Maintenance of Heater

The heater in 52S units is located directly behind the evaporator coil. The heater assembly includes the heater (2.3 KW, 3.4 KW or 5 KW), a primary limit switch, and a secondary limit switch. The primary and secondary limit switches are safety switches that regulate heater operation. The primary limit switch disables current to the heater when the temperature exceeds the setting on the switch.

For example, a 3.4 kW heater may have a primary limit switch setting of 150 F. If the temperature at the switch exceeds 150 F, the primary limit switch will electrically open and shut off power to the heater. Once the temperature drops below 110 F, the primary limit switch will automatically reset.

The secondary limit switch works in the same manner as the primary limit switch, except it is a one time switch and has a slightly higher temperature setting. Once this switch is thermally opened, it must be replaced. Together the primary and secondary switches help maintain safe temperature limits for unit operation.


Heater failure may result from broken heater coils, primary limit switch failure, or an open secondary limit switch. These conditions are generally caused by low or no airflow.


Before working on the heater, read the instructions thoroughly and gather the necessary tools for the job. Review all safety information on the unit and in the product literature.


Before performing any cleaning, servicing, or maintenance to the unit, Disconnect All Power to avoid the possibility of electric shock and personal injury.

The manufacturer reserves the right to discontinue, or change at any time, specifications or designs without notice and without incurring obligations.

Consider the following safety issues:

Prior to performing any service or maintenance on electrical equipment you must Disconnect All Power.

New and unfamiliar tasks should be performed under the supervision of an experienced service technician.

Personal protective equipment, such as safety glasses and work gloves, should be worn.

The floor around the work area should be clean and free of debris.

Make sure tools are the correct tools for the job, and that they are working properly and in good condition.


The following list includes recommended tools and devices for working on the heater section of 52S units.


Perform the following steps to remove the Heater Assembly.


2. Remove the front panel of the unit by grasping panel on each side, lifting upward and out. See Figure 1.

3. Remove the unit from the sleeve as detailed in the GENERAL DISASSEMBLY section.

4. Remove the discharge air grille and the discharge air deck as detailed in the GENERAL DISASSEMBLY section. Save screws.

5. Once the air discharge deck is removed, the heater assembly is exposed. After noting positions of wires and labeling them, use a pair of needle nose pliers to carefully remove the wires connected to the heater assembly.

6. Remove the two 1/4-in. screws mounting the heater assembly to the inside of the partition. Save the screws. See Figure 42.

7. Remove the heater assembly.

Examine the heater as follows to determine if it is operational:

Perform a visual inspection. The heater coil should be free of breaks. If there are any breaks in the coil, replacement of the heater assembly is necessary.

Coil resistance must also be checked to verify the heater is operating correctly. The resistance of the heater coils must meet approximate levels for the heater to perform at its optimum efficiency. See Table below for approximate resistance for heaters at 75 F. Before checking the heater coil resistance, be sure all power to unit is off.

To check resistance, set the volt-ohmmeter selector switch to the lowest ohms value. Next connect the voltohmmeter leads to each side of the heater coil at the studs that hold the limit switches in place. Incorrect readings can be obtained if the wires are not removed from the limit switches on the heater assembly. If the resistive reading is infinite or zero, the heater is failed and replacement is necessary. Reinstall the heater assembly by reversing steps 1 - 7.

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