Every hydronic system in a freeze-prone climate needs glycol. But glycol is not free performance, it thickens the fluid, reduces heat transfer capacity, and degrades over time. The goal is the minimum concentration that provides adequate freeze protection with an acceptable safety margin, not the maximum concentration the system will tolerate.
This guide covers propylene glycol (the HVAC standard for potable and occupied-space systems) and ethylene glycol (industrial-only, toxic but better thermal performance). The data comes from the ASHRAE Handbook, Fundamentals, Chapter 31 and manufacturer product data from Dow Chemical (DOWFROST and DOWTHERM SR-1 lines).
Freeze Point vs. Burst Protection
Glycol solutions do not freeze like water. They form a slush that gets progressively thicker as temperature drops. The freeze point is the temperature where ice crystals first form. The burst protection point is significantly lower, the temperature where the slush becomes rigid enough to actually damage pipes and equipment.
For propylene glycol:
| Concentration | Freeze Point | Burst Protection |
|---|---|---|
| 20% | +18°F | ~+5°F |
| 30% | +5°F | ~−10°F |
| 40% | −12°F | ~−30°F |
| 50% | −28°F | ~−55°F |
In most of the northern United States, 30–40% propylene glycol covers the realistic freeze risk. Going to 50% or above adds cost, pump penalty, and heat transfer loss for protection you likely do not need unless the system is exposed to sustained sub-zero conditions.
Glycol Freeze Protection Calculator
Determine required glycol concentration for freeze protection with performance penalties. Propylene and ethylene glycol data from ASHRAE Fundamentals Ch. 31 and Dow Chemical.
The Performance Penalties of Glycol
Every percentage point of glycol concentration costs you pump performance and heat transfer capacity. The penalties are not linear, they accelerate as concentration increases, especially at lower temperatures.
Viscosity: At 40% propylene glycol and 100°F operating temperature, viscosity is roughly 2–3 times that of plain water. At 180°F, the penalty is smaller (about 1.5×) because heat thins the fluid. But during cold startup, viscosity can spike to 5–10× water, which is when the pump works hardest.
Specific heat: Water has a specific heat of 1.0 BTU/lb·°F. A 40% propylene glycol solution drops to about 0.87 BTU/lb·°F at 180°F. That means you need roughly 15% more flow to deliver the same BTU/hr.
Density: Glycol solutions are slightly denser than water (about 1.03–1.06 specific gravity at typical concentrations). This partially offsets the specific heat loss in BTU-per-gallon terms but adds to the head the pump must overcome.
The combined effect: at 40% propylene glycol, plan on 15–20% more GPM to deliver the same heating capacity, and expect 20–40% more pump head loss through the piping. Size your pump for glycol, not for water.
Glycol Freeze Protection Calculator
Determine required glycol concentration for freeze protection with performance penalties. Propylene and ethylene glycol data from ASHRAE Fundamentals Ch. 31 and Dow Chemical.
Propylene vs. Ethylene Glycol
Propylene glycol is the default for HVAC systems in occupied buildings. It is classified as GRAS (Generally Recognized As Safe) by the FDA and is used in food processing, cosmetics, and pharmaceutical applications. If a heat exchanger fails and glycol enters the potable water supply, propylene glycol is non-toxic at the concentrations involved.
Ethylene glycol is toxic. It has a sweet taste that makes it attractive to children and animals, and ingestion can cause kidney failure and death. It should only be used in industrial systems with no possible cross-connection to potable water.
The performance advantage of ethylene glycol is real but modest: at the same concentration, ethylene glycol provides a lower freeze point, has lower viscosity, and has better heat transfer properties than propylene glycol. A 30% ethylene glycol solution gives roughly the same freeze protection as 40% propylene glycol, with less pump penalty.
For any system in an occupied building, commercial facility, school, hospital, or food service operation: use propylene glycol. The performance difference does not justify the liability.
Glycol Freeze Protection Calculator
Determine required glycol concentration for freeze protection with performance penalties. Propylene and ethylene glycol data from ASHRAE Fundamentals Ch. 31 and Dow Chemical.
Testing and Maintenance
Glycol degrades over time. Heat, oxygen exposure, and dissolved metals (especially copper ions) break down the inhibitor package that prevents corrosion. Degraded glycol becomes acidic and attacks the metals it was supposed to protect.
Test glycol annually with a refractometer (for concentration) and pH strips or a pH meter (for condition). Fresh inhibited propylene glycol has a pH of 9.0–10.5. When pH drops below 7.5, the inhibitor package is exhausted and the glycol should be replaced.
Concentration can change over time as well. Makeup water dilutes the glycol. Evaporation through air separators or expansion tanks concentrates it. A system that started at 35% may test at 25% after a few seasons of makeup water additions, and 25% propylene glycol only protects to about +22°F.
Replace glycol every 3–5 years in residential systems, or when pH drops below 7.5, whichever comes first. In commercial systems with proper inhibitor maintenance programs, glycol can last longer, but annual testing is non-negotiable.
Glycol Freeze Protection Calculator
Determine required glycol concentration for freeze protection with performance penalties. Propylene and ethylene glycol data from ASHRAE Fundamentals Ch. 31 and Dow Chemical.
Glycol Freeze Protection Calculator
Determine required glycol concentration for freeze protection with performance penalties. Propylene and ethylene glycol data from ASHRAE Fundamentals Ch. 31 and Dow Chemical.