Catalogue

UV-PRC004-EN 37
Performance Data
Glycol in an HVAC System
Because the detrimental effects of glycol are lower at high temperatures, little concern is given to
capacity loss or increased pump power when glycol is added to heating systems. This is why it is
not uncommon to see glycol percentages up to 40 percent in the heating loop of a system.
However, the same is not true for cooling systems. Concentrations of this level are intolerable in
cooling systems where fluid temperatures are lower. The viscosity of the glycol increases as the
temperature of the mixture drops. This not only decreases the effectiveness of the heat transfer,
but it also makes the mixture more difficult to pump. To make things worse, as the percentage of
glycol increases, the risk of having laminar flow in the coil increases. This again is because glycol
is more viscous than water.
With these effects in mind it is important to use a minimum amount of glycol to protect the HVAC
system.
Burst Protection vs. Freeze Protection
Burst protection is sufficient in systems where there is adequate space to accommodate the
expansion of an ice/slush mixture. The protection works as follows: As the temperature drops
below the solution’s freeze point, ice crystals begin to form. Because the water freezes first, the
remaining glycol solution is further concentrated and remains fluid. The combination of ice crystals
and fluid make a flowable slush. The volume increases as this slush forms and flows into the
available expansion volume (usually an expansion tank). When a sufficient concentration of glycol
is present, no damage to the system will occur.
Freeze protection is required in cases where no ice crystals can be permitted to form or where there
is inadequate expansion volume available. HVAC systems intended to start-up in cold weather after
prolonged winter shutdowns may require freeze protection. Table 27 is provided by Dow Chemical
Co. for its ethylene and propylene glycol products.
Ta bl e 27 shows that a 30 percent ethylene glycol solution is enough to protect a system down to
-60°F. Because of the benefits of burst protection, excessive glycol only degrades the heat transfer
and i
ncreases the pressure drop of the fluid without providing additional system protection. Use
glycol correctly.
Table 27. Percentage volume glycol concentration
Temperature (°F)
For Freeze Protection For Burst Protection
Ethylene Glycol Propylene Glycol Ethylene Glycol Propylene Glycol
20 16% 17% 11% 11%
10 25% 26% 17% 18%
0 33% 34% 22% 23%
-10 39% 41% 26% 28%
-20 44% 45% 30% 30%
-30 48% 49% 30% 33%
-40 52% 51% 30% 35%
-50 56% 53% 30% 35%
-60 60% 55% 30% 35%
Glycol Correction Factors
UV-PRC004-EN.book Page 37 Monday, February 4, 2013 11:01 AM