ASHRAE 2015 Snow Melt Design Loads
Chapter 51 of the ASHRAE Applications Handbook sets design heating loads for snow melting systems. The load depends on snowfall rate, wind speed, outdoor air temperature, and how "clean" the surface must be kept (Class I = no snow on surface at any time, Class II = thin layer OK, Class III = accumulation allowed). For residential driveways and sidewalks the Class I load is typical.
Typical Heating Loads by Climate
Values for Class I residential snow melt systems, BTU/hr per square foot of surface, plus 15% back-loss:
| Climate zone | Cities | BTU/hr/ft² | W/m² | 600 ft² driveway |
|---|---|---|---|---|
| Mild | Seattle, Portland OR, Vancouver BC | 100 | 315 | 69,000 BTU/hr (20 kW) |
| Moderate | Chicago, Denver, NYC, Boston | 125 | 395 | 86,250 BTU/hr (25 kW) |
| Cold | Minneapolis, Buffalo, Fargo | 150 | 475 | 103,500 BTU/hr (30 kW) |
| Severe | Anchorage, Int'l Falls, Yellowknife | 200 | 630 | 138,000 BTU/hr (40 kW) |
Hydronic vs Electric
Hydronic (PEX + glycol + boiler) is standard for residential. 1/2" PEX at 9" OC delivers about 140-170 BTU/hr per sq ft at 140 °F supply — plenty for the loads above. Electric snow melt cable or mats delivers 30-50 W/ft² (about 100-170 BTU/hr/ft²) — also adequate but 3-4× more expensive to run. Use electric for small areas (steps, entryways, under 100 sq ft) and hydronic for driveways and sidewalks.
Annual Operating Cost
Typical run time: 50-150 hours per year depending on climate and snowfall frequency. A moderate-climate 600 ft² driveway at 86,000 BTU/hr running 100 hours uses about 8.6 million BTU per year. At $1.50/therm natural gas with a 95% AFUE boiler that is about $136 per year. Colder climates easily double or triple this. Always budget the operating cost before committing — snow melt is convenient and safe, not cheap.
Frequently Asked Questions
Do I need under-slab insulation?
Yes. R-10 minimum foam insulation reduces downward heat loss from 40-50% to 10-15%. The calculator assumes insulated installation.
What water temperature for hydronic snow melt?
130-150 °F supply with 20 °F ΔT. Higher temps risk cracking the slab from thermal shock when snow first contacts the warm concrete.
Can I use my existing boiler?
Only if it has enough extra capacity — add the snow melt load to your heat loss and check. Dedicated snow melt boilers avoid this problem.
How long does the system take to warm up?
1-3 hours to bring the slab from cold to operating temperature. Most systems use a slab sensor + weather forecast to pre-heat before a storm.
Is snow melt worth the cost?
For safety in elderly-access areas, steep inclines, and liability-sensitive commercial walkways — yes. For pure convenience on a flat driveway — often not.
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