Turning Up the Heat
Manipulation is key to your turf care program. You manipulate turf height with mowing; you manipulate turf growth patterns with fertilization. Sports turf managers of fields used by professional teams may take that manipulation one notch higher with temperature control of the root zone.
Ross Kurcab, CSFM, is Turf Manager for Invesco Field at Mile High, the game field of the Denver Broncos football team. After serving that same role for the Broncos' practice facility, he applied his experience with extensive research to assist with the field development specifications for the new facility. Soil heating is one element of that design.
Kurcab says, “The main reason we heat is to prevent ground frost, which generally occurs in late November or early December in the Denver area. Once the ground is frozen, the surface becomes as hard as concrete. That's not a safe field for play. An added benefit of our soil heating is we are keeping grass living, growing and recuperative, which provides a better playing surface. In more southern areas, where the primary warm-season turf used on athletic fields is bermudagrass, the major purpose of soil heating is to keep the bermudagrass growing longer into the winter for field safety and playability.”
Professional level athletic fields generally are constructed with a gravel or crushed stone sub-layer topped by an 8- to 12-inch layer of a sand-based soil profile. The sand is selected to precise specifications to provide stability and pre-determined growing conditions. Sand percentages range from 80 to 100 percent, with the non-sand materials organic or a combination of organic and inorganic as pre-specified to create the desired level of environmental control.
The two basic types of athletic field heating systems are water-based (hydronic) and electrical. An electrical system is installed in one of the fields of the Denver Broncos practice facility. Hydronic systems are used in one section of the fields of the Chicago Bears Halas Hall practice facility and in Invesco Field at Mile High. Both electrical and hydronic systems operate from a sub-surface location with a network of tubing or coils that are heated to an adjustable temperature level. The heat is conducted from this heat source through the sand-based profile to the field surface. As the distance the heat must travel through this conduction method increases, the temperature achieved within the sand-based profile gradually drops in proportion to the distance from the heat source.
A third type of system uses forced air, with or without added heat, and may be equipped to operate in both suction and pressure modes. It moves the air throughout the sand-based profile through forced convection. This type of system combines the heating capabilities with rootzone air and moisture control. One of these systems is installed at Safeco Field, in Seattle, Wash., home of the Seattle Mariners.
Kurcab notes, “The Invesco Field system incorporates 20 to 21 miles of heat tubing based on 9 inch centers located 10 inches below the field surface. That's at the bottom of sand-based profile and just on top of the pea rock. The tubing was tied down to a wire grid during the construction to keep the tubes from floating upward into the rootzone. Being secured at this depth helps us avoid hitting the tubes during the aerification processes. It also helps eliminate ‘waves’ in the tubing that could trap air like bubbles in a straw. Our system is equipped with an air separator to help force out any trapped air that would impede the water movement through the tubing.
“Our system uses only water with no added glycol. Because we will keep it running throughout the winter, freezing is not a factor. We have two boilers just for the system. These are heated with natural gas. We could also tie in to one of the building's boilers, if necessary. This gives us a safety net of triple redundancy. The heated water is pumped from the heat chargers and circulated through the tubing as automated by the computerized system. The heating begins on the west side of the field and is pumped to the east and then goes into the return lines. When we first turned on the system we had a 4- to 5-degree temperature discrepancy across the field. We balanced that by adjusting the rate of flow. The faster the water moves, the less heat it drops.”
The field is divided into five heating zones, with two sensors per zone located at the 6-inch level. Temperatures vary within the sand-based profile, with a 7- to 10-degree difference from the top half-inch to the 10-inch depth. Kurcab's goal is to set the soil temperature to achieve optimum root growth temperatures for the cool-season turf, in the 55- to 65-degree range. The top heating temperature he's used is 65 degrees at the tubing level.
He says, “We focus on the living zone, the top inch or two of the soil. That's where the vast majority of the grass roots are located. The top level has the greatest effect on the game and also on the grass plant. It's important to know the field, to recognize that the surface temperatures will vary with more shade and with the sun angles. The ideal heating system layout would take into account the microclimates, including the role of the turf as an insulator, and be engineered to compensate for them. We check the temperature at different levels using a 9-inch soil probe that is tip-sensitive to read with precision within ½ to 1 inch of the tip. That let's us know exact temperatures throughout the soil profile rather than an average, so we can make the desired adjustments.
“Because we're attempting environmental control, trying to delay winter, we use tarping in conjunction with the heating system to generate heat during the day and minimize the radiational cooling at night. These two tools also will allow us to get a head start on spring growth, working under the tarps during the post-season to prep the turf.”
Kurcab notes that the electric system at the Broncos practice facility was installed around 1990. The heating units are 10 inches deep on 6-inch centers. He says, “Installation costs on the electrical systems generally are less than half the price of hydronic system installations, but operational costs are much higher compared to heating the water with natural gas. That was a key consideration in choosing the hot water system for the new stadium.”
Ken Mrock, Head Groundskeeper for the Chicago Bears, says, “We have a heated footprint 210 feet by 400 feet at the Halas Hall practice facility. It's a hot water/glycol system with 14 miles of tubing located at an 8-inch depth on 10-inch centers. Our sensors are 6-inches deep. We use three 2-million BTU boilers just for field heating. The extended area provides heated space for the lined football field and a surrounding area for the individual position drills.
“This is the sixth year for our system. We start heating in mid-October, when the soil temperature drops into the low 50-degree range, and maintain it in the 50s gradually raising the level to a high of 65 degrees as winter advances. This keeps the surface from freezing, staying at 40 degrees right below the thatch level by the end of the playing season. Because of the conditions we're able to maintain with the heating system, the team only practiced indoors five times this past year, out of a possible 140 practices. The team practiced on the heated field for the last 8 weeks of their season. We core aerified on January 22 to get a head start for spring's mini-camps, which is a huge tool for our maintenance program. We then shut down the heating system to allow the turf to go dormant. We feel it has earned the rest after the extended season.
“After our first year with the system, we noticed that the heated field was a lighter green and behind the other fields in development as they came out of dormancy in the spring. We determined that the grass plants were taking up the available nutrients in the soil and developed a separate fertilization program for that field to equalize conditions by the start of spring.”
Kurcab and Mrock agree that the heated field is a living lab with a big learning curve. Though the principles are relatively simple: to keep the playing surface from freezing and to keep the turfgrass growing as long into the season as possible, the realities are complex. It's not a matter of setting the temperature once for the season and letting the system run on its own. Mrock collaborates on the heating levels with Assistant Head Groundskeeper, John Berta, who then sets the controls. Kurcab and his staff constantly monitor conditions and manipulate temperature levels to accommodate microclimates and weather conditions. The rewards are worth the effort. Mrock says, “We are going to have a heated field with a similar system at the reconfiguration of Soldier Field.”
Steve Trusty is executive director and Suz Trusty is communications director for the Sports Turf Managers Association, www.sportsturfmanager.com
THE SNOW FACTOR
Kurcab notes the biggest misconception with a heated field is that it will prevent snow from accumulating. With a heated field, as with all other surfaces, snow accumulation is based on the rate of the snowfall. A warm grass surface at 60 degrees F generally will prevent accumulation if the snow falls at less than a half-inch per hour. If the rate is higher, snow will begin landing on snow, rather than the grass surface, and accumulation will occur.
But the heated field keeps working on accumulated snow. Mrock reports that the Bears' heated field has melted 14 inches of snow in 3 days. And, though three inches of snow fell in about 3 hours in January of 2002, the snow was gone and the field ready for play the following day.
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