Glycol Mix for Hydronic Snow-Melt Systems

Why this matters

A snow-melt loop runs in 5 F supply temperatures and sits idle for months. Plain water freezes, ruptures the slab loop, and the next call is a concrete saw. Use the wrong glycol (automotive antifreeze), and you get a heat-exchanger acid burn within two heating seasons. Use the right glycol but mix it wrong, and you either lose freeze protection (under-strength) or lose heat transfer (over-strength) and burn extra fuel for the life of the system. The glycol decision is one of the few install choices that, once it goes into the slab, you cannot easily undo without flushing the entire loop, so getting it right at install is worth the hour of math.

Glycol type: propylene vs ethylene

• Propylene glycol (PG) is the standard for HVAC and snow-melt. Lower toxicity (LD50 around 20 g/kg in rats), approved for incidental potable contact (FDA GRAS), tolerated by most boiler manufacturers without warranty issues. Higher viscosity than EG; system pump head increases.
• Ethylene glycol (EG) has better heat transfer, lower viscosity. Higher toxicity (LD50 around 4.7 g/kg), banned in many AHJs for any potential potable cross-connection. Industrial chiller plants use EG; residential snow-melt uses PG.

Use inhibited HVAC-grade glycol only. Approved brands: Dowfrost HD, Noble Noburst HG, Hercules Cryo-Tek 100/-100, Camco Boiler Antifreeze, Fernox Alphi-11. Never automotive antifreeze (silicate inhibitors precipitate at hydronic temperatures and clog the boiler heat exchanger). Never uninhibited "lab-grade" glycol (no corrosion package, acid generation begins within months).

Concentration: freeze protection vs heat transfer

Two thresholds matter:

• Freeze point. Temperature at which the first ice crystals form. The slab loop must never freeze, but a slush of crystals at the lowest design temp is acceptable; full solid freeze is not.
• Burst point. Temperature at which the frozen mix expands enough to rupture pipe. Typically 10 to 20 F below the freeze point because the crystal slurry has room to expand into the warmer fluid.

Snow-melt design supply temp is typically 100 to 130 F; minimum loop temp during ambient soak is the local design dry-bulb min. For a Zone 5 climate (winter design 0 F), specify mix that protects to -10 F freeze point with margin. For Zone 6 (-10 F design), specify -20 F. For Zone 7 (Minneapolis class, -20 F design), specify -30 F.

Concentration by volume (propylene glycol):

Going above 50 percent PG costs significant pump head and heat-transfer capacity for diminishing freeze-point gain. Above 60 percent PG, viscosity at low temp can stall a marginally-sized circulator.

Heat transfer penalty

Compared to plain water at the same flow rate, 50 percent PG transfers about 80 to 85 percent of the heat per gallon. To deliver the same BTU/hr to the slab, the loop needs about 18 percent more flow, which means higher pump head, often the next pump size up. Account for this in:

• Pump selection: use the glycol-corrected flow curve from the pump manufacturer
• Pipe sizing: bumping flow can push the loop into turbulent velocity zones (>4 ft/sec) that cause erosion
• Heat exchanger sizing on hydronic separator installs: derate plate count for glycol service

The Caleffi Idronics 5 manual publishes correction tables for PG/EG at common mixes; use those, not water tables, for snow-melt design.

Mixing procedure

1. Calculate system volume. Add the slab loop volume (tube length x volume per foot from PEX manufacturer table), boiler/heat-exchanger water side, piping, expansion tank water side.
2. Multiply by target concentration. 100 gal system at 40 percent PG = 40 gal pure PG + 60 gal water. Order 5 gal extra for trim.
3. Premix in a clean drum or large bucket. Mix water and glycol thoroughly before injecting; partial-mixed slugs leave stratified zones. Use deionized or low-TDS water; high-mineral well water deposits scale that the glycol cannot dissolve.
4. Pump in via the boiler drain using a transfer pump or hand-pump. Larger systems: rent a glycol fill cart (most plumbing-supply rental fleets have one) that includes a pressure gauge and bypass for the boiler relief.
5. After fill, run the system for 30 minutes to circulate and homogenize. Take a sample from a system low point (boiler drain) with a calibrated refractometer (Mira PG-G50 or equivalent). Confirm concentration matches design. Adjust by adding small charges of pure glycol or pure water until refractometer reads target.

Maintenance and testing

Annual test:

• Refractometer reading to confirm concentration has not drifted (leaks topped off with city water dilute the mix)
• pH test strip. Healthy inhibited PG runs pH 8.0 to 10.0. Below 7.5, inhibitor is depleted and the mix is turning acidic; add inhibitor concentrate per manufacturer, or schedule a full flush and replace.
• Visual: clear, slight color cast (most makers add a dye), no flocculent material or oily film.

Replace glycol every 5 to 7 years on a snow-melt system; sooner if pH testing shows depletion. Spent glycol must be disposed per local hazardous-waste rules; never to storm drain.

References

• ASHRAE Handbook, HVAC Applications 2023, Chapter 51 (Snow Melting and Freeze Protection)
• Dow Heat Transfer Fluids: Glycol-Based Fluids Engineering and Operating Guide
• Caleffi Idronics 5: Hydronic Snow and Ice Melting Systems
• ASTM E202: Standard Test Methods for Analysis of Ethylene Glycols and Propylene Glycols
• ASHRAE Standard 188-2021: Legionellosis Risk Management (potable cross-connection rules)
• IAPMO Uniform Plumbing Code 2021, Appendix M (Hydronics)