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Snow Melting
Manual Controls     
Idle/On Controls
System Performance
Area Free Ratio
Frequency Percentile  


snow melt guidelines
The Radiant Panel Heating and Snow Ice Melt System Guidelines

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Snow Melting - Area Free Ratio
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snow melt for drivewayThe Ar, or the Snow Free Area Ratio, is of importance to client expectation. The quantity of sensible and latent heat flux will determine the condition of the surface based on the following Ar criteria.

When Ar=1, there is to be no accumulation of snow, i.e. the snow melt system must melt, evaporate and drain off all precipitation.  Projects where this is critical include medi-vac applications, steep (sloped) access to parking facilities and bridge decks where loss of control over vehicles can lead to severe property damage or causalities. When Ar=0, there can be 100 per cent accumulation of sufficient thickness to prevent heat and evaporation losses. Projects where this is tolerable might be patios, decks, non-pedestrian walks and or level driveways in non-commercial or non-critical applications.

When Ar=0.5, there can be some accumulation in the form of slush but not ice. Projects where this is tolerable might be driveways, walks, steps and ramps.

The Snow Free Area Ratio is defined by:

Ar = Af / (Af + As) = Af / At


Af  = equivalent snow free area, ft2
As = equivalent snow-covered area, ft2
At  = Af + As = total area, ft2

Frequency Percentiles

Once the designer has determined what coverage is acceptable based on applications and the client’s needs, there must then be some discussion as to practical expectations of the system. Designers should then ask if it is fine to have some slush in the most severe conditions, which may only occur five per cent of the year. This magnitude of severity is described by frequency percentiles (Class I, II and III Methodology has been replaced with percentile method). ASHRAE offers guidance for 75, 90, 95, 98, 99 and 100 percentiles. By definition, the frequency percentile indicates the percentage of time that the required snow melting surface heat flux does not exceed the design value for a given snow free area ratio. It is used to design a snow melting system for a given level of customer satisfaction depending on criticality of function. For example, “...a heliport at the rooftop of a hospital may require an Ar = 1.0 for the 100 percentile storm, a residential driveway may be considered satisfactory at the 90 percentile at an Ar = 0.5 design conditions.”1 The load calculations, equipment and controls designed for 100 percentile satisfaction can be vastly different for the same application with 90 or 75 percentile expectation. For example, a system designed for 100 frequency percentile may require 170F, whereas only 140F is required for a 75 percentile. A non-condensing boiler with return temperature protection might be required in the 170F scenario while a condensing boiler would be used in the 140F scenario without worrying about return temperatures.

1 ASHRAE, 2003 Applications Handbook, Chpt. 50, Snow Melting and Freeze Protection.   

Click here for part I, Introduction
Click here for part II, Manual Controls     
Click here for part III, Idle/On Controls & System Performance      
Click here for part IV, Area Free Ratio and Frequency Percentile  
Click here for part V, Conclusion
Click here for part VI, Storm Data and Loads   


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