Cottonwood Drive Snowmelt.

Scope of work

A retrofit snowmelt installation onto an existing 1996 custom home. The homeowner had been managing the driveway with a combination of plowing (contracted), shoveling (themselves), and salting (themselves, sparingly, given the well water and a stand of mature aspen along the property line). The driveway is 220 feet long from the garage to the road, with a 6% downhill grade for the last 140 feet — a section that froze hard most mornings between November and March and was the single biggest concern.

Scope: 1,400 sf of driveway snowmelt (the downhill 140-ft section plus a 60-ft turnaround), 320 sf of front walk, 130 sf at three sets of steps off the front porch. Excluded: the upper driveway closest to the road, which the homeowner elected to continue having plowed (the public road is plowed first, the driveway second, so heavy snowmelt at the road end would have to compete with plow piles and the homeowner accepted that trade).

Why a dedicated plant

The existing house plant was a 2008 atmospheric Burnham boiler — 130,000 BTUH input, no modulation, set to a fixed 180°F supply. Sharing the snowmelt with the existing plant would have required a plant replacement and a significant rework of the heating distribution to accommodate the snowmelt's primary-secondary piping and outdoor-reset requirements. The homeowner declined the heating rework; we sized and installed a dedicated snowmelt plant in a new wall-hung position adjacent to the existing boiler in the mechanical room. The two plants run independently, sharing only the gas line trunk and the condensate drain (via a separate neutralizer for the new boiler).

Dedicated plant: Triangle Tube Prestige Solo 250, 250,000 BTUH input, 5:1 turndown, helical stainless heat exchanger. Vented in 3″ CPVC through the south wall at 6 feet AFG, with a horizontal termination keeping the vent and intake separated per the manufacturer's instructions. Condensate to a Caleffi NeutraSafe XL inline neutralizer, then to the existing washer standpipe with backflow protection.

The slab and the trunk run

The slab was poured in October 2023 by Bozeman Concrete Co., 4″ thick, 4,500 psi mix with fibrous reinforcement and welded wire mesh, over 2″ of XPS rigid insulation, with the tubing tied to the mesh at 9″ o.c. The slab was poured around our tubing in two pours, separated by a control joint at the turnaround/driveway boundary; both pours were photographed top-down before placement and water-tested at 100 psi for 24 hours before cure.

The trunk run from the mechanical room (located in the basement, 320 feet from the slab edge) was the single most expensive part of the install. The run leaves the mechanical room through a wall sleeve, drops into a buried polystyrene insulating chase under the lawn, and re-emerges at the slab manifold cabinet on the garage exterior wall. The pipe in the chase is 1¼″ PEX-AL-PEX (chosen for its low expansion coefficient) wrapped in 1″ closed-cell elastomer insulation and an HDPE outer jacket; the heat loss across the chase at design conditions is approximately 3.4°F end-to-end, accounted for in the supply-temperature commissioning.

Commissioning record · 2023-12-04

Outside temperature on commissioning day: –2°F at 06:30 MT (system start), warming to +14°F by 15:20 MT (commissioning complete). Building (the house, not the slab) was occupied. The slab had cured 47 days at start of commissioning.

Snowmelt loop pressurized to 100 psi with city water for the leak test, then drained and refilled with the prepared 32% propylene glycol mix (Reichert refractometer reading at fill: 32.4%). Heat exchanger isolated from the heating side via the brazed-plate. Boiler started at minimum firing rate; combustion tested with Testo 320 at 25%, 50%, 75%, 100% firing rates; CO < 14 ppm air-free across the range; CO₂ 8.9–9.3%; O₂ 4.9–5.4%; stack temperature 122–164°F across the firing range. Snowmelt curve set: 150°F supply at –10°F outside, 110°F supply at 30°F outside, idle-on temperature 33°F slab surface.

Each of the six loops was purged separately via the manifold purge valves with the loop isolated; Δt at each loop measured at the manifold against design (18°F design, 16.4–19.1°F as-purged); actuator preset adjusted on loops C-1, C-3, and W-2 to bring Δt within 1°F of design. The moisture sensor on the slab (Tekmar 090) was tested by misting the sensor with water at 07:48 MT and verifying control activation within 22 seconds.

Operating profile · two heating seasons in

Through two complete heating seasons (2023–24 and 2024–25), the system has logged 1,224 hours of idle-on operation and 187 hours of active-melt operation, with 24 detected snow events. Average glycol concentration at annual service has held within 0.6% of the commissioning value, indicating no significant glycol loss. Average natural-gas consumption for the snowmelt plant: 1,840 therms in 2023–24, 1,520 therms in 2024–25 (the second winter was milder). At the homeowner's NorthWestern Energy residential gas rate, that is approximately $1,750 (2023–24) and $1,440 (2024–25) in fuel.

The homeowner's report after the first season was that the driveway has been clear for every morning commute since commissioning, with one exception: a 14-inch storm on 2024-01-08 that exceeded the system's active-melt capacity. The slab was clear within 90 minutes of the storm ending, which the homeowner accepted as design-as-intended.

Credits

Engineer of record · Iris Cottonwood. Slab layout + tubing pull · Kel Begay (lead) + Marlowe Krenn. Mechanical room install · Lars Treadway + Kel Begay. Trunk run + chase · Kel Begay + Marlowe Krenn. Commissioning · Wendell Pratt + Lars Treadway. Binder · Halla Bjørndal. Concrete pour by Bozeman Concrete Co. (separate contract). Signed off by Lars W. Treadway, 2023-12-04.

Read across

See the snowmelt scope for the design framework, the manifold sheet for the Uponor ProPEX manifolds we used, and the January cold-snap entry for the night this slab and its sister at the Bridger Canyon residence carried the brunt of a –22°F event.