Warming up to Snowmelt Technology

J. B. Hunt, one of the largest trucking companies in North America, takes transportation seriously. Their reputation for safety and efficiency played into a decision they made to install an extensive snowmelt system for their new headquarters building in Lowell, Ark.

The 22,000-sq.-ft. snowmelt system has been hard at work for the trucking firm for the past three winters, easily dealing with the rigors of Midwestern ice and snow. Yet, in a sad irony that hit them hard, founder Johnnie Bryant Hunt slipped on ice at a nearby restaurant in early December 2006. He died in the hospital five days later. Had the restaurant installed a snowmelt system to protect its patrons, perhaps he’d be alive today.

Barricade to liability claims

Most of us in the trade readily see the value of snowmelt systems. Yet, many building and facility owners have yet to embrace them.

Frequently, a key obstacle to winning their interest is the up-front cost for installation. It’s not relatively cheap. But there are benefits that balance out the equation. These include convenience, environmental enhancements and the greatly reduced labor and hardware costs that are otherwise needed to do the job.

One of the key obstacles is neutralized if a facility has waste heat that can be exchanged. If excess heat is available, or if all that’s needed is an additional geothermal or water-sourced heat pump during the design phase, an upgrade may be feasible.

Snow and ice removal employs tubing buried outside in a mass (concrete, asphalt, stone pavers) to gently melt off winter precipitation to keep pathways, driveways and other areas dry and clear. For commercial applications, especially those areas deemed critical, radiant heat performs a valuable, perhaps life-saving, function.

According to Keith Whitworth, regional manager for Watts Water Technologies, another advantage is the proactive heading-off of liability claims and added safety overall. “Given today’s litigious society, snowmelts don’t cost money; they can save it,” he said. “The cost of the system is more than returned with one avoided lawsuit. Also, some insurers recognize the value of these systems, rewarding building owners with reduced insurance rates.”

Snowmelt classifications

For the sake of easy reference, snowmelt uses are grouped into classifications. These allow us to quantify the level of snowmelting a system is designed to perform. There are two essential systems of classification—old ASHRAE (American Society of Heating, Refrigeration and Air-conditioning Engineers) and new ASHRAE.

The old ASHRAE classifications split snowmelting systems into three groups. These classes split systems into the amount of snow actually melted at design conditions:

• Class I – Systems designed not to melt snow while it is falling, but afterwards
• Class II – Half the snow is melted during snowfall, the rest afterwards
• Class III – All snow and ice is melted continuously

The key to these classifications are the design conditions. If a system were designed as a Class I for 36 inches of snow per day, it could act as a Class III system with a minor snowfall of eight inches.

Conversely, a Class III system designed for around six inches of snow per day would act as a Class I system with 36 inches of snowfall. So, to guide your decisions, know your snowfall.

New ASHRAE standards still keep the Class I, II and III designations, but ASHRAE now calls it 0, 0.5 and 1 for ratio of snow melted. They also add a new twist, a percentage to quantify how often the maximum amount of snow occurs. Some professionals tend to over-design a system to handle worst-case scenarios. ASHRAE snowmelt percentages are essentially classified into 75%, 90% 95%, 98%, 99% and 100%, with 100% being the maximum snowfall foreseeable for an area.

It takes a lot of energy to melt snow, about five to six times the load required to heat a building of similar size. For example, it generally takes 25 – 40 Btu/hr per square foot to radiantly heat a structure. But a snowmelt system may require up to 150 Btu/hr-square foot or more. Why so much energy? Chiefly, the components make up the load. There are five basic parts to a snow/ice melt system:

Sensible heat (Qs): The first load factor is the sensible heat required to raise the snow or ice from ambient temperatures to 32 F. The colder the ambient conditions are when precipitation is detected, the higher the sensible load will be.

Heat of fusion (Qm): Once the mass has reached 32 F, the second phase of the snowmelt process can begin. This phase is called the heat of fusion, which is the amount of energy required to change states from a solid to a liquid. This phase generally requires the most energy.

Heat of evaporation (Qe): As the mass temperature increases, natural evaporation will begin to take place directly from the snow to the atmosphere. This phase is generally a small part of the overall process.

Heat loss to the atmosphere (Qh): Atmospheric losses are the fourth phase of the snowmelt process. Once snow starts melting, there will begin to be voids in the snow cover; areas that may not have initially contained as much snow as other areas due to drifting or solar gain. These areas clear faster in patches, allowing for greater losses to the atmosphere. The cold atmosphere will literally “suck” the heat from the slab — energy that must be continually replaced.

Back and edge losses (Qb): Back and edge losses refer to losses not directly associated with snow- and ice-melting. This includes the ground below the mass as well as to the side. Energy in a snowmelt system is such that heat moves from a warm source (tubing) to a cold source (the mass). When a snowmelt first starts, energy moves in all directions equally, since the surrounding mass is of equal temperature. Conditions change the longer the system runs. Since the ground is not an exposed surface, it will begin to retain energy, thus allowing its temperature to rise. Conversely, the exposed surface of the mass continuously loses energy to the snow and atmosphere. Surprisingly, heat loss to the ground is generally only three to five percent of the total system load.

Typical snowmelts employ tubing buried in a concrete slab. The most popular tubing used is either synthetic rubber (EPDM) or cross-linked polyethylene (PEX). EPDM is derived from synthetic rubber and is cross linked, much the same way PEX is. Both types of tubing have a long history of performance and longevity in high temperature apps.

Tubing comes in a variety of sizes; typically ½" ID (inside diameter) to ¾" ID will be used in a snowmelt system. The tubing ties into the supply and return piping via twin distribution manifolds. The layout is usually easiest if these manifold pairs are located together next to the zone, the area to be snowmelted. Manifolds can be located away from the zone, but then more tubing will be required to get to and from the manifold pair. Tubing lengths vary according to manifold placement.

