Why doesn’t adjusting the High Temperature Limit dial turn the unit on?

The High Temperature limit dial is used to set the maximum temperature that the unit will work at. When the High Temperature limit sensor detects a temperature higher than what is selected on the dial it overrides the system and shuts it off. This function can be used to further regulate slab temperatures in pavement applications and as a warm weather override when the system is used for roof and gutter melting applications.

High Limit Thermostat

The calibrated 40°F to 90°F (4°C to 32°C) high limit thermostat prevents excessive temperatures when using constant wattage and MI heaters. It also permits safe testing at outdoor temperatures too high for continuous heater operation. The temperature sensor is included and must be connected to the system for proper operation.
There are two DIP switch configurable operation modes for the high limit thermostat. The factory default operation mode uses the high limit thermostat as a slab temperature regulator, preventing heater operation at temperatures above the High Temperature Limit setting. The optional operating mode uses the high limit thermostat as an ambient air sensor, preventing heater operation at temperatures above the High Temperature Limit setting until the temperature drops back to within the set limits.
The details of operation in each mode are as follows:

Slab Regulating Thermostat Mode

• High temperature causes unit to turn off heaters, if running, and to ignore any call for heater operation from the panel, RCU, or EMC
• High temperature continues any hold-on cycle that was initiated before the high temperature condition. If the slab temperature drops within limits during the hold-on time, the heater will be turned back on
• In a high temperature condition, an APS will still initiate operation of connected SC–40C contactor(s)
• The Heater Cycle functions normally

Ambient Temperature Thermostat Mode

• High temperature causes the unit to turn off heaters, if running, and to ignore any call for heater operation from the panel, RCU, or EMC
• High temperature cancels any Hold-On cycle that was initiated before the high temperature condition
• In a high temperature condition, an APS will not initiate operation of connected SC–40C contactor(s)
• If the Heater Cycle switch is operated in a high temperature condition, the heater(s) will be turned on for a maximum of 30 seconds. A new Heater Cycle cannot be initiated for another two minutes after that.