Glycol snowmelt, driveway & walk.

What it is, and what it isn't

A hydronic snowmelt system circulates warm propylene-glycol-and-water mix through PEX-a tubing embedded in a poured slab. The slab absorbs the energy from the loop and emits it upward, melting snow and ice on contact and evaporating the resulting melt-water. A snowmelt slab is not de-icing — it is not designed to clear a snowpack on its own — it is designed to be on through the storm, idling at the surface freezing temperature, so accumulation never accrues.

The single most important number on the spec sheet is the design BTU per square foot. For southwest Montana at the elevations we work on, that number is between 100 and 200 BTU/sf: 100–120 for low-elevation walkways under low-wind exposure, 130–150 for typical driveways in the Gallatin Valley, 160–200 for driveways at altitude (Big Sky, Yellowstone Club, the higher canyons) where the wind correction is severe. We pull the design load from ASHRAE Applications Chapter 51's wind-corrected tables — which are public-knowledge tables, available through the ASHRAE Handbook in any university library — and we publish the number on the binder.

Why glycol, why dedicated, why hydraulically isolated

The snowmelt loop runs glycol because it lives partly outside the building envelope; if the system loses power, the loop must survive the freeze. We specify 30 to 40 percent propylene glycol by volume, food-grade where the homeowner has elected it, with a corrosion inhibitor package. Propylene rather than ethylene because propylene is non-toxic if a fitting leaks where pets or children may encounter it. The glycol percentage is sized for the design temperature plus a margin — 30% glycol freezes around about –10°F, 40% around –22°F, 50% around –35°F. The percentage is verified on commissioning day with a refractometer; the reading is stamped on the binder.

The snowmelt loop is hydraulically isolated from the building's heating system using a brazed-plate heat exchanger, almost always a SWEP B12 or Alfa Laval CB14. The reason for isolation is two-fold: it keeps glycol out of the domestic heating loop (glycol is more viscous than water and reduces heat-transfer coefficients), and it allows the snowmelt loop to operate at a different supply temperature curve than the floor loops.

We size snowmelt as a dedicated plant whenever the design BTUH exceeds about 200,000, which is most driveways larger than 1,000 sf. The dedicated plant means a separate mod-con boiler, vented separately, with its own outdoor-reset curve tuned to the snowmelt's idling-on curve rather than the house's heating curve. For smaller systems, we share the boiler with the house with proper primary-secondary piping and a priority sequence at the controller.

How we install it

The slab assembly is built on the rough excavator's and foundation contractor's schedule. We work from rebar-up: 2″ XPS rigid insulation on the prepared subgrade, perimeter insulation to frost line, vapor barrier where required, welded wire reinforcement, tubing tied to the wire at 9″ on-center, slab edge insulation, sensor wells set for the slab thermistor and the slab-mounted moisture sensor. Tubing is pressure-tested at 100 psi for 24 hours before the pour, photographed top-down and side-on, and turned over to the concrete crew. We re-test after the cure. We are present at the pour.

The mechanical room work — boiler, heat exchanger, snowmelt pump, controls — is on its own schedule, in parallel with the house heating mechanical. We use Taco 0011-F4 cast-iron circulators for snowmelt where the design flow exceeds 25 gpm, and Grundfos Alpha2 ECM circulators for smaller systems. The control is almost always a Tekmar 654 Snow Melting Control with a slab sensor, an outdoor sensor, and an optional moisture sensor.

The honest cost conversation

Hydronic snowmelt is the most-asked, second-most-expensive scope we install. Installed cost runs $22 to $38 per square foot in our service area, all-in, including the dedicated plant if required. A typical 1,500 sf driveway lands around $50,000–$65,000. A typical 2,500 sf driveway with a turnaround and approach lands around $75,000–$95,000.

The operating cost is the second honest conversation. A snowmelt slab idling at 33°F surface temperature through a typical Bozeman heating season — about 1,200 idle hours plus maybe 80–120 active-melt hours — consumes roughly 5 to 8 therms per square foot per season at the boiler. At NorthWestern Energy's residential natural-gas rate (approximately $0.95 per therm in 2026), that is about $7 to $11 per square foot per season in fuel, plus the electrical draw of the circulators. For a 1,500 sf driveway, plan on $10,500–$16,500 per heating season in operating cost. This number gets smaller in mild winters and larger in cold ones.

When this is the wrong scope

For a 30-foot front walk from the garage to the front door, an electric resistance mat on a dedicated 30A circuit is usually the right answer, not a hydronic loop. We will refer you to an electrical contractor on our list. For a steep driveway where the homeowner's actual problem is ice rather than snow load, a roof-runoff diversion and a heated trench drain may resolve the problem at one-tenth the cost. For a commercial property with a maintenance contract that includes plowing, snowmelt is rarely cost-justified.

Read across

Read the radiant floor scope for the closely-related warm-floor loop; the boiler scope for the plant; the manifold sheet for the parts that hang on the wall; the Cottonwood Drive job for a 1,850 sf worked example; and the January cold-snap daybook for a record of one running through a –22°F event.