May 2014 Newsletter (vol.29)   >>   Proper Use of Heat Cables



Incorrect use and/or improper installation of Redi-Heat Soil Warming Cables can cause danger of fire, electric shock, electrocution or damage to planted material. Redi-Heat Soil Warming cables are to be buried in soil or soil-based mixtures so that no portion of the heated section is visible while in use. They are not to be used with flammable materials like dry peat moss or with insulation materials like vermiculite or sand, with the exception listed below. We strongly recommend that a ground fault circuit interrupter (GFCI) be used in the cable circuit to provide additional safety and protection, as well as installing a thermostat to regulate temperature.


Be sure the cable is disconnected when installing, planting, cultivating or harvesting and when not in use. Do not allow the cable to crossover or otherwise touch itself. To do so could result in overheating, failure or other hazardous conditions. Do not attempt to modify the cable to lengthen, splice or shorten it. Such modifications may create a hazardous condition and will void the warranty. Heat cables contain no serviceable parts.


Basic Installation

Cut plastic or metal hardware cloth, with ½” openings, to fit the planting area. Arrange the cable on the hardware cloth in a pattern so that it is a least 3” from the sides of the flat and at least 4 to 5” away from other portions of the cable. Loosely attach the cable to the hardware cloth using string, tape or plastic ties. The thermostat sensor should be located approximately half way between the center and edge of the pattern.


Cables can be used either indoors - within a contained box or bed, or outside in the field. For use indoors, construct your box from wood or insulation board. Be sure to include adequate drainage holes in the bottom of the box. For outdoor use, dig a hole or trench deep enough for drainage, as well as extra (non flammable) media and plants. Flammable media is anything that is 100% organic, like peat moss or a soiless mix, it can dry out and start to smolder. Place the hardware cloth and cable assembly in the flat or hole with the warming cable side down. This will protect the cable from being damage by tools used to cultivate or move plants.


Soil Use

To use the cables in a bed that plants will be grown in, add an additional 2-3” of pre-moistened soil/mixture over the hardware cloth; moisture conducts and distributes the heat evenly.


Pots and Flats Use

If inserts or pots are to be used, add only 1” of pre-moistened soil/mixture over the hardware cloth. Place the pots on the soil/mixture layer and then add additional soil/mixture around the pots to within ½” of the lip of the pots. Without this additional soil/mixture to conduct heat to the sides of the pots, there will be little or no heat transmitted through the bottom of the pots. Add water and check temperatures and uniformity daily.


Desert Type Plants and Sand Beds

For desert type plants which do not require as much moisture, again add only 1” of soil/mixture over the hardware cloth. Add water and check for temperature and uniformity. Then add 2-3” of a sand-soil mixture for planting. Water should be added to keep the bottom soil/mixture moist and conducting heat. The water should simply pass through the sand-soil mixture.

For a sand bed you would use just sand instead of a mixture over the 1” soil/mixture layer. Using just sand is only recommended when being used for propagation where frequent mist will keep the sand moist. Sand is basically an insulating material and it will not conduct heat as effectively as soil or sand-soil mixture.


Heating Capacity

Use a soil thermometer to check temperature and uniformity. Adjust the sensor location as required. The Redi-Heat cable has limited heating capacity. When the air temperature drops below 60F, protection should be provided over and around the planting flat to conserve the cable heat. If such protection is not provided, all of the heat generated by the cable will be lost to the surrounding air, and the temperature of the soil and air could drop to the point where the plantings will be damaged. Without protection from cool air, the plants roots may stay warm enough to survive but the leaves and stems may be damaged.

Spacing the cable as mentioned at the beginning of this article will give about a 5F increase over ambient temperature without protection to conserve heat. This spacing and the chart in the Hummert catalog are for 14 watts of heat per square foot of heated area. The University of Missouri Extension recommends 12 watts for the state of Missouri except the bootheel region where 10 watts per square foot will be sufficient. These recommendations are for use in a cold frame outside to keep the ground from freezing. It is not recommended to space the cable closer than the 14 watts per square foot.

Heat cables produce such a large amount of heat that if it can’t pass it on will cause overheating and failure of the cable. It is important to follow the above directions so that the cable is surrounded by moist soil. It is the water in the soil that will conduct heat away from the cable, to the plants and prevent overheating.