40% glycol solution will result in a 14.5% loss of
heat transfer. Ethylene glycol should not be used where it could
contaminate potable water, food processing or other products meant for
Propylene glycol was developed to replace ethylene glycol where possible
contact with potable water and food could occur. Propylene glycol does
not have the heat transfer efficiency that ethylene glycol has. It also
takes slightly more propylene glycol to provide the same freeze
protection as ethylene glycol.
It should be realized that all glycols oxidize when exposed to air and
heat. When this occurs an organic acid is formed. If not properly
inhibited, this fluid is very corrosive. Inhibitors are added to the
glycol to act as buffers preventing low pH attack on system metals.
Certain types of inhibitors also passivate the metal surfaces protecting
them from corrosion. These inhibitors can be tested for activity level
with a basic test called Reserve Alkalinity. This test checks the
buffering capacity of the inhibitor. If complete breakdown has not
occurred fresh inhibitor can be added to restore corrosion protection.
Glycol based automotive anti-freeze is different because it is inhibited
with silicates. This type of inhibitor is excellent for protection of
aluminum at high temperatures and where an agitated environment is
present. In a HVAC system where circulation is low and copper and steel
are present, it can gel causing loss of heat transfer and system
plugging. It is designed to be changed every three to four years which
cannot be done in most HVAC systems.
Besides inhibitor breakdown, biological fouling can also occur.
Bacterial slime will grow by feeding on the organic carbohydrates of the
glycol. Certain inhibitors also provide nutrients for bacterial growth.
Once a bacterial slime starts system corrosion will increase.
In systems where a glycol solution is maintained on a continuous basis
an extra corrosion inhibitor such as borate-nitrite and molybdate should
still be added. This extra protection will help prevent corrosion if
basic inhibitor breakdown occurs. This system should be monitored for
freeze protection, reserve alkalinity, inhibitor level and biological
contamination on a routine basis.
In systems where coils need to be drained and glycol flushed through to
protect low areas against freezing, more serious problems tend to occur.
In most cases the main loop becomes contaminated with glycol that has
broken down when spring start-up occurs. Even if the glycol is properly
inhibited at the start, exposure to air for 4 to 6 months will result in
oxidation of the glycol. Not only will this glycol have broken down to
form an organic acid but bacterial contamination is more likely. Before
the spring start-up, each coil should be flushed with fresh water at
least three times to remove as much of the residual glycol as possible.
If the system is started up and glycol contamination occurs resulting in
a low pH excursion and biological contamination, the system should be
drained and flushed if possible. If this is not possible because of the
size or design of the system further steps need to be taken. A by-pass
filter system should be installed to remove the corrosion by-products.
Filters as low as one micron maybe required to remove contaminants.
An appropriate biocide such as isothiazalone should be slug fed to
provide biological control. The system pH should be gradually brought up
to at least 8.5 with the addition of an alkalinity builder. If the pH is
brought up too quickly or too high the iron in solution may precipitate
and cause system plugging. Your corrosion inhibitor should be brought up
to maximum strength. Continued monitoring is required to make sure
recontamination doesn't occur.
Closed loop systems cannot be neglected. It is just as important that
they be maintained corrosion and foulant free as it is with open