IN LAST MONTH’S ARTICLE (February, pg. 28), I talked about coordination for a difficult snow-melt job at a retail store. During the next few months, I will cover the many facets involved with the design, installation, operation and maintenance of snow-melt systems.
Probably one of the first questions is, “Why have a snow-melt system?” The reasons can vary from a need to maintain critical continuous access, to avoiding slip- and fall-related injuries, to lowering maintenance costs and can include municipal code requirements.
In the case of the recently completed retail job, the snow-melt system was a municipal requirement. The township has certain snow storage requirements based upon the parking lot surface area. This means that certain areas of the parking lot are dedicated to snow storage for snow removal during major storms. This can result in a loss of numerous potential parking spaces for paying customers. This particular town has a provision to allow a reduction in snow storage area based on the area of active snow melt. Hence, my company got involved in the design, installation, operation and maintenance of this snow-melt system.
People from the retailer originally contacted our firm during the early design phases of their store project. They had no idea what a snow-melt system would cost, what was involved with the installation or even where the mechanical room would be located. They had an idea of which areas they wanted to snow-melt based on their planners’ discussions with the township, but they had no idea what kind of boiler plant would be needed to make it work.
I consulted with the planners and gave them an estimate of what it would cost to install the system, based on my idea of where the system should be located. Unfortunately, my idea of where it should be located was completely different from theirs.
It seems that almost every square foot of store space is allocated long before pen touches paper in the design process. Eventually, the store connected me with its mechanical design department, which had little to no experience designing a hydronic snow-melt system. I coached the staff, told them of my experiences and preferred products and lined them up with appropriate component manufacturers.
Basic design parameters
The sizing parameters for a typical snow-melt system are based on the overall reaction time of the system as defined by ASHRAE as Types I, II and III. These classifications cover the differences between allowing a little accumulation of snow to allowing no accumulation of snow.
In addition to the three types, the ASHRAE guidelines cover actual loads per square foot based on inches of snowfall per hour, temperature of the snow, density of the snow, wind loading in miles per hour and lowest ambient temperature of operation.
For this store, the design engineer chose a value of 185 Btuh per square foot. While I thought this is exceptionally high in my experience, I installed it as designed.
In any case, it is a good idea to consult your tubing manufacturer and have the people there assist you in the design of the system. They will help you determine tubing density per square foot, tube size, recommended flow rate, maximum recommended tube circuit length and overall anticipated resistance to flow. Many of the tubing manufacturers have low-cost or no-cost software programs to expedite the design process.
Insulation below the snow-melt slab and on the edges should be regarded as a requirement, not as an option.
Controlling the system
Over the years, I have been exposed to just about every type of snow-melt control logic that has ever been conceived. They have varied from a simple pilot-lighted on/off switch to highly sophisticated computer-controlled systems. Over the next few months I will give you a brief description of each along with my view of advantages and disadvantages.
The first one is the lighted pilot switch: This is basically an on/off switch assembly. The theory of operation is when you see that it is snowing outside, or you know it’s going to be snowing shortly, you turn on the switch and the snow-melt system begins heating up the slab. A neon LED in the toggle switch glows orange when the switch is on. The only advantage of this system is that it’s the least expensive option.
The disadvantages are many. Where do you place the switch so the system isn’t inadvertently left on for long periods of time? If the switch is near the front door and the occupants leave via the attached garage, they may be surprised by a large utility bill. This has actually happened more times than I care to remember in our mountain communities; people closed up their vacation homes but left the snow-melt system running. In some cases, the utility bill was so high that the snow-melt system was shut off and never used again.
We have a saying in the business: “If you need to know how much it costs to operate a snow-melt system, you can’t afford it.” It’s easy enough to do the math, and if your customer really wants to know you can figure it out.
Another disadvantage of a lighted pilot switch is that it doesn’t reference the actual slab conditions and can result in over- or under-melting, neither of which is good. I’ve seen some slabs that were running at an excessive 80°F. A target slab temperature of 35°F to 40°F is generally adequate for melting snow.
Tune in next month as we continue to delve into the many facets of snow-melt design, installation, operation and maintenance.
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.