Protecting Glycol-Water Closed Systems
Technical Bulletin 3-004
Closed Systems
Updated 12/15/11
Glycol-Based Heat Transfer Fluids
Glycol-water mixtures are used to provide freeze protection for HVAC closed loop heating and cooling systems
and other industrial processes. Typically ethylene glycol or propylene glycol is used, with the type of glycol and its
concentration determining the freeze point and other physical properties.
Ethylene vs. Propylene Glycol
In industrial applications, ethylene glycol is more commonly used due to lower cost, lower viscosity (lower pumping
cost), and better heat transfer properties. However, due to its toxicity, the EPA classifies ethylene glycol as a hazardous
substance per SARA 313, which means it is subject to strict reporting requirements in the event of a spill or discharge.
Propylene glycol is much less toxic than ethylene glycol. Because it is not subject to EPA SARA reporting requirements,
it is also considered more environmentally responsible. Propylene glycol is typically used in food processing facilities or
other applications where there is potential for contamination of potable water or foodstuffs. Using it in a system designed
for ethylene glycol can adversely affect performance. For example, the higher viscosity of propylene glycol (about two
times ethylene glycol) directly translates to higher pumping costs and the poorer heat transfer properties can reduce
cooling capacity. Automotive antifreezes are not suitable for HVAC and industrial applications because the high levels of
silicate inhibitors used can cause evaporator fouling and pump seal failures.
Corrosion and Degradation
Properly treated glycol-water solutions will last 20+ years. However, untreated glycol solutions are extremely
corrosive and will eventually degrade to form acids that depress the pH and accelerate corrosion. Systems
containing glycol can have serious, long-term problems unless proper treatment measures are taken to minimize
corrosion and degradation. A comparison of the corrosivity of uninhibited and inhibited glycol-water solutions can be
found in Table One. An uninhibited ethylene glycol solution is 4.5 times more corrosive towards carbon steel than
plain water. If the glycol is allowed to degrade, corrosion is even more severe.
Inhibited vs. Uninhibited Glycol
Glycol systems will not be adequately protected using conventional nitrite- or molybdate-based closed system
treatment programs. They require high levels of specific inhibitors and buffers for protection against corrosion and
glycol degradation. These inhibitors can come premixed with the glycol or can be added as supplemental treatments.
Glycol sold with the inhibitors and buffers in the formulation is known as “inhibited glycol.” Inhibited glycols typically
contain 4-6% inhibitors with the remainder being glycol. When using inhibited glycol products, a minimum of 25%
ethylene or propylene glycol should be added to ensure the proper inhibitor levels are achieved.
Table One
Corrosion Rate of Inhibited and Uninhibited Glycol- Water Mixtures
Corrosion Rate (MPY)
Metal Type
Water
Ethylene Glycol
Propylene Glycol
Inhibited
Ethylene Glycol
Inhibited
Propylene Glycol
Mild Steel
9.69
44.50
9.80
0.04
0.04
Cast Iron
21.20
55.70
16.20
0.13
0.05
Copper
0.08
0.16
0.16
0.12
0.04
Brass
0.23
0.46
0.20
0.12
0.08
Aluminum
13.20
19.80
1.80
0.44
+0.36 (weight gain)
Study and test results by Dow Chemical. ASTM Test D1384 - 190ºF for two weeks, 30% by volume glycol, air bubbling used
When using inhibited glycol, it is necessary to compensate for the percentage in the product that is not glycol. For
example, a 20% by volume mixture of ethylene glycol and water is required for freeze protection
to 16ºF. If the inhibited glycol is 96% active, it will take 20.8 gallons per 100 gallons of system
water (20 gallons divided by 0.96) to achieve a 20% glycol concentration.
©2011 Chem-Aqua, Inc.
Water Quality
To prevent the inhibitors from precipitating, high-quality water should be used for glycol-water solutions. In
general, the water quality should follow these guidelines, especially with higher temperatures
•
<50 ppm calcium hardness
•
<50 ppm magnesium hardness
•
<25 ppm chloride
•
<25 ppm sulfate
If the hardness or other parameters are significantly higher than the recommended guidelines, softened or
de-ionized water should be considered.
Testing and Monitoring
Glycol-water solutions should be tested regularly to determine the percent glycol, pH, alkalinity, inhibitor
levels, and degree of contamination. A specially calibrated refractometer is the most reliable and practical
means of determining the percent glycol in the field. Periodic laboratory analysis of the glycol solution
is recommended. Glycol degradation is a serious problem that can be difficult to address once started.
Symptoms of glycol degradation include pH depression, presence of a sharp aldehyde or septic odor,
severe steel corrosion often accompanied by high copper levels, and microbiological problems. Depending
on its severity, salvaging the degraded glycol may not be practical.
Precautions
Automatic water-only makeup addition is generally not recommended for glycol systems, especially if
used in low temperature applications (<40ºF). Any makeup addition may require glycol and inhibitors
to maintain the desired level of protection. Low-level glycol contamination, which can occur when coils
are not thoroughly drained and flushed following winter lay-up, can cause very severe corrosion and
microbiological problems. When using inhibited glycol solutions, galvanized piping is not recommended at
>120ºF and aluminum is not recommended at >150ºF.
Table Two
Percent Volume vs. Freeze Point for Ethylene and Propylene Glycol Solutions
Ethylene Glycol
Propylene Glycol
% Volume
Freeze Point (ºF)
% Volume
Freeze Point (ºF)
% Volume
Freeze Point (ºF)
% Volume
Freeze Point (ºF)
0
10
20
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
32.0
24.2
14.9
9.3
8.1
6.9
5.7
4.4
3.0
1.6
0.2
-1.2
-2.8
-4.3
-6.0
-7.6
-9.4
-11.2
-13.1
-15.0
-17.0
-19.1
-21.3
-23.5
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
70
80
90
100
-25.9
-28.3
-30.8
-33.5
-36.2
-39.1
-42.0
-45.1
-48.3
-51.6
-55.1
-58.7
-62.4
-66.3
-70.3
<-70
<-70
<-70
<-70
<-70
NA
NA
NA
-12.3
0
10
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
32.0
28.4
20.0
19.0
18.0
17.0
16.0
15.0
14.0
13.0
12.0
11.0
9.0
8.0
7.0
5.0
4.0
2.0
1.0
-1.0
-3.0
-4.0
-6.0
-8.0
-10.0
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
100
-12.0
-14.0
-16.0
-18.0
-20.0
-22.0
-25.0
-27.0
-29.0
-32.0
-34.0
-36.0
-39.0
-41.0
-44.0
-47.0
-50.0
-53.0
NA
NA
NA
NA
NA
-60
©2011 Chem-Aqua, Inc.