Upgrade your knowledge on snow- and ice-melt systems
Not your grandpa’s snow melt.
The ASHRAE folks develop most of the standards used in determining the energy requirements of snow- and ice-melt systems. These requirements are based on numerous variables affected by geographic location and system performance expectations. The actual loads are separated into sensible and latent requirements. Once these have been determined, based on location, the expectations portion comes into view.
This is a very important conversation that needs to happen between the system designers and the system operators in order to establish the consumer’s expectations, as well as to manage installed costs. In some cases, if a qualified engineer has been retained for the job, many of these questions will have been asked and energy demand calculations will already have been performed.
There used to be three basic categories of snow-melting capabilities. They were listed in classes, such as class 1, 2 or 3. Those classes have been superseded and are now considered snow-free area ratios.
These ratios are broken down into three basic groups.
Any application designated as 1.0 would indicate that 100% of the controlled area is completely free of snow for 99% of the time that snowfall occurs. Under this scenario, accumulation of snow on the melting surface might occur for less than 2% of the snow season, and should be completely melted in short order. This scenario would typically be used for a critical operation such as the drive-up entrance to a hospital emergency room or a heliport landing pad. This also will result in the highest energy demands per square foot of melted area.
A .5 system design essentially means it is acceptable to allow a slight accumulation of snow before melting takes place, possibly covering as much as 50% of the melted surfaces, hence the .5 designation. This category could be used for any noncritical areas but would be someplace between a 1 and a 0 rating.
Duration of the snow accumulation is an obvious function of wind speed, snowfall intensity, snow temperature, moisture content, runoff, evaporation and latent energy required to physically melt the snow. This type of design will use less energy than a type 1, but more energy than a type 0.
A type 0 system design anticipates the accumulation of snow, which is melted over a longer period of time. This would be a common residential or commercial noncritical area of control, such as melting the snow around a trash can container enclosure or a noncritical entrance to a building. This system design will have the lowest energy demand per square foot of surface melted, but also could show more snow accumulation, and will take longer to completely clear the melted surfaces. Time, in this case, does equal money.
The evolution of SIM controls
Numerous and varied methods of operating a given snowmelt system exist today. It has been proven that, regardless of the design of the system, controlling the system properly is critical to performance. It also has been proven over time that having a human interface device is a good step to avoiding energy waste and maximizing SIM performance. These systems work proactively better than they work reactively.
There are numerous manufacturers of SIM control systems, and these controls have progressed significantly over the years. When I first started in the trade many years ago, a “nice” system had a lighted pilot switch. If the switch was lit, it meant the SIM was operational. It was fraught with problems.
We’d typically put this switch near the main entrance to the home, so as the residents were leaving, they’d realize they needed to shut the SIM system off. The only problem was that if the residents left via the garage, there was a good chance the SIM would remain on until their return. In a setting where the house is only occasionally occupied, which is fairly common in ski resort towns, this can result in the fuel bill being greater than the mortgage for the same home.
Enter the 12-hour twist timer. Although a significant improvement over the simple lighted light switch, people found that if you wedge a toothpick or matchbook behind the dial, it stays on all the time. So much for automatic shut-off features.
Then came automatic on-site controls. These controls have two or more sensors. One senses the actual slab temperature (between two tubes, for example) and the other senses the outside north-facing air temperature. Some models even have a moisture detector. They also have a feature that will limit the operating temperature of the actual snow-melt surface. The only real fall back on these controls is cost over function. You can run them in a full hands-off automatic role or you can have a push-button feature for either melt or idle.
Idle is a condition whereby the slab is maintained at a preset condition to ensure a short reaction time when the snow begins falling. Idling a slab can be a very expensive proposition. At night, when there are no clouds to act as insulation to the night sky, a warm surface transfers its radiant energy to a cooler regime. In the case of night sky re-radiation, outer space is around -325° F. The Btu will power vault from a warm slab toward the colder surrounding night sky.
Next were Internet-active controls. At least one manufacturer/programmer (Climate Automation Systems) I am aware of has interfaced its controls with the National Weather Service. This allows for multiple zone control ability and the possibility of having a three-tiered operation for the SIM. You could have an idle condition that is well below a melt condition, which would reduce energy consumption for the hot idled slab. Let’s call it a “cold idle.” When the Internet-active control gets an alert that a snowfall is imminent, the control can take the slab from a cold idle (20°) to a warm idle (30°).
This cold idle would be a temperature above whatever the slab would normally be at based on shadowing the ambient conditions, say 20°, but less than the temperature required for melting. When moisture is detected at the onsite location, the slab operating temperature is then moved to the melt function.
The three-tiered operation will not only reduce the amount of time to get a slab from a full cold idle to a full-on melt situation, it can make a .5 system act like a 1.0 system, or it can make a 0 system act like a .5 system. It also can be used to harvest free solar and ambient energy from the slab during nonsnow-melting seasons for use as a DHW preheater, to heat a swimming pool or hot tub, or wherever there is a demand for thermal energy.
In addition, some ground-source heat pump systems use the night sky’s re-radiational cooling effect to lower the operating temperature of a given loop field in commercial settings, where heat rejection is the dominant thermal load.
This is not your grandpa’s snow-melt system. Times have changed, and people who deal with these systems on a regular basis need to upgrade their knowledge base and offer the end-use customer more than just a hot slab that can melt snow. We have options, but we must ask the consumer what his wishes, wants and needs are in order to fulfill the need.
And do not forget to insulate the back sides and edges of the SIM slab. It’s not just a good idea, it’s the code in most jurisdictions.
This article was originally titled “Not your grandpa’s snow melt” in the August 2016 print edition of Plumbing & Mechanical.