Heat Loss Calculations and System Design
heat loss, time to freeze, fluid outlet temperature, minimum flow rates, tracing wattage required, heat gain, etc., all based on the specific requirements of each project. Basic information needed for typical heat trace design includes: project name/location; minimum ambient temperature; above and/or below ground; depth of bury if applicable; core pipe material and diameter; pipe length per circuit; insulation thickness; required maintain temperature; flow direction for sensor positioning; power point location (one or both ends, middle etc.); voltage available.
It is recommended that a safety factor of 10 to 25% be added to allow for such field conditions as voltage drop, under voltage condition, etc.
Heat Flow Chart Watts/ft/hr/100°F T(2)
Dia.^{(1)} 
Urethane Insulation Thickness 

25 mm (1 in) 
40 m (1½ in) 
50 mm (2 in) 
63 mm (2½ in) 
75 mm (3 in) 

1 
1.8 
1.4 
1.2 
1.1 
1.0 
2 
2.9 
2.2 
1.8 
1.6 
1.4 
4 
4.9 
3.6 
2.9 
2.5 
2.2 
6 
6.9 
4.9 
3.9 
3.3 
2.9 
8 
8.9 
6.3 
4.9 
4.1 
3.6 
10 
n/a 
7.6 
5.9 
4.9 
4.2 
12 
n/a 
9.0 
6.9 
5.8 
4.9 
14 
n/a 
10.4 
7.9 
6.5 
5.6 
16 
n/a 
n/a 
8.9 
7.4 
6.2 
18 
n/a 
n/a 
9.9 
8.3 
6.9 
20 
n/a 
n/a 
10.8 
9.0 
7.6 
22 
n/a 
n/a 
11.8 
9.9 
8.3 
24 
n/a 
n/a 
12.7 
10.8 
9.0 
Heat flow in watts per lineal foot
The table is based on the application of the following formula:
Where:
W = Watts/ft/hr (W x 3.414 = Btu/hr)
K = Btu/ft²/hr/1°F/ft = 0.0108 for Urethane
T = temperature differential°F
D = outside diameter of the insulation
d = outside diameter of pipe 1. Diameters for (D/d) taken as 3/1 for 1″ pipe +1″ insulation and is typical for all other combinations.
2. For other than 100°F T, divide by 100 and multiply by required T.
Formula
The heat loss for an externally traced pipe may be calculated by the following formula:
Where:
W = Watts per foot of pipe
Tm = maintained temperature°F
Ta = ambient temperature°F
Ln = natural log
Di = outside diameter of insulation (in)
Dp = outside diameter of pipe (in)
Ki = K value of insulation (BTU • in / hr • ft² •°F)
Dj = outside diameter of jacket (in)
Kj = K value of jacket (BTU • in / hr • ft² •°F)
Sf = Safety Factor
For more information on time to freeze and raw data requirements
Ambient Temperatures – Below Ground
Frost Depth
The frost depth can be fairly accurately calculated as it is usually directly related to the number of freezing degree days for a given geographic location. The exact frost depth will vary depending on the specific soil type and condition, elevation, as well as other variables. The following map indicates the number of freezing degree days for Canada by zone. It is provided courtesy of Environment Canada. The table below provides the average frost depth in meters for any given number of freezing degree days. By consulting the map and then the table, the average frost depth can be obtained.
Normal Freezing Index in Degree Days
1500 
1.57 
3100 
2.59 
8.5 
1550 
1.62 
3150 
2.62 
8.6 
1600 
1.66 
3200 
2.64 
8.7 
1650 
1.70 
3250 
2.67 
8.8 
1700 
1.74 
3300 
2.69 
8.9 
1750 
1.78 
3350 
2.72 
8.9 
1800 
1.82 
3400 
2.74 
9.0 
1850 
1.86 
3450 
2.77 
9.0 
1900 
1.90 
3500 
2.79 
9.1 
1950 
1.94 
and more 
2.80 
9.2 
Soil Temperature
The soil temperature at a given depth will vary depending on the soil type, moisture content, etc. The following table which is provided, courtesy of the Ontario Ministry of the Environment, can be used as an approximate guide to determine soil temperature. Refer to the map below to obtain the freezing index degree days for the location being studied, then plot to obtain the approximate soil temperature at a given depth.