The actual calculation of the power necessary for obtaining a non-freezing surface is complicated and depends on a lot of variables. To simplify the projection of heating systems, the table of recommended areal power density for the most common surface compositions can be used.
| Table of recommended areal density | |
| Outdoor surface | power [W/m2] |
| Surfaces cooled only from above: | |
| concrete surface | 250 … 280 |
| flooring up to 3cm laid on a layer of glue | 250 … 280 |
| flooring up to 4cm laid into powdery materials | 280 … 310 |
| stone, lock flooring of 4 to 8cm laid into powdery materials | 300 … 330 |
| asphalt of 2 to 4cm, HC placed in a concrete foundation layer | 290 … 330 |
| asphalt of 3 to 8cm, HC placed into an asphalt layer | 300 … 350 |
| Surfaces cooled from both above and underneath: | 300 … 400 |
The power density was determined based on the the assumption of laying the heating cable in a depth of 3 to 6cm under the surface. In lock flooring 2cm under the surface. A general applies, that the closer to the surface we install the heating cables, the higher will the efficiency of the heating system be.
Calculation example:
Calculation of the necessary output for obtaining a non-freezing temperature of the surface:
where
k – coefficient of increasing dynamics in heating system, we mostly choose ca. 1.8
Qpovr – heat transfer through boundary layer air – upper surface,
Qnad – heat loss of layers above the heating cables,
Qpod – heat loss in the concrete below the heating cables,
Qspod – heat transfer through bottom layer - subbase.
Other quantities needed for the calculation of the partial values:
tvenk – minimal outside temperature above the ramp, e.g. -30°C,
tžád – required temperature on the ramp surface, typically 3°C,
tkab – is the minimal temperature around heating cable (6.5°C),
α – coefficient of heat transfer between air and upper surface, for concrete 1.18
λ – coefficient of heat transmission, for concrete 0.75 W/mK,
dnad – thickness of concrete layer above the heating cable.
