Frost Heave Design Guide - Thermon Manufacturing Company

Freezer Frost Heave ProtectionStep 4: Determine System LayoutHeating cables used for frost heave protection systems can beinstalled in metallic or nonmetallic conduits under the insulatedfloor. These conduits allow the heater cable to be replacedwith new cable should the need arise. Conduits usedfor heating cable should be a minimum of 3/4" inside diameter.Where the size of the freezer permits, conduit layout can utilizea looping design allowing all junction/pull boxes to be locatedalong one wall. If the length requires, straight line runs locatingjunction boxes along walls opposite each other may be used.The conduit layout and number of bends should be arrangedin order that the pulling force of the heating cable does not exceed667 newtons (150 pounds) (refer to Details 4.1 and 4.2).As per the National Electrical Code, Article 348-10, “Thereshall not be more than the equivalent of four quarter bends(360 degrees total) between pull points, e.g., conduit bodiesand boxes.”Location of junction boxes must allow accessibility to the wiringwithout removing any part of the building or other substanceused to establish finished grade. Typically junctionboxes are located 12" to 24" (30 to 61 cm) above the finishedfloor (refer to Detail 4.3).Once the locations of the junction boxes are known, beginconduit layout using the information provided in the earliersteps.• Determine layout using center-to-center spacing selectedin Step 2.• Do not exceed continuous circuit lengths shown in Table3.0.• For installation in concrete subslab, secure conduit to reinforcingsteel using steel tie wire.• For installation in gravel subslab, secure conduit using theaddition of rebar or steel spikes to maintain recommendedspacing.Step 5: Establish Control Method to Operate the SystemAll frost heave protection systems should be controlled. Thereare two basic ways to control and monitor a frost heave protectionsystem:• Recommended: Proportional control using a ResistanceTemperature Detector (RTD) with HeatChekSystem—Turnssystem on proportionally based on subfloor temperatures.At maximum heat loss system is 100% on. RTDs, to be locatedin a minimum of 1-1/4"conduit between electricheat tracing conduits, have the ability to limit the cable’spower output to closely match heat loss (refer to Detail 5.1).• Alternate: On/Off Thermostatic Control—Turns system onand off based on subfloor temperatures. Sensor is locatedin 1-1/4" conduit between electric heat tracing conduits(not shown).Detail 4.1 Looping DesignRTD Sensor ConduitConduit with Heating CableDetail 4.2 Straight Line DesignRTD Sensor ConduitConduit with Heating CableDetail 4.3 Junction BoxDetail 5.1 HeatChek System6