Glycol freeze points by concentration
How cold an ethylene or propylene glycol solution can go before ice, against how much glycol is in it, 0 to 60 % by volume. The table rows are the published ASHRAE figures; the curves are drawn through them.
The shape is the lesson. Freeze protection deepens quickly to about 50 %, more slowly to 60 % — and then the curve turns. Pure ethylene glycol freezes at -12.9 °C, warmer than a 30 % mixture. Topping a loop up “to be safe” past the minimum is actively removing protection, which is why nobody should charge by eye.
| Glycol by volume | Ethylene °C | Ethylene °F | Propylene °C | Propylene °F |
|---|---|---|---|---|
| 0 % | 0.0 | 32.0 | 0.0 | 32.0 |
| 10 % | -3.2 | 26.2 | -2.9 | 26.8 |
| 20 % | -7.8 | 18.0 | -7.1 | 19.2 |
| 30 % | -15.3 | 4.5 | -13.1 | 8.4 |
| 40 % | -24.3 | -11.7 | -21.1 | -6.0 |
| 50 % | -36.8 | -34.2 | -33.5 | -28.3 |
| 60 % | -52.8 | -63.0 | -51.1 | -60.0 |
| 100 % (pure) | -12.9 | 8.8 | -59* | -74* |
*Propylene glycol does not freeze crystalline at high concentration — it thickens into a glass; −59 °C is where pure PG sets, not an ice point. Between 60 % and pure glycol the published phase diagrams genuinely disagree — about the freezing points and even about how many eutectic points the ethylene system has — so this page tabulates the span the sources agree on and says plainly what happens beyond it.
Enter the design temperature — the coldest the fluid must still flow at — in either scale, and read the required concentration for both chemistries. The charts follow the input: the same published anchors as the table above, drawn the way the question is actually asked, concentration against target temperature.
| Ethylene glycol | 41 % by volume (exact 40.6 %, rounded up — protection is a floor) |
| Propylene glycol | 44 % by volume (exact 43.6 %, rounded up — protection is a floor) |
Freeze protection, not burst protection — a dormant loop survives colder than it can run. Below about 20 % the inhibitor package is too dilute to protect steel.
Ethylene glycol: Past the tabulated span the curve reaches its minimum and turns: pure ethylene glycol freezes at −12.9 °C, warmer than a 30 % mixture. The published phase diagrams disagree about exactly where the minimum sits — even about how many eutectic points the system has — which is why the line stops rather than guessing.
Propylene glycol: Above the tabulated span propylene glycol solutions stop freezing crystalline at all: instead of forming ice they thicken into a glass, so there is no freeze point to tabulate. Pure propylene glycol sets at about −59 °C. Long before that the viscosity has made the mixture unpumpable, which is the practical limit.
Freeze protection is not burst protection
Two different questions. Freeze protection is the temperature above which the fluid still flows and the system can run. Burst protection is the colder temperature down to which a dormant system survives without splitting pipe: below the freeze point a glycol solution forms a slush of ice crystals in liquid that expands far less than solid ice, so the pipe lives even though the loop cannot run.
Burst points sit well below freeze points and are published per product rather than per chemistry — inhibitor packages differ — so this page does not tabulate them. For scale, Dow’s published example at −12 °C (10 °F): freeze protection needs about 26 % ethylene glycol but burst protection only about 18 %; propylene needs about 30 % against 21 %. Check the manufacturer’s chart for the actual fluid in the loop.
One more number worth knowing: below roughly 20 % the inhibitor package is too dilute to protect steel, and the mixture turns corrosive within a few seasons. The floor matters as much as the ceiling.
Freeze point is only the first cost of glycol. The same concentration that protects the loop also raises viscosity and lowers specific heat — the full temperature curves for each mixture are on their own pages:
ASHRAE Handbook—Fundamentals, Ch. 31 (Secondary Coolants); pure-glycol point from manufacturer data. Concentrations are by volume, which is how glycol is sold, mixed and read on a refractometer; a mass-percent mixture is a different fluid with a different freeze point. Typical values for inhibited product — the data sheet for the specific fluid in the specific loop beats this table every time.