How to integrate SIM into almost any outdoor surface.
August 1, 2015 by Lance MacNevin
Applying radiant heating technology to outdoor surfaces, (hydronic snow and ice melting (SIM) systems) provides a safe and reliable way to keep surfaces clear of snow and ice. With proper design and installation, these systems offer long-term performance and reliability as well as saving the time and energy spent on traditional snow and ice removal using mechanical equipment.
SIM systems operate in a variety of outdoor areas such as sidewalks, steps, driveways, ramps, parking lots, loading docks, carwashes, roadways, bridges, and even helicopter landing pads.
Whether the system is always on or uses a moisture sensor, a significant amount of heat can be conducted to the frozen earth below the SIM surface if appropriate insulation is not installed. In some cases, downward losses can exceed 50 per cent of all the energy supplied to the SIM area. CSA B214 requires at least R-5 insulation below SIM areas, but many designers specify R-10, since insulation also improves response time. The type of insulation may be extruded polystyrene (XPS), polyurethane (PU) or even expanding foam that is sprayed onto existing concrete or the earth to follow contours. Be sure the insulation is rated for outdoor use and meets the expected compressive loading from vehicles, or settling can occur.
TUBING SIZE AND SPACING
SIM systems require high flow rates to deliver lots of heat in demanding winter conditions, so ¾ in. nominal PEX tubing is the most popular size. Larger projects may even require one-inch tubing, while ½- and -in. tubing are used when tighter spacing or low profile is a consideration. To help with rapid and even melting most designs use 20 to 23 cm. (eight to nine inch) tube spacing. While some special cases use tighter spacing, going wider than 23 cm. runs the risk of snow strips between tubes.
In poured concrete the PEX tubing is simply embedded within the concrete, strongly resembling slab-on-grade radiant heating installations. This is very popular in stained concrete. It is recommended that the tubing be located five to 7.5 cm. (two to three inches) below the surface for faster response time, but this is not always practical. Tubing is often stapled directly onto the insulation board or tied to rebar or wire mesh within the 10 to 15 cm. (four to six inches) of structural concrete. A great technique is to pour the structural slab first, cover it with insulation, then install the SIM tubing within a 7.5 cm. (three inch) concrete overpour. By placing the tubing closer to the surface, the heated slab thickness is reduced, making the system more responsive.
INTERLOCKING CONCRETE PAVERS
The most common technique for installing SIM with pavers is to embed the tubing in the sand bed above well-supported insulation. The sand bed should be laid approximately 40-mm. (1 ½-in.) thick, or 28-mm. (1 -in.) after compaction. More sand than this will slow down heat transfer and may allow too much movement of the pavers.
According to Robert Bowers, P. Eng., director of engineering with the Interlocking Concrete Pavement Institute (ICPI), insulation used under pavers must be very rigid to prevent movement under load. ICPI does not recommend stone dust media under pavers, as it interferes with drainage, loses strength as it holds water, and can refreeze when wet, potentially heaving. Technical specifications and drawings of SIM systems with pavers can be found at www.icpi.org.
When heated above 120C (250F) most PEX tubing will soften and flatten if compressed. This would surely happen if hot asphalt, which can be more than 175C (350F), is placed directly around the tubing. The tubing must be protected in two ways:
1. Tubing is installed within a 7.5 cm. (three inch) thick compacted bed of stone dust below the asphalt to prevent direct contact with the asphalt. Tubing is usually stapled to insulation laid beneath the stone dust. The stone dust conducts heat well and allows paving equipment to drive above it.
2. Cold water is flushed through the distribution manifold and all tubing circuits to carry the heat away from the tubing until the asphalt cools. The flow of fresh water is regulated to be below 65C (150F) at the manifold outlet. If it is hotter than that the flow rate must be increased and the water just goes to a drain. On a hot, sunny day, this flushing may take more than 12 hours to cool the asphalt.
Obviously, outdoor stairs are dangerous in winter, so well-installed SIM systems increase pedestrian safety. Just as there are different ways of building outdoor stairs, there are different installation techniques for the tubing. It is important that the tubing is within five cm. (two inches) of the surface and the nose of each step, and that it is not buried deep inside concrete, as this could lead to icy spots. Tight tube spacing is also crucial for even melting and rapid response. Due to the low profile and tight spacing requirements, ½-in. PEX tubing is often used in steps. When concrete stairs are poured in place, the tubing needs to be held in place using rails or other fasteners so it is properly positioned after the concrete is poured.
It is even possible to retrofit SIM tubing above existing stairs. When concrete steps are pre-existing or pre-cast, tubing can be anchored on top of the structural concrete and embedded within an overpour. Insulation is sometimes sprayed onto existing steps before the tubing is placed, for better efficiency and response.
When snow and ice melt, the resulting water must drain. Otherwise, the SIM system must also evaporate the moisture, which requires more energy than simply melting snow. Allowing gravity to carry the water away to landscaping or a drain will improve safety and reduce operating costs. Drains are especially important at the bottoms of steps or ramps. Water left standing can refreeze, so be sure that any installed drains have heating tubes nearby and direct the run-off to avoid creating hazards or ice dams on unheated areas. Drains may be connected to drain tile piping or storm sewers, where regulations allow, or even to a ditch or pond, if there is one nearby.
The location of distribution manifolds for supplying warm glycol to the SIM piping circuits may be a conundrum, since the designers want the manifolds close to the SIM area, yet the SIM area is often far from the heat source location or mechanical room. Therefore, SIM manifolds are often installed in remote cabinets or vaults designed to protect the manifold and pipes from sun, vandalism and accidental damage, while allowing good drainage through an open bottom. Various styles of utility boxes have successfully been used. Warm glycol may be transferred to the manifolds through buried pre-insulated PEX pipes, to deliver the heat with minimal heat loss and high reliability.
Hydronic snow and ice melting systems are effective, efficient and reliable. There are many situations where public and private buildings require this level of protection outdoors. Being aware of the tips presented here and working with an experienced designer will help to ensure that installations go smoothly. <>
Lance MacNevin, P.Eng., is manager of REHAU Academy where he is responsible for training across North America. With over 20 years of hydronic experience, he is on the technical committee for CSA B214. Editor’s note: Shortly after press deadline, Lance joined the Plastic Pipe Institute as director of engineering, building and construction (BCD) and conduit (CD) divisions.