Tubing is spaced six to 12 inches on center and circulates water that’s been heated to 110 to 140 F. Tube spacing is varied according to the degree of snowmelting required. More snowfall that needs to be melted at a faster rate will require closer spacing of tubes. More material over the top of the tubing increases resistance to heat transfer, requiring a higher supply water temperature.

Common snowmelt applications include:

Helipads. Helipads are excellent examples of snowmelts. With space becoming more and more precious, many hospitals, for example, are forced to install helipads on building roofs. These helipads can become extremely slick and dangerous when covered with ice and snow.

Sidewalks. Convenient and more inviting to passersby, sidewalk snowmelts can increase business and decrease liability. Customers are probably more apt to shop stores with cleared sidewalks.

Stairs. Of course, stairs can be dangerous. With snow-melt, pedestrians can use steps safely. The spacing of tubes for stairs varies according to application, but they’re usually installed with two lengths of tubing in the tread and one in the riser.

Car washes. Water is always present in car washes. Using snowmelt, property owners can keep car washes open and ice-free. The control strategy for car washes is simple. Either air temperature or slab temperature is monitored. If the temperature of the slab or the air drops below 35 F, the system is activated. When temperatures exceed 35 F, the system is disabled.

Hospital entrances/sidewalks. Because they are usually considered Class III systems, tube spacing for hospital entrance ramps are usually set closely, at 6" OC. Further, these systems are idled, or operated at a reduced output, to decrease system lag time. When sensors detect precipitation, the system is operated at full output. Hospitals may have waste heat from steam or condensate readily available, greatly reducing or eliminating energy needs.

Parking garage ramps. Snowmelting systems ensure cars driving in off the street can safely navigate parking garage ramps. One note of caution: Be sure to place sensors for these controls where they can detect snowfall or precipitation and temperature.

Loading docks. Moving goods is important work, even during winter months.

Large area “hot pads.” Instead of melting an entire area, which is sometimes cost prohibitive, smaller areas where snow may be deposited are melted. This technique is often used for airport runways and large parking lots. Typically, tubing for hot pad slabs is spaced at 4" to 6" OC to accommodate a large amount of snow. Remember, six inches of snow from a runway or parking lot will be collected and deposited on the pad. It’s not uncommon to have a hot pad of perhaps 30' by 30' with snow piled four to six feet high.

Hot pads are usually operated manually, activated whenever the need arises. Twist timers can be used in place of on-off switches, so the operator doesn’t have to remember to switch the system off.
Principles of operation

On-off operation. Some snowmelts are operated only when there is ice or snow. These systems are operated in the presence of precipitation, when the ambient temperature is below 35 F. While less costly to operate, these systems take longer to melt precipitation, because they must first increase the slab temperature.

Controls. These include sensors to detect precipitation when temperatures fall below 35 to 38 F. Because these controls are simple, their cost is relatively low.

After precipitation and temperature conditions are met, the system will operate until precipitation stops. Most controls will continue to operate the snowmelt for a period of four to six hours after precipitation has ended, ensuring an ice-free surface.

Twist timers can also be used in parallel with the snowmelting control to allow some manual control. If it’s known that a winter storm is approaching, the system can be started several hours before its arrival to reduce system lag time. Conversely, if snow happens to drift onto the snowmelting surface but does not engage the precipitation sensor, the system can be started manually.

Sophisticated controls sense slab temperature, outdoor temperature and precipitation. These are more costly than on-off controls but allow for much greater system control. They usually have settings for warm- and cold-weather shutdown and slab idle temperature. Cold weather shutdown is necessary, because snowmelt systems cease to be effective below about 0 F. We simply can’t provide enough energy below 0 F to get the job done. This is rarely a challenge, however, because snow below 0 F contains very little water.

In order to help them respond faster, some systems are idled, or operated at a reduced output, until precipitation is sensed with a temperature below 35 to 38 F, when the system is operated at full output. These systems allow for a faster response, and no snow or ice accumulates.

Operating cost. Snowmelts themselves are not that expensive to operate, since they’re only activated occasionally. The biggest cost incurred with a snowmelt system is the up-front cost. Glycol antifreeze is required for all systems, because system fluid is either dormant or could go dormant for a period of time. Relatively large pumps may be required to move slushy water-glycol mixture on initial system startup. Larger heat sources are required to deliver the 100 – 300 Btu/hr-sq ft. Supply and return piping is required to get the energy from the boiler to the manifolds for the tubing buried in the slab. With all these factors, including a larger heat source, a snowmelt system can typically cost between $6 – $12 per square foot.

• On-off systems. The cheapest systems to operate are on-off snowmelts, because they are only used five or 10 times a year. As an example, a Class II system in Buffalo, N.Y. may cost about $0.21 per square foot per year. The same system in Chicago may cost $0.12 per square foot per year. In Minneapolis, cost of operation might be $0.25 per square foot per year.

• Idled systems. Idled systems cost more to operate, because they operate any time the temperature is below 38 F. These typically consume up to 100 Btu/hr-sq ft whenever they are idling and up to 300 Btu/hr-sq ft. during full operation.

Whether you’re trying to eliminate snow in Lowell, Ark. — as is the case for J.B. Hunt — or warming a hospital entrance in Nome, Alaska, a snowmelt system, could be the answer you are seeking. n

John Vastyan owns Common Ground, a trade communications firm based in Manheim, Pa. He has researched and written about plumbing and mechanical, HVAC, solar, geothermal and radiant heat systems for decades.