Snow melting system design

Snow melting systems keep sidewalks, entrances, parkade ramps, loading docks and helipads clear by circulating warm fluid through tubing embedded below the surface. In Canada they are common at hospital entrances, emergency ramps, accessible routes, steep parkade ramps and high-end commercial frontages where plowing and salting are impractical or undesirable.

They are also large energy users. A snow melting slab can demand as much heating capacity as the building it serves. Good design balances the level of performance the owner actually needs against capital and operating cost, and uses controls to run the system only when necessary.

Defining the performance level

The ASHRAE Handbook, HVAC Applications volume, classifies snow melting systems by how clear the surface must stay:

Higher classes require higher heat output. Design heat flux depends on the class, the local climate (snowfall rate, air temperature and wind during snowfall) and the free area ratio, which is how much of the surface must be snow-free. ASHRAE tabulates design heat fluxes for many North American cities, including several Canadian ones, and the resulting values commonly range from a few hundred to several hundred W/m2. Windy, cold locations need considerably more than mild, calm ones.

Choosing a lower class or a lower free area ratio is the single largest lever on cost. A hospital ambulance bay may justify Class III; a retail sidewalk often does not.

Heat losses to account for

The design heat output must cover:

• Melting snow and warming the meltwater.
• Evaporation of water from the surface.
• Convection and radiation from the surface to the cold sky and air.
• Back and edge losses into the ground and surrounding slab.

Insulation under the slab reduces back losses, particularly over unheated spaces such as parkade ramps exposed below, or over soils where frost and drainage matter. Edge insulation helps on smaller areas where perimeter losses are a large fraction.

System components

Heat source

Common options include:

• Dedicated condensing boilers, which suit the low return temperatures of snow melt loops well. See condensing boilers.
• Building heating plant with a heat exchanger, when capacity is available during snow events.
• Waste heat from chillers, data rooms or process loads.
• Geothermal or heat pump sources, often supplemented for peak conditions.

Waste heat sources can be attractive because snow melting loads coincide with cold weather, when the building may already have surplus heat from cooling loads in interior spaces.

Fluid

Snow melt loops are filled with an inhibited glycol solution, typically propylene glycol, separated from the building heating system by a plate heat exchanger. Concentration is chosen for freeze protection at the lowest expected slab temperature when the system is idle. See glycol systems.

Tubing

• Cross-linked polyethylene (PEX) tubing with an oxygen barrier is the most common choice.
• Tubing is usually placed at a consistent depth in concrete, often around 50 to 75 mm (2 to 3 in) below the surface, and secured to reinforcing.
• Spacing is commonly in the range of 150 to 230 mm (6 to 9 in), with tighter spacing for higher outputs.
• Circuits are kept to similar lengths so they balance naturally at the manifold.
• Pavers are installed over a sand bed with tubing below; asphalt requires tubing that tolerates installation temperatures or protection during paving.

Manifolds and pumping

Manifolds with flow meters and isolation valves let each circuit be balanced and isolated. Locate manifolds in accessible, heated spaces where possible.

Design procedure

1. Define the areas and performance class with the owner, considering accessibility and safety priorities.
2. Determine design heat flux from ASHRAE data for the nearest climate location, adjusted for exposure.
3. Calculate total load including back and edge losses.
4. Select supply and return temperatures, delta-T and flow, taking glycol properties into account.
5. Lay out circuits and manifolds, keeping circuit lengths and pressure drops similar.
6. Size the heat exchanger, pumps, expansion tank and heat source.
7. Specify controls, sensors and the sequence of operation.
8. Coordinate drainage: meltwater must drain away or it will refreeze at the edges.

Controls

Controls make the biggest difference to operating cost:

• Automatic snow and ice sensors embedded in the slab detect moisture and temperature and start the system when snow falls.
• Idling mode keeps the slab just above freezing during cold weather so it can respond quickly. Idling improves response but uses significant energy; it should be optional and scheduled.
• Outdoor temperature cut-out stops operation in extreme cold when melting is impractical and energy use would be very high.
• Slab temperature limits protect concrete from thermal shock. Supply temperature and the rate of change during start-up should be limited.
• Run-on timers keep the system on after snowfall ends to dry the surface and prevent refreezing.
• BAS integration for monitoring, alarms and energy tracking.

Operation and maintenance

• Test glycol concentration and inhibitors annually.
• Check sensors before the snow season.
• Verify pumps and heat exchangers, and clean strainers.
• Inspect surfaces for cracking or damage that could expose tubing.
• Review energy use after each season and adjust idling and setpoints.

Frequently asked questions

Hydronic or electric snow melting?

Electric cable systems cost less to install for small areas but cost more to operate in most Canadian markets. Hydronic systems are generally preferred for larger areas and where a heat source or waste heat is available.

Can a snow melt system be added to an existing walkway?

Only by removing and replacing the surface. It is best planned during new construction or a full surface replacement.

Key takeaways

• Define performance class and free area ratio first; they drive capacity and cost.
• Design heat flux depends heavily on local climate and exposure, and ASHRAE data is the standard reference.
• Use inhibited glycol in an isolated loop, with consistent tubing depth and balanced circuits.
• Automatic sensors, limited idling and cold cut-outs control operating cost.
• Waste heat or heat pump sources can reduce carbon emissions, and energy modeling helps compare options.