Snow Melt Systems

NOTE This standard governs the elements that heat an outdoor pavement from within: the hydronic tubing or the electric heating cable or mat embedded in it, the manifolds or the power connections at its edge, the sensors and the controller that decide when it runs, the insulation beneath it, and the tests that prove the embedded element before the pavement closes over it. It begins at the manifold connections or the junction boxes at the pavement edge and ends at the finished surface. (1.1.1)

NOTE The surfaces in scope are walks, plazas, entrance aprons, exterior stairs and ramps, accessible routes, drives, loading dock approaches, and parking aprons, whether finished in cast-in-place concrete, asphalt, or unit pavers over a concrete or aggregate base. (1.1.2)

NOTE A snow melt element is the one part of a pavement that cannot be reached after the pavement is placed. Every other defect in a hardscape is repaired by cutting out a panel; a tube or cable that was crushed by a vibrator, nicked by a saw, or never tested is found in the first storm and is repaired by removing the pavement. The weight this standard gives to layout, attachment, cover, and testing before placement follows from that. (1.1.3)

NOTE The plant that supplies a hydronic circuit, the piping that reaches the manifold, the electrical distribution that reaches the junction box, and the pavement itself are each governed by their own standard. This standard states what it needs from each at the boundary and does not restate them. (1.1.4)

NOTE The following terms carry the meanings given here throughout this standard: (1.2.1)

NOTE The shop drawing with the joints overlaid is the submittal that prevents the common failure. A contraction joint sawn across a tube cuts it, an isolation joint that a cable bridges without a sleeve shears it when the panels move, and neither is visible once the pavement is finished. The overlay is where those crossings are found while the layout can still be moved. (3.1.3)

NOTE The record drawing and the photographs are the only map of an element that no one will see again. A future core, a bollard footing, a sign post, or a utility repair in a heated pavement is located against them, and a pavement without them is a pavement no one can safely cut. (3.2.2)

NOTE The listing is what proves the element survives the pavement. An asphalt lift is placed at a temperature that softens the insulation on a cable listed only for concrete, and a mat whose listing names concrete has not been evaluated for the compaction loads of a paver bed. The listing names the material because the material is the test condition. (4.2.6)

NOTE No heat source is the norm across projects, because the answer follows the plant and the area. A hydronic system has a higher first cost in tubing, manifolds, and the plant that serves it, and a lower operating cost wherever a fuel-fired or waste-heat source is available, and it is the usual choice for a large area on a site that already has a hydronic plant or where one is being installed. An electric system has a lower first cost, no plant, no fluid, and no freezing risk when it is off, and a higher operating cost at any electricity price above the cost of fuel heat, and it draws a connected load that a large area can push past the available service. It is the usual choice for a small area, for a site without a hydronic plant, and for a retrofit where tubing cannot be run back to a plant. (5.1.2)

NOTE The class sets the snow-free area ratio and, with the design frequency, the design heat flux, so it is the first design decision and the one the output follows from. Class I, with a snow-free area ratio of 0, lets snow accumulate during the storm and melts it afterward, at the lowest output and the lowest operating cost, and suits a surface that can be closed during a storm. Class II, with a ratio of 0.5, keeps the surface passable during all but the heaviest snowfall at a moderate output, and suits walks, plazas, and drives that are in use through a storm. Class III, with a ratio of 1.0, keeps the surface free of snow and ice throughout the storm at the highest output, and suits a surface where any accumulation is a hazard or a stoppage: an emergency entrance, an accessible ramp, a loading dock on a slope, a helipad approach. No class is the norm across projects, because the same project often holds a Class III ramp beside a Class II walk. (5.2.2)

NOTE The design frequency is the share of the snowfall hours in the handbook record for which the installed output is enough. A system designed to 75 percent is undersized in one snowfall hour in four, which is acceptable where the surface can wait out a heavy storm; a system designed to 99 percent is sized for the storm of the record and runs at a fraction of its capacity in every other one. The output difference between the two is often a factor of two, and the frequency is chosen with the class because a Class III surface at a low frequency is not Class III in the storm that matters. (5.2.4)

