In recent years, the interest and demand for hydropic snowmelt systems has increased. Layout and installation of these systems have become fairly straightforward. Several tubing manufacturers have developed or adapted specialized software programs to help with system design. In its simplest form, a hydropic snowmelt system is nothing more than a radiant floor system with a higher BTU/hr output per square foot. The key challenge in implementing a snowmelt system is in properly controlling it.
In order to put together a high quality and efficient system, a few control issues must be addressed:
• Slab and boiler protection
• Slab temperature control
• Snow detection
• Boiler staging
Slab and boiler protection
A hydropic snowmelt system is a classic example of extremes! Here’s a system in which the heat source (most commonly a non-condensing boiler/s) typically needs to operate at high temperatures and, at the same time, the snow-melting slab can produce very low temperatures.
Introducing high temperature into a frigid slab can cause damaging stresses because of the unevenness of thermal expansion, risking severe damage to the slab. At the same time introducing extremely cold temperatures into a hot boiler can cause detrimental stresses as the metal cools and contracts rapidly and unevenly. This can severely threaten the boiler components and operation.
To maximize the life of the snow-melt slab, and everything connected to it, a control package should be able to sense how fast heat is being applied to the snow-melting slab and have the ability to limit the rate of heat transfer. In order to do this, the control must have a mixing device of some kind (variable speed injection pump or mixing valve) and the ability to sense the slab supply and slab return temperatures (delta T).
BTU/hr = Delta T x GPM x Constant
This allows the control to determine the delta T of the slab, and by limiting the delta T, the rate that heat is applied can be controlled. At the same time, the control should be equipped with a sensor to measure the boiler temperature. If the boiler temperature begins to drop rapidly or cannot be maintained, the control should respond by limiting the amount of cold fluid being returned to the boiler. The control can achieve this by modulating the same mixing device that is used to control the heat supplied to the slab.
Slab temperature control
Accurate slab temperature control is a very important aspect in operating an energy efficient snowmelt system. Most people feel that the hotter you operate the system, the faster the snow will melt. This is true. However, you must keep in mind that the hotter you operate the slab, the more energy you are driving out the back of the slab and off of its edge.
This energy is not being used to melt the snow on the surface of the slab; rather, it’s being used wastefully to heat the ground beneath and around it.
The most efficient temperature to operate a snow melting system at is just above the freezing point. Though, this caution: a potential disadvantage at this low temperature is the possibility of the slab icing over if outdoor temperatures drop suddenly.
Ideally, the control package should be able to maintain an accurate slab surface temperature just above the freezing point while monitoring the outdoor temperature. In the event of a sudden drop in outdoor temperature, the control package could respond, ensuring that the surface temperature does not drop and permit freezing.
An in-ground snow and ice detector is the most accurate method of detecting snow. This device detects the actual conditions on the surface of the heated slab. It senses the presence of blowing or drifting snow, and also knows - if the slab is dry - when it can shut down system operation. The in-ground sensor does not rely on timers that may turn off too early, causing icing problems, or run too long, wasting energy. An in-ground sensor does not need to be adjusted for slabs that respond quickly or slowly or for slabs that are being idled.
Large snowmelt areas require large amounts of heat and large boiler capacities. If a system can be broken into smaller sections or zones and melted sequentially, it’s possible to use a smaller heating plant. This results in a less expensive installed cost for the system. But, for optimal operation, the control package must be able to properly prioritize the zones. The control package must also be able to ensure that the highest priority zone always receives enough heat in order to avoid potential freezing problems. Ideally the control should be able to determine if the system is running at less than the design load. In this case, there may be enough boiler capacity available to melt more than one zone at a time. The control package should be able to determine this and respond accordingly.
Boiler staging is an important aspect of snow melting that is often overlooked. Just as with heating systems, snow-melt systems are designed for their maximum loads. However, design loads only happen at design conditions.
This means that they are oversized most of the time. By using multiple boilers and proper staging control, the heat output of the boiler can be better matched to the heat required by the system. This minimizes short cycling of the boiler and increases the overall efficiency of the system.
Overall, the operation of a snowmelt system has several very important aspects that need to be addressed. The control package selected to operate the snowmelt slab should be capable of dealing with all of the many variables in getting the job done properly.
Articles reprinted with the approval of the Radiant Panel Association,
800-660-7187. Articles reprinted with the approval of the Radiant Panel Association,
800-660-7187. More information on radiant heating, including standard guidelines, is available at www.radiantpanelassociation.org.
The Blake Residence
The Blake residence in Whitefish, MT, is a 5,000 sq. ft., 15-year-old, 4-story custom log home fashioned from massive spruce logs. The home was built above one of the last switchbacks on the road to Big Mountain Ski Resort. Its decks and grand windows offer a spectacular view of the whole Flathead valley. (Photo: Tube is laid for a snow melting system at the Blake residence in Whitefish, Montana.)
The owners, Rick and Alicia Blake, settled on a plan shortly after buying the home to have the Simensen’s company improve the domestic hot water heat and install 700 sq. ft. of hydronic snow melt in front of the garage. However, as the job progressed,
the owners became more intrigued at the idea of extending the snowmelt areas. Before long, the Radiant Specialties crew found themselves with a need to extend the snowmelt around the house to the hot tub area and down the rest of the driveway. (Photo: Snow comes to Montana, but the driveway stays clear.)
The Blake’s snow melt area was divided into six zones that can be activated individually or simultaneously, directed by timers on each zone. The system consists of two EBL 300 Lochinvar sealed combustion, 2-stage boilers, controlled by a tekmar 262 boiler control that also monitors the 80-gallon indirect-fired water heater for domestic water. A tekmar 362 variable speed mixing control sees to consistent system temperature. A variety of Grundfos circulating pumps circulate a glycol/water mix through almost two miles of Wirsbo 1/2" PEX tubing with 6" to 8" spacing.
Excerpted from Radiant Panel Report, October 2001-11-26.