Why some closed loops run glycol, which glycol to use, how to keep its concentration and protection right, and why a glycol loop must never be topped up with plain water.
Glycol concentrate and inhibited glycol are chemicals. Read the supplier’s Safety Data Sheet before handling, and dispose of drained glycol as local regulations require. See Chemical Handling.
Glycol lowers the freezing point of the loop fluid. It is used where a loop, or part of it, can see freezing temperatures: outdoor coils and dry coolers, rooftop units, snow melt, and process chillers running below 0 °C (32 °F).
| Ethylene glycol | Propylene glycol | |
|---|---|---|
| Heat transfer | Better | Somewhat lower; needs slightly higher concentration for the same freeze point |
| Toxicity | Toxic | Low toxicity |
| Use where | No risk of contact with potable water or food | Any risk of contact with potable water, food, or people |
Follow your local code for backflow protection wherever a glycol loop connects to the potable supply. See Material Compatibility — glycol systems.
Glycol slowly breaks down in service, especially when hot or exposed to air, forming organic acids that attack metal. Inhibited glycols carry a buffer and corrosion inhibitors to neutralize those acids; as they are consumed, the loop’s protection falls.
Never top up a glycol loop with plain water. Every gallon of water dilutes both the glycol and its inhibitor. A loop that loses fluid and is refilled with water gradually loses its freeze protection — often discovered the first cold night.