NOTE The surface flux follows from the class, the frequency, and the station, so it is derived rather than chosen. The handbook tabulates, for each listed station and each class, the surface flux not exceeded at each frequency, and a calculation from the local snowfall rate, air temperature, wind speed, and humidity gives the same answer for a site that is not listed. The value is the flux at the surface; the installed output of the tubing or the cable is that flux plus the losses from the back and the edges of the pavement, which depend on the insulation selected under this standard. (5.3.2)

NOTE Requirements in this section apply where the heat source selected under Design Basis is hydronic. (6.1)

NOTE The three PEX types differ in the crosslinking process and in the result. PEX-a is crosslinked in the melt and is the most flexible of the four, bends to the tightest radius, recovers from a kink when heated, and is joined by cold expansion fittings under ASTM F1960; it carries the highest price. PEX-b is crosslinked after extrusion by moisture cure, is stiffer, is joined by crimp or clamp fittings under ASTM F1807, and costs less. PEX-c is crosslinked by electron beam and sits between the two in flexibility and price. PE-RT is not crosslinked, is as flexible as PEX-b, can be joined by heat fusion, and is rated under ASTM F2623 for the temperature and pressure of a snow melt circuit. No material is the norm across projects, because the choice turns on the fitting system the manifold uses and on the bend radius the layout needs at the circuit ends. (6.2.2)

NOTE Oxygen diffuses through the wall of an unbarriered polyolefin tube and into the fluid, where it corrodes every ferrous component in the loop: the pump volute, the expansion tank, the heat exchanger shell, the manifold where it is steel. A barrier layer in the tube wall stops that, and nearly every snow melt circuit is connected to a plant that contains ferrous parts, which is why barrier tubing is the default. Non-barrier tubing is chosen where the whole loop, plant included, is non-ferrous or where the fluid treatment program under HVAC Water TreatmentHVAC Water TreatmentResolves to the current adopted revision.sync/hvac-water-treatment is designed to scavenge the oxygen. (6.2.4)

NOTE Size and spacing are selected together with the supply temperature and the depth, and the heat flux calculation is where the combination is proved. A larger tube carries more flow at the same pressure drop and allows a longer circuit and a smaller manifold, and costs more per foot; a smaller tube bends more easily at the circuit ends. Closer spacing delivers more output at a given supply temperature and evens the surface temperature between the tubes, at more tube per square foot; wider spacing suits a low output class and a deeper embedment, where the pavement spreads the heat before it reaches the surface. Neither value has a norm across projects. (6.2.7)

NOTE The circuit length is a consequence of the tube size and the flow the output demands, which is why it is derived. A circuit that is too long for its tube either starves at the design flow or needs a pump head the plant was not sized for, and a manifold that serves circuits of unequal length delivers most of the flow to the shortest unless each is balanced. Keeping the circuits on a manifold within a narrow band of length is what makes the balancing valves able to even the flow without throttling one circuit to a trickle. (6.3.3)

NOTE The supply temperature and the temperature drop are system-sized quantities that the mechanical engineer sets with the plant, the tube spacing, and the depth, and they sit on the schedule beside the flow and the head the plant has to deliver. A high supply temperature lets wider spacing or deeper embedment reach the output, and is limited by the tube rating and by the thermal stress it puts into the pavement; a large temperature drop reduces the flow and the pump but leaves the return end of each circuit cooler than the supply end, which shows as a strip that melts last. (6.3.6)

NOTE Propylene glycol is the default because a snow melt loop lies under a surface that people, pets, and stormwater cross, and a leak from it reaches the ground. Propylene glycol is of low toxicity and is the fluid most jurisdictions accept near a storm inlet or a planting bed; it is more viscous than ethylene glycol at the same freeze point, so it carries more pumping power and a lower heat transfer coefficient. Ethylene glycol transfers heat better and pumps more easily, and is toxic if ingested, so it is chosen where the loop is isolated from the ground and from any potable connection and the Owner accepts the handling requirements. (6.4.2)

NOTE The concentration follows from the site's minimum temperature and the fluid supplier's table, so it is derived rather than chosen. A snow melt loop is unlike a building loop in that it sits idle in a pavement at the outdoor temperature for days at a time, so it is protected to the freeze point, at which no ice crystals form, rather than to the burst point, at which slush is tolerated, because slush in a tube under a pavement blocks the circuit at the moment the system is called on. Each additional five percent of glycol lowers the heat transfer and raises the pumping power, which is why the concentration is taken from the table rather than rounded up. (6.4.4)

