In addition to outdoor degree days, winter design temperatures and other weather-related factors included in standard heat loss calculations, the following should be considered:
Snow - Light snow melts more quickly than heavy snow, and the more quickly it falls, the more energy is required to keep an area clear. Also, be sure to consider wind-blown snow accumulation, which often can double the amount of snow in a given area.
Air Temperature - Snow may fall when the air temperature is lower than 4ºC (39ºF). In addition, air temperature affects refreezing of melted snow or ice. To be effective, a system must be able to maintain a temperature of at least 1ºC (34ºF) until the surface is completely dry.
Wind Velocity - Cold air moving very quickly will blast a surface with freezing air and moisture. To melt ice and snow in locations with high wind velocity requires a system with higher heat output than the area might otherwise warrant.
Slab surface temperature - Although a surface need only reach a temperature of 1ºC (34ºF) to prevent ice formation, in practice it must be higher because of several factors. When the system turns on the slab is often insulated by a layer of snow, however after the snow melts the system must be able to compensate for the wind velocity.
A higher surface temperature also prevents ice bridging – air gaps created when only the bottom part of the snow layer melts, leaving unmelted snow and ice on top.
Drainage - The system must include a drainage system so water from melted snow leaves and does not refreeze. More energy is needed if water does not drain away. It’s a good idea to heat the drains themselves as well so ice doesn’t form and clog the drain.
Insulation - Insulation below the heated surface and around its edges minimizes heat loss and directs as much heat as possible to the top layer where the snow and ice accumulate.
Not only is less energy needed, but the system works more quickly.
The best insulation choice is high-density, rigid foam that can handle the typical pressures of dynamic and static forces created while the system is operating. One inch of rigid foam insulation used in concrete slabs typically has an R-value of 5 with a load rating of 25 psi.
Surface materials - The most common surface materials are concrete, asphalt, brick, stone and pavement. Piping, insulation and installation accessories to secure the pipe are needed.
Heating fluid - Normally, the fluid heated and pumped through the pipes is a propylene glycol (PG) and water mixture. The percentage mixture is determined by how low the outside temperature is expected to be.
Normally, a 50-50 mixture is sufficient. It is important to remember the hydraulic properties of the heating fluid affect the pump, pump size and piping-section lengths. The viscosity of a 50-50 PG mixture is greater than 100% water.
As PG viscosity increases, flow decreases. Also, viscosity of glycol varies inversely with temperature: the warmer the glycol, the lower the viscosity.
Components of an ice melting system
Ice melting systems can be augmented and adapted for any situation with some or all of the following components:
General hydronic components - To avoid boiler shock when very cold water is returned to the boiler for reheating, many systems use bypass piping or a four-way mixing valve.
Concerns about internal boiler corrosion can be addressed with an oxygen barrier pipe, which keeps oxygen from entering the water, thus reducing or eliminating internal corrosion.
Instead of barrier piping, which can be expensive, many systems rely on a plate-to-plate heat exchanger and the use of non-ferrous components to avoid corrosion.
The longer the piping system, the more likely a heat exchanger will prove to be the most cost effective choice.
Circulating Pumps - Circulating pumps move the heating fluid through the piping. They are designed to provide specific flow rates and head pressure per circuit. Use the manufacturer's pump flow and head chart to determine which circulating pump is right for your system.
Circuit components - PEX piping is the industry standard, offering durability and performance for decades. It comes with or without an oxygen barrier.
Supply and return manifolds - These components connect the radiant circuit piping to the boiler system’s supply and return. Brass manifolds are used for ½ inch piping, and copper manifolds are used for all larger sizes.
Outdoor components - The performance of an ice melting system is improved by using control systems to monitor slab and boiler temperature, heating fluid temperature and heat input. Proper controls prevent thermal shock in the boiler and the slab.
Outdoor controls are available to turn the ice melting system on when needed. They can be as complex as temperature and humidity sensors to detect freezing temperature and humidity as it occurs, or as simple as a timer to turn the system on at a regular time each day.
It also is possible to use an idle start approach that keeps the surface at a chosen temperature to minimize the change in heat needed to melt snow and to reduce the potential of thermal shock to the boiler and slab. However temperature is controlled, the slabs should never be subjected to water temperatures above 65ºC (150ºF).