Times change, but until recently there have been few developments in the commercial application of boilers and related hydronic heating equipment. The basic uses of this technology are all fairly familiar––producing steam for industrial processes, heating water in bulk, and, of course, space and process heating.
But the last decade has seen the growth of a previously obscure hydronic application––radiant heating. In radiant heating jobs, a boiler is used to heat a warm fluid that’s circulated through flexible piping imbedded in a solid surface, such as a concrete floor, or driveways or walkways (most often referred to simply as snowmelt systems).
In the 1970s and 1980s, uses of radiant heat were often confined to a few critical areas, especially those where snowmelt was important––like hospital emergency ramps and helipads. However, with the development of flexible hydronic piping, radiant heat is showing up in more commercial and industrial jobs. No longer limited to critical areas, radiant heating is used in car wash and service bay floors, airplane hangers, garages, pedestrian mall walkways, handicap access ramps, stadiums, and even under football fields.
In commercial applications, radiant heating systems melt snow and ice to reduce liability and improve accessibility. Compared to typical mechanical and chemical methods of snow removal, a radiant snowmelt system is more economical and certainly more environmentally acceptable by avoiding salt and chemicals. At industrial sites, snow- and ice-free zones are maintained with a radiant system on call or on duty 24 hours a day. And, when used to heat large warehouses or production areas, a radiant floor is far superior to forced air.
With so many uses, the radiant market is growing fast––nearly 25% a year according to the Radiant Panel Association. In 1998, nearly 122 million feet of piping was sold in the U.S.––or 23,063 miles.
The most critical component in a successful radiant heating system is the piping. Pipes made from polyethylene (PE) have been on the market for years in the U.S., but PE lacks the physical properties for radiant heating systems because it can withstand only minimal temperature and pressure changes.
To improve performance, cross-linked polyethylene, or PEX pipe, was developed. This material withstands temperatures up to 200° F at 80 psi while also exhibiting strength and durability. A seamless barrier (thin outer layer) of ethylene-vinyl-alcohol copolymer (EVOH) on the pipe blocks oxygen from entering the fluid and corroding boiler components. In addition, PEX pipe also handles the weight and compressive forces when encased in concrete or asphalt.
With advanced radiant pipe, more radiant heating solutions are possible, but are they practical? The cost per square foot for commercial radiant installations (including pipe, manifolds, fittings, and the necessary pipe attachment accessories) averages $1.00 to $1.50 per square foot. Two workers can lay 500 square feet in one hour (using a pipe uncoiler), with a third worker laying out the manifold. System operating costs are also reasonable, since the boilers are fired by economical oil or gas fuels.
Becoming Comfortable with Radiant
The growing popularity of radiant heating systems means that engineers and heating contractors should become comfortable with radiant applications. Fortunately, these applications are not much different than traditional hydronic heating jobs. The primary difference is that on a radiant project, the medium that delivers heat is a large-scale, heat-emitting surface––a floor, concrete slab, or other object.Outdoor applications are typically snow melting projects, where a slab is heated to a surface temperature of 38° to 40° F. Indoors, where large open areas (such as an airplane hanger) are impractical to heat by forced air, a radiant system can heat the space comfortably with a floor temperature of about 80° F. To get a better understanding of radiant heating, it helps to consider the most prevalent commercial application––outdoor snow melting. ASHRAE defines three classes of snowmelt performance in its 1995 Handbook:
- Class 1: Residential/Light Commercial, with a load of 75-125 BTU/sq. ft. for residential walks or driveways and interplant areas.
- Class 2: Heavy Commercial, with a load of 125-175 BTU/sq. ft. for store and office sidewalks, facility driveways, loading docks, and steps of hospitals.
- Class 3: Institutional/Industrial, with a load of 175-200 BTU/sq. ft. for highway toll plazas, bridge decks, fire station areas, stadium exits, airport taxiways and hospital emergency entrances.
These classifications determine an allowable rate of snowfall and accumulation. For example, a Class 3 system will not allow any accumulation during the coldest day in that geographical area’s weather history. A Class 1 system, on the other hand, will allow some accumulation for more economical system operation.
Outdoor radiant systems typically use a 50% propylene glycol (anti-freeze) solution in the loop. Indoor applications may use water in the piping loops if there is absolutely no chance of freezing––but a glycol/water mix is preferred for safety’s sake.
Performance Calculations
Successful application of a radiant system is based on proper performance calculations. There are many factors that go into figuring the number of BTU/hr required by a radiant system. In addition to outdoor degree days, winter design temperatures, and other weather-related factors included in standard heat loss calculations, other site variables include:
- Snow: The rate and density of falling snow is directly related to the performance requirements of the snow melting system. Light snow takes less energy to melt than heavy snow. The rate at which snow falls, including wind-blown accumulation, will also affect the system’s capabilities.
