Defrost may be initiated by any one of the following methods:

1.The manual defrost switch (MDS) is closed by the user.

2.The user sends a defrost command by communica- tions.

3.The defrost interval timer (controller function code Cd27) reaches the defrost interval set by the user.

4.The controller probe diagnostic logic determines that a probe check is necessary based on the tempera- ture values currently reported by the supply and re- turn probes.

5.If the controller is programmed with the Demand Defrost option (Future) and the option is set to “IN” the unit will enter defrost if it has been in operation for over 2.5 hours without reaching set point.

ENERGIZED

 

DE--ENERGIZED

CONTROL TRANSFORMER

FOR FULL DIAGRAM AND

 

LEGEND, SEE SECTION 7

 

POWER TO

CONTROLLER

T6 PB PA

 

 

 

 

 

 

 

TD

 

 

T6

 

 

 

 

 

 

 

PA

PB

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SIGNAL TO

CONTROLLER TU USV

TSESV

Figure 4-7 Defrost

Defrost may be initiated any time the defrost temperature sensor reading falls below the controller defrost termination thermostat set point. Defrost will terminate when the defrost temperature sensor reading rises above the defrost termination thermostat set point. The defrost termination thermostat is not a physical component. It is a controller setting that acts as a thermostat, “closing” (allowing defrost) when the defrost temperature sensor reading is below the set point and “opening” (terminating or preventing defrost) when the sensor temperature reading is above set point. When the unit is operating in bulb mode (refer to paragraph 3.3.9), special settings may be applicable.

If the controller is programmed with the Lower DTT setting option the defrost termination thermostat set point may be configured to the default of 25.6_C (78_F) or lowered to 18_C (64_F). When a request for defrost is made by use of the manual defrost switch, communications or probe check the unit will enter defrost if the defrost temperature thermostat reading is at or below the defrost termination thermostat setting. Defrost will terminate with the defrost temperature sensor reading rises above the defrost termination thermostat setting. When a request for defrost is made by the defrost intermale timer or by demand defrost, the defrost temperature setting setting must be below 10_C (50_F).

When the defrost mode is initiated the controller opens contacts TD, TN and TE (or TV) to de-energize the compressor, condenser fan and evaporator fans. The COOL light is also de-energized.

The controller then closes TH to supply power to the heaters. The defrost light is illuminated.

When the defrost temperature sensor reading rises to the defrost termination thermostat setting, the de-icing operation is terminated.

If defrost does not terminate correctly and temperature reaches the set point of the heat termination thermostat (HTT) the thermostat will open to de-energize the heaters. If termination does not occur within 2.0 hours, the controller will terminate defrost. An alarm will be given of a possible DTS failure.

If probe check (controller function code CnF31) is configured to special, the unit will proceed to the next operation (snap freeze or terminate defrost). If the code is configured to standard, the unit will perform a probe check. The purpose of the probe check is to detect malfunctions or drift in the sensed temperature that is too small to be detected by the normal sensor out of range tests. The system will run for eight minutes in this condition. At the end of the eight minutes, probe alarms will be set or cleared based on the conditions seen.

When the return air falls to 7_C (45_F), the controller checks to ensure the defrost temperature sensor (DTS) reading has dropped to 10_C or below. If it has not, a DTS failure alarm is given and the defrost mode is operated by the return temperature sensor (RTS).

If controller function code CnF33 is configured to snap freeze, the controller will sequence to this operation. The snap freeze consists of running the compressor without the evaporator fans in operation for a period of four minutes with the suction modulation valve fully open. When the snap freeze is completed, defrost is formally terminated.

4-7

T-309

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Image 59
Carrier 69NT20-531-300 manual Defrost
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69NT20-531-300 specifications

The Carrier 69NT20-531-300 is a cutting-edge rooftop unit designed to meet the demanding requirements of commercial heating and cooling applications. This model embodies Carrier’s commitment to energy efficiency, reliability, and advanced technological features that enhance indoor comfort while minimizing operational costs.

One of the standout features of the Carrier 69NT20-531-300 is its high energy efficiency. It is equipped with advanced compressor technology that optimizes performance while reducing energy consumption. This ensures that businesses can maintain a comfortable indoor climate without incurring exorbitant utility bills. With a high Seasonal Energy Efficiency Ratio (SEER) and Energy Efficiency Ratio (EER), users can expect not only enhanced comfort but also significant cost savings over time.

The cooling capabilities of the 69NT20-531-300 are complemented by its robust heating options. This unit utilizes a gas heat exchanger, ensuring quick and even heating throughout the space. The combination of cooling and heating capabilities in one compact unit makes it a versatile choice for various applications, including retail spaces, offices, and warehouses.

In addition to its impressive performance specifications, the Carrier 69NT20-531-300 incorporates innovative technologies that further enhance its functionality. The unit comes with advanced controls, allowing for precise temperature regulation and zoning capabilities. This ensures that different areas of a building can maintain their desired temperatures based on usage, improving overall energy efficiency.

Durability is another hallmark of the Carrier 69NT20-531-300. The unit is constructed with high-quality materials and features a weather-proof cabinet that protects it from the elements, ensuring long-term reliability even in harsh conditions. Furthermore, its modular design simplifies maintenance, making it easy for technicians to access critical components.

The 69NT20-531-300 is also designed with low noise operation in mind. It employs sound-dampening technology to ensure that operations are quiet, making it an ideal choice for locations where noise reduction is a priority, such as schools or healthcare facilities.

Overall, the Carrier 69NT20-531-300 stands out as a premier choice for businesses looking for an efficient, reliable, and feature-rich rooftop HVAC unit. Its blend of energy efficiency, advanced technology, and durability positions it as a leader in commercial heating and cooling solutions, making it a valuable investment for any commercial property.
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