What is the difference between ethylene glycol and propylene glycol?

Ethylene glycol generally provides slightly better heat transfer and lower viscosity at the same freeze protection level, but it is toxic and is not used where accidental contact with potable water is possible. Propylene glycol is less toxic and commonly selected for food processing, snow melt, and potable-water-adjacent systems, though it usually requires more pumping energy.

How often should I replace glycol in an HVAC system?

Glycol is not changed on a fixed calendar alone. It should be tested periodically for concentration, pH, reserve alkalinity, and inhibitor condition. Many operators test annually. Replacement or re-inhibition is needed when the fluid degrades, becomes contaminated, or no longer meets the system manufacturer's chemistry targets.

Glycol Freeze Point Calculator

Hydronic loops, chilled water, heat pump circuits. Estimate freeze point, burst point, specific gravity, viscosity factor, and mixing volumes for ethylene and propylene glycol solutions.

Typical HVAC range

25–40%

Practical upper limit

60%

30% EG freeze pt

3°F

30% EG burst pt

−15°F

Glycol type PG for food-contact / potable-adjacent systems

Concentration (%) 0–70% (EG). PG table tops out at 60%.

System volume (gallons) Total fluid volume in the loop

Units Switches inputs and outputs

Freeze point

3.0°F

30% ethylene glycol

Burst point

−15.0°F

Glycol volume needed

30.0gal

Water volume needed

70.0gal

Specific gravity

1.039

Viscosity factor

1.5×

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Tip Use inhibited HVAC-grade glycol, not automotive coolant — different corrosion package, different metal/seal compatibility. Verify field concentration with a refractometer matched to the glycol type.

Worked example

100 gallon hydronic loop, 30% ethylene glycol. Find mixing volumes and freeze-protection margin.

1. Mixing volumes Glycol vol = 100 × 30 / 100 = 30 gal Water vol = 100 − 30 = 70 gal 2. Lookup — 30% EG Freeze point = 3 °F Burst point = −15 °F Specific gravity = 1.039 Viscosity factor = 1.5× 3. Design margin If coldest expected fluid temp ≥ 15 °F, freeze-point margin ≈ 12 °F — adequate for light winter exposure. Mix = 30 gal glycol + 70 gal water

Glycol concentration reference

Conc. (%)	EG freeze (°F)	EG burst (°F)	PG freeze (°F)	PG burst (°F)	Notes
0	32	32	32	32	Plain water baseline.
10	26	20	26	20	Minimal protection for mild climates.
20	17	5	18	6	Brief cold exposure.
30	3	−15	7	−10	Common light-commercial hydronic start.
40	−13	−35	−8	−30	Good winter margin, noticeable viscosity penalty.
50	−34	−60	−28	−55	Strong protection, pump energy rises.
60	−52	−80	−55	−80	Usually the upper practical limit.
70	−64	−100	—	—	EG only. PG above 60% is rarely practical.

Common mistakes

Using automotive antifreeze. Wrong inhibitor package for HVAC metals and seals — can cause copper, aluminum, or elastomer damage.
Maximizing concentration "to be safe." Viscosity rises sharply above ~50%. Heat transfer drops, pump horsepower climbs. Size from actual minimum fluid temperature.
Not testing annually. Inhibitor package depletes. pH, reserve alkalinity, and concentration drift. Test each season before the cold months.
Mixing EG and PG. Different refractometer scales, different inhibitors. Drain, flush, and recharge with a single fluid type.

Warn Ethylene glycol is toxic. Do not use in open-loop or potable-adjacent systems. Follow local plumbing code for backflow protection and labeling. Burst point ≠ safe operating temperature — freeze point is the design target.

FAQ

What is the difference between ethylene and propylene glycol?

EG usually gives better heat transfer and lower viscosity at the same freeze-protection target, which is why it is common in closed industrial and commercial loops. Its drawback is toxicity. PG is less toxic and is preferred in food plants, ice rinks, schools, and potable-water-adjacent systems. The safer fluid often comes with a larger pumping penalty.

What glycol concentration should I use?

Select concentration from the minimum expected fluid temperature, then add a reasonable design margin. Most HVAC systems end up between 25% and 40%. Avoid blindly maximizing — very strong mixtures raise viscosity, hurt heat transfer, and add pump horsepower.

What is the difference between freeze point and burst point?

Freeze point is when crystals first appear; the fluid becomes slushy and loses performance. Burst point is lower and relates to the temperature at which expansion of partially frozen mixture can break a closed pipe or coil. Designers size concentration to stay safely above the freeze-point margin, not just above burst.

How do I measure glycol concentration in the field?

Refractometer calibrated for the glycol type being tested. Take a representative sample, confirm the scale matches EG or PG, and compare the reading to the manufacturer chart. Hydrometers also work in some programs, but refractometers are faster and less sensitive to operator error.

How often should I replace glycol?

Condition, not calendar. Test concentration, pH, reserve alkalinity, inhibitor condition, and contamination annually. Replace or re-inhibit when fluid has degraded or lost inhibitor protection. A loop still within spec can stay in service much longer than one overheated, diluted, or exposed to oxygen ingress.

Sources

ASHRAE Handbook — HVAC Applications, Ch. 13 (2023) Dow DOWTHERM / DOWFROST engineering data MEG Ethylene glycol property tables NIST Aqueous glycol thermophysical data

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