NOTE A manifold inside the building is protected from freezing, flooding, and traffic and is reached from the mechanical room, and it needs the supply and return legs of every circuit to run back through the wall, which lengthens each circuit by the distance from the pavement to the manifold and fixes the circuit count the wall penetration can carry. A below-grade vault at the pavement edge keeps the circuits short and the manifold out of sight, and it sits at the outdoor temperature, collects water unless it drains, and is reached through a lid that must carry the traffic over it. An above-grade enclosure is reached without lifting a lid and stays above the water, and it stands in the hardscape where it is seen and struck. No location is the norm across projects. (6.5.2)

NOTE The isolation valves are what let a circuit be found and shut off when it leaks, the balancing valves are what even the flow across circuits of unequal length, and the purge valves are what let each circuit be filled and bled on its own; without them a manifold of ten circuits is filled by chasing air from one circuit into the next. The thermometers show the operator whether the loop is delivering the design temperature drop, which is the one reading that distinguishes a plant problem from a circuit problem. (6.5.4)

NOTE Requirements in this section apply where the heat source selected under Design Basis is electric. (7.1)

NOTE A single-conductor constant-wattage cable is a series resistor whose output depends on its length, so each circuit is cut to the length the design calls for and both ends return to the junction box; it gives the designer the most freedom and the installer the most to get wrong. A twin-conductor cable has both conductors in one jacket, delivers a fixed output per foot at its rated voltage, and terminates at one end, which simplifies the layout. A self-regulating cable reduces its output as it warms, which limits the slab temperature and lets the cable be overlapped, at a higher cost per foot and a startup inrush the circuit has to carry. A mineral-insulated cable has a metal sheath and is rated for the highest asphalt placement temperatures and for direct burial, and it is factory terminated to length and cannot be cut in the field. A mat carries cable pre-spaced on a mesh and is unrolled over the area, which is the fastest installation on a regular shape and the hardest to fit around an obstruction. No form is the norm across projects. (7.2.2)

NOTE Ground-fault protection of equipment is set at 30 mA, not the 5 mA of personnel protection, because a long embedded heating cable has a capacitive leakage that trips a 5 mA device on a dry day. A 30 mA trip still opens the circuit when the insulation on a cable in the pavement breaks down, which is the fault the protection exists to catch before the cable burns open and the pavement is cut to find it. (7.3.4)

NOTE A heated area at the watt density Class II or III demands draws a load that is easy to underestimate: a few thousand square feet at the upper end of the common density range is a load in the hundreds of amperes, running through the night of a storm. The confirmation is required before ordering because a shortfall found after the cable is in the pavement is corrected by a service upgrade. (7.3.8)

NOTE A manual switch with a run timer costs the least and starts the system only when someone decides to, which on an unstaffed site means after the snow is on the ground. An aerial sensor on a pole or a roof detects precipitation and air temperature and starts the system as the snow begins to fall, before it reaches the pavement, and it cannot see drifting snow, blown snow, or meltwater that refreezes on the surface. A pavement-mounted sensor reads the surface temperature and the moisture on it, so it responds to whatever is on the pavement from any source and holds the system on until the surface is dry, and it starts only once moisture reaches the sensor. The combination starts on the aerial sensor and runs on the pavement sensor, which is the earliest start and the surest finish, at the cost of two sensors and the wiring to both. Initiation from the building automation system puts the start decision in the operator's hands or the system's weather feed with the pavement sensor to confirm the condition, and suits a campus that already supervises its snow operations. No type is the norm across projects. (8.1.2)

NOTE A sensor that reads the wrong place runs the system for the wrong reason. A pavement sensor at a drain reads wet long after the surface is dry, one at the edge reads the unheated ground, and one under a snow pile reads a storm that ended a week ago. The location is a design decision because the sensor is the only thing the controller knows about the pavement. (8.1.5)

NOTE An activation temperature a few degrees above freezing starts the system while the surface is still above the freeze point so that the first snow lands on a warm pavement, and 38°F is the setting controllers are shipped with and the one nearly every project keeps. The after-run has no norm because it depends on the surface: a sloped drive that drains dries in minutes, and a flat plaza with a sensor at its high point holds meltwater in its low corners for an hour after the sensor reads dry, and that water is the ice the system was built to prevent. (8.2.3)

