Let’s continue to look at the different options for controlling snow-melt systems.
Remote snow cup controllers: This control logic, while better than no control logic at all, leaves a lot to be desired due to its lack of slab monitoring capabilities. The device is remotely mounted above the ground. It has the ability to sense ambient air conditions and the presence of moisture. It will activate a snow-melt system typically when the outside air temperature is below 38°F and moisture is detected.
In this scenario, the moisture detected may not necessarily be in the form of snow, so it is entirely possible that the snow-melt system may be started under conditions that would not warrant it.
Conversely, if it is below 38° F and moisture is detected, there is a good chance that snowfall will occur or is imminent, but not always.
I have also seen this logic interfaced with a simple setpoint controller to limit the maximum operating temperature of the slab.
The snow-sensing element is a heated grid, with approximately 150 Btuh/sq.ft. being delivered to the grid/sensor. This is to emulate the actual slab conditions and allows the device to tell when snowfall has ceased. When snowfall has ceased, the control begins a countdown of a specific time period to make sure that all areas of the slab are snow-free.
Location of the snow-sensing switch is critical. It should not be placed where it may be exposed to drifting snow coming from a roof. The sensor requires regular cleaning and should be located where it is easily accessed.
While light-years ahead of the simple piloted light switch, this is still not the best control logic available.
Total control/monitoring system: This is the ultimate in snowmelt controls. The device has a sensor located in the slab, strategically placed midway between the tubes transporting the heated fluid. It also has sensors watching the outside ambient air, snow-melt fluid supply temperature, snow-melt return temperature and either heat source supply or heat source return temperature.
The slab sensor monitors moisture at the slab, as well as actual slab temperatures.
The control can be programmed to maintain idle conditions, steady state conditions or reactive conditions. It can lock out the system when the outside air temperature is above or below a given condition. It can control and reset the slab operating temperature based on actual field conditions. It can protect the slab from thermal shock. It can protect the boiler/heat source from thermal shock and long-term condensation production problems. It can control motorized mixing valves, as well as variable-speed injection pumps. It can be remotely monitored and controlled. It can control more than one area or zone of snow melt. It can stage and sequence boilers. It monitors run time of the snow-melt system. It monitors and catalogs maximum and minimum temperatures seen on all parts of the system.
The only disadvantage is that it is the most expensive control alternative, but it is worth every penny.
Let the fun begin
Six long months went by before I eventually heard back from the general contractor that the national big-box retailer had hired to actually build the store for them. They had a set of engineered drawings showing the physical plant’s proposed location, as well as preliminary areas for snow melting.
The drawings had a piping diagram showing the size of the piping leaders between the mechanical room and the targeted snow-melt areas on the opposite end of the store, but it did not provide any information as it pertained to the sub distribution manifold boxes to serve the actual areas being snow melted.
When I asked the engineer/designer about this glaring deficiency, he told me that it was up to me to figure out how to make the transition from the 4-in. steel mains that he had designed to the 58-in. PEX tubing that would be buried in the cement sidewalks. He reminded me that once I had made the decision on how I proposed to do this, I needed to run it by him for his approval before actually performing the installation.
This required me to take a full set of blueprints and determine the best locations for the distribution boxes. Then I also had to determine the best route, manner and method to get the smaller supply and return piping from the 4-in. mains in the ceiling of the store down the outside walls of the store, and then through the wall and foundation assemblies and out to the in-slab distribution boxes.
I decided that the system would best be served with five different manifold distribution boxes, strategically located throughout the 10,000-sq.-ft. snow-melt area.
Sizing of the smaller branch lines was dictated by the size of the area served by each manifold. The size of these areas was dictated by common sense and space available to locate the manifold distribution boxes.
The actual distribution boxes that we used were made of concrete and plastic and are commonly used for containing electrical connections for traffic control systems.
Tune in next month as we get into the dynamics of snow-melt system sizing.
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303/778-7772.