- Air Temperature: Snow will develop when air temperatures range between -10° and 40°F. Air temperature also affects refreezing of melted snow or ice. As a result, the system must be designed to maintain a surface temperature of at least 33°F until the surface is completely dry.
- Wind velocity: “Wind chill” is a common meteorological term that takes into consideration air temperature and the speed of air, referred to as wind velocity. Wind velocity will change based on weather conditions and the terrain. For example, wind velocity will increase as the air blows down certain streets due to the Venturi effect. The wind and air temperature may then combine to blast a surface with freezing air and moisture. These harsh conditions require a design with greater heat output than would normally be expected in that locale. In the heat loss formula, wind velocity is assigned an “h” value that combines rates of conduction and convection for a surface area. This value is used to calculate the heat capacity or surface temperature required to maintain a dry and/or unfrozen surface.
- Humidity: The relative humidity or moisture content of the air directly impacts the ability of the wind to “dry” the surface. Moist air is defined as air with a moisture content of 80-90% water. When the slab surface temperature is fixed, the evaporation rate varies with changes in air temperature, relative humidity, and wind speed.
- Slab surface temperature: To prevent ice formation until the exposed area is dry, slab surface temperature must be maintained at least 33°F. Yet, the slab temperature is not sufficient to determine the desired heat load. At start up, the slab is insulated by a layer of snow. But after the layer melts, the design must be able to withstand the wind load. Also, a higher surface temperature is often advisable to fully melt the snow. This will prevent ice bridging––air gaps created when only the bottom part of the snow layer melts, leaving unmelted snow and ice on top.
- Drainage: A well-designed drainage system is critical for peak system performance. When melted snow is removed through proper drainage, less energy is needed to prevent refreezing. Drains may also require heating to prevent clogs.
- Insulation: The use of insulation, below and at the edges of the heated surface, maximizes efficiency by minimizing heat losses on the backside and at the edges. The response time of the heated surface is greatly enhanced by insulation. The insulation used must handle anticipated dynamic and static loads. High-density, rigid foam insulation is often specified. One inch of rigid foam insulation used in concrete slabs typically has an R-value of 5 with a load rating of 25 psi.
- Surface materials: The most common surface materials are concrete, asphalt, brick, stone and pavement. In addition to typical construction procedures, piping, insulation, and installation accessories to hold the pipe in place must be used.
- Heating fluid: The common fluid for snow melting is a propylene glycol (PG) and water mixture. The percentage of glycol to water depends on how extreme the cold temperatures are in the geographical area. Normally, a 50% mixture ratio is satisfactory. The hydraulic properties of the heating fluid also affect the pump, pump size and piping-section lengths. The viscosity of a 50% PG mixture is greater than 100% water. As PG viscosity increases, flow decreases. Also, viscosity of glycols vary inversely with temperature––the warmer the glycol, the lower the viscosity.
System Components
With the basic design parameters in mind, the various system components can be considered.General hydronic components. Boiler shock due to low temperature return water can be handled by bypass piping or a four-way mixing valve. Using an oxygen barrier pipe will lessen concerns about internal boiler corrosion (meeting DIN 4726 requirements). Use of a heat exchanger––such as a plate-to-plate unit––and non-ferrous components, with non-barrier pipe, also meets the same requirements. This is perhaps the most common commercial and industrial installation practice. Use of a heat exchanger and non-barrier pipe is often more cost effective when the length of pipe exceeds 3000 lineal feet. But I encourage you to evaluate it for yourself.
Circulators. Circulators are required to move the heating fluid through the piping. Circulators are selected based on their ability to provide sufficient flow rates and head pressure per circuit. A Pump Flow and Head Chart provided by the manufacturer can help determine the proper selection.
Circuit components. PEX piping is available in four diameters: ½”, 5/8”, ¾”, and 1”, with the larger sizes used for outdoor applications. Piping is also available with or without an oxygen barrier.
Supply and return manifolds. These components connect the radiant circuit piping to the boiler system’s supply and return. Brass manifolds are used for ½” piping; copper manifolds are for larger sizes. The manifold must have a body size that will provide the flow required to support all the circuits. Since flow always takes the path of least resistance, it is important that all circuits and manifolds (when there is more than one) have equal resistance so the system remains balanced. The goal is to have all circuit lengths within 10% of each other, which will equalize head between each circuit to facilitate even heat distribution, thereby eliminating the need for balancing valves.
Outdoor components. Control systems that monitor slab and boiler temperature, heating fluid temperature and heat input will optimize performance and prevent thermal shock in the boiler and slab. Sensors are available to monitor temperature and humidity conditions, which will switch relays to detect freezing temperature and precipitation. These sensors can work as an off/on control, or a simple timer can turn the system on and shut it off two to three hours later. Finally, an “idle start” approach can be used to keep the surface at a predetermined temperature below freezing to minimize the change in heat needed to melt snow and to reduce the potential of thermal shock to both the boiler and slab. No matter the controls used, slabs should never be subjected to water temperatures above 150°F.