NOTE A system that starts cold heats the whole mass of the pavement from the outdoor temperature before the surface reaches the melting point, which on a thick slab in deep cold takes hours, during which the snow accumulates; it uses no energy between storms. Idling holds the pavement at a setpoint near freezing between storms so that melting begins within minutes of the call, at an idle energy use that over a season can exceed the energy used for melting. Outdoor reset lowers the idle setpoint as the outdoor temperature rises so that the pavement is held only as warm as the next storm needs, which recovers part of that idle energy. Class III surfaces are usually idled because the response time is the point of the class; Class I surfaces usually are not. No mode is the norm across projects. (8.2.5)

NOTE The controller of a hydronic system usually sits in the mechanical room beside the plant, and the controller of an electric system is as often mounted outdoors near the junction boxes as it is in the electrical room, so no rating is the norm across projects. Type 1 is the indoor dry-location enclosure; Type 3R sheds rain and is the outdoor minimum; Type 4 resists hose-directed water for a location that is washed down; Type 4X adds corrosion resistance for a location exposed to deicing salt spray; Type 12 excludes dust and dripping oil in an industrial interior. (8.3.2)

NOTE Heat that leaves the bottom of a heated pavement warms the ground, and without insulation that loss is a large share of the installed output in the first hours of a call and a smaller but continuous share of an idled system. An insulation board under the pavement cuts the back loss and shortens the response time, and it costs the board, the labor, and a pavement section that is deeper by the board thickness, and it has to carry the pavement loads without crushing. A value of zero is a real selection on a Class I surface that melts after the storm, where the response time does not matter and the ground beneath acts as a heat store that returns part of the loss. No value is the norm across projects. (9.1.2)

NOTE An exposed pavement edge, a curb, or a building wall draws heat sideways out of the heated panel and shows as a strip along the edge that melts last. Edge insulation stops that where the edge abuts unheated ground or an unheated slab, and it is omitted where the edge abuts another heated area or where the layout places the first tube or cable run close enough to the edge to carry the loss. (9.1.5)

NOTE Depth trades response against protection. An element close to the surface heats the surface quickly at a low supply temperature or watt density and sits within reach of a contraction joint saw cut and of the surface loads; an element set deeper is protected by the pavement above it and heats the surface slowly, through more mass, at a higher output. The depth is also what the installer has to hold during placement, and a depth the reinforcement does not naturally give is a depth that needs chairs or rails to hold. Neither value has a norm across projects. (9.2.3)

NOTE Air or nitrogen is the usual medium on a snow melt circuit because the test is made outdoors, often in freezing weather, and a water-filled circuit that is left under test overnight freezes and bursts the tube the test was meant to prove; air shows a leak as a pressure drop and shows its location poorly. Water shows a leak where it is and reads a pressure drop with less sensitivity to temperature, and it suits a test made in warm weather on a circuit that will be drained or charged immediately after. Testing with the glycol solution proves the circuit and charges it in one operation, and it puts the fluid in the tube before the pavement is placed, where a tube damaged during placement spills it into the base. No medium is the norm across projects. (10.1.3)

NOTE A reading that falls between the test before placement and the test after it is the signature of a cable damaged by the pavement work, and the stage at which it fell is what tells the Contractor where to look. The test before energizing catches the moisture that entered a connection during the weeks the circuit sat waiting for the pavement to cure. The 20 MΩ default is the minimum IEEE 515.1 sets for a commercial heat tracing circuit, and a manufacturer's higher published minimum governs where one is stated. (10.2.5)

NOTE The elements are the only part of the pavement that is finished before the pavement is started, and every trade that follows works over them. Naming the release as a hold point and the pavement contractor as the party who does not start without it is what keeps the pour from being the first time the two trades talk. (11.1.5)

NOTE The unassigned cost in a snow melt warranty is the pavement. The tube or the cable is a few dollars a foot, and the slab over it is what a warranty argument is about, which is why the pavement is named here and why the record drawings are named as the evidence. (13.5)

NOTE A pavement sensor is the part that fails first, by physical damage from a plow blade or by water entry at its lead, and it fails during a storm. A spare on the shelf is what lets the Owner's staff restore the system the same day instead of waiting for a part in the week the pavement is iced. (14.2)