Install Instructions
IM-PR 566442 1114 (Design Manual)
98
5.2 Calculating the snow melting load
The methodology Viega recommends to calculate
the snow melting load is based on that described
in the 2008 ASHRAE Handbook – Systems and
Equipment. Use Table 5-1 to find the heat flux
output requirement in Btu/hr/ft². The heat flux
output requirement given in Table 5-1 values does
not include back and edge heat losses, which must
be accounted for separately to get the final heat
output requirement (refer to Table 5-2 for back and
edge loss multipliers).
Procedure:
1. Find the location of the snow melting system
in the rst column.
2. Determine the snow-free area ratio required.
3. Follow to the right and read the snow melting
load under the column representing the
appropriate condence level.
Example:
Find the snow melting load for a Level 2 system in
Boston, MA, with an area-free ratio of 1 and a 98%
confidence level.
By referencing Table 5-1, determine that a 98%
confidence level system with a snow-free area ratio
of 1 in Boston will provide need to supply 202 Btu/
hr/ft², not including back and edge losses. A system
with this area-free ratio and confidence level is
expected to perform as follows in Boston:
• 100% of the surface area will be snow free
98% of the time snow is falling
• In a typical year in Boston, there are 112
snowfall hours, meaning that 100% of the
surface area will be snow free for 109.8 hours
of snowfall (98%*112=109.8 hours)
Of that remaining 2% of the snowfall hours, or
approximately 2.2 hours:
• 1% of the snowfall time (that time between
the 98% and 99% non-exceedance values
given in Table 5-1), snow will accumulate on
the slab to a thickness at which the snow
blanket insulates the slab, but the thickness
will not increase beyond that level. Snow
melts on the underside of the blanket at the
same rate at which the snow is falling: 1.1
hours of snowfall (1%*112=1.1 hours)
• For the last 1% of the snowfall time (that time
between the 99% and 100% non-exceedance
values given in Table 5-1), the system cannot
keep up with the snowfall, and buildup may
occur: 1.1 hours (1%*112=1.1 hours)