Hydronic snowmelting essentials, part 2

Part one, which appeared in the last issue of Phc News, introduced the snowmelt system and its various forms and uses. Part two of this article looks in greater detail at the need for inhibited glycol solutions when installing snowmelt systems. It also examines the type of heat losses that will occur, how to anticipate the influence of heat loss, basic types of operation and control, and cost of system operation.

Avoid uninhibited glycols

Plain glycol solutions, because they lack corrosion inhibitors, can increase the threat of corrosion in a hydronic snowmelt system. Moreover, putting them into your system cold will eventually cost you far more than the initial fluid price. Uninhibited glycols are less expensive, but become an ongoing threat to your system components.

Heat, oxygen, chlorides, sulfates, metallic impurities and other contaminants can increase the rate of corrosion in the system. Combined, these are likely to create unscheduled system shutdowns, maintenance issues and reduced system life. Glycols produce organic acids as they degrade, especially when heated. If left in solution, these acids lower the fluids’ pH.

With no corrosion inhibitors to buffer these acids and protect the metals in the system, the corrosion rate of a solution of plain ethylene or propylene glycol can be greater than that in plain water, highly corrosive in its own right.

The industrial inhibitor packages used in products like Dow Chemicals dowfrost propylene glycol based fluids and dowtherm SR-1 ethylene glycol base fluids are specially formulated to help prevent corrosion in two ways. First, the corrosion inhibitors “passivate” the surfaces of the metal, so they are less susceptible to corrosion. Second, the inhibitors buffer the organic acids formed as a result of glycol oxidation to keep the fluid from becoming acidic.

Melting snow with hydronic snowmelt system

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 may only take 30-40 Btu/hr per square foot to radiantly heat a structure. But it can take up to 150 Btu/hr-square foot or more to melt snow and ice from a surface.

Many variables interact to affect system effectiveness:

Sensible heat: The first load factor is the sensible heat required to increase the temperature of snow or ice from ambient temperatures to 32 F. The lower the temperature when precipitation is detected, the higher the sensible load will be.

Heat of fusion: Once the mass of snow or ice has reached 32 F, additional energy is required to change its state from solid to liquid. This stage of snow/ice melting generally requires the most energy.

Snow-free area ratio: The insulating effect caused by the presence of a layer of snow or ice has a huge effect on heat transfer and evaporation occurring at the snow melt surface.

Heat of evaporation: As the mass temperature increases, natural evaporation will begin to take place.

Heat loss to the atmosphere: Atmospheric losses are the fourth phase of the snowmelt process. Once we start melting snow off our system, we will begin to have voids in the snow cover -- areas that may not have initially contained as much snow as other areas due to drifting or may be exposed to solar gain effects. These areas clear faster causing clear patches to form, allowing for greater losses to the atmosphere.

Back and edge losses: Back and edge losses refer to losses not directly associated to the melting snow. These include the ground below the mass as well as to the side. Energy in a snowmelt system behaves like any other radiant system: heat moves from a hot source (the tubing) to a cold source (the mass). When a snowmelt system first starts, energy moves in all directions equally since the surrounding mass is of equal temperature. This condition changes the longer the system runs.

System basics

Normally, pex tubing can handle up to 180 to 200 F, but there are variations. The maximum temperature rating can also be affected by the concentration and type of glycol used in the snowmelt system.

Tubing comes in a variety of sizes with 1/2" ID (inside diameter) to 1" ID being typical for a snowmelt system. Supply and return manifolds are often made of steel, copper or stainless steel. The layout is usually easiest if these manifold pairs are located together next to the “zone,” or area to be melted. Manifolds can be located away from the zone, but then more tubing will be required to get to and from the manifold pair.

The tubing is normally spaced from six to 12 inches on center and circulates heat transfer fluid that has been heated to 110 to 140 F. Tube spacing can be varied according to the performance of the snow melting required. Higher snowfall requires closer spacing of tubes. Higher slab thickness or tubing, which is buried further below the slab surface, increases resistance to heat transfer which may require higher supply water temperatures.

Inhibited glycol-based heat transfer fluids are more viscous than water and this means higher system pressure drop and higher pumping cost. This will be most noticeable during system start-up when the fluid temperature is coldest. The viscosity of ethylene glycol based fluids becomes excessively high below fluid temperatures of -20 F, whereas propylene glycol fluids reach excessively high viscosity below a temperature of 0 F.

System costs

Cost to operate depends on location and system specs like energy source -- access waste heat or steam, natural gas or oil versus electricity and also equipment design, required performance, need for redundancy, slab geometry, materials of construction, etc.

The concentration of glycol used for a system must have a freezing point which is 5 F below the lowest anticipated winter temperature to ensure adequate protection. Failure to use sufficient freeze protection can lead to bursting or rupturing of system piping caused by formation of ice crystals. Fluid heaters must be capable of providing required heat loads, which typically range between 100-300 Btu/hr-ft2. Add the cost of supply and return piping required to get the energy from the boiler to 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. The cheapest system to operate is with an on-off mode. These systems are only used five or 10 times each year.
• Idled systems. Idled systems, because they operate any time the temperature is below 38 F, cost more to operate.

These systems typically consume up to 100 Btus per hour, per square foot whenever they are idling and up to 300 Btus per hour per square foot whenever they are operating. Hospitals may have waste heat from steam or condensate that may be readily available, greatly reducing or eliminating energy needs.

So, whether you’re seeing an occasional need to eliminate snow, or warming an emergency room entrance, a snowmelt system, properly installed and protected, will readily answer the call.