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M#25VNA8 Troubleshooting Tips

Bubba Moore

Bubba Moore

Technical Services Manager
Southeast District

M#25VNA8 Troubleshooting Tips

The M#25VNA8 communicating unit has several devices that aid in the operation and protection of the unit. These devices are the suction pressure transducer, outdoor coil thermistor, outdoor air thermistor, suction thermistor, and the discharge thermistor. Sometimes these devices can start to fail, but not completely fail. When this happens the M#25VNA8 unit tries to correct the operations or protect itself with the info it is receiving from these devices. The info the unit receives can cause the unit to shut down on a safety or run inefficiently.

    The suction pressure transducer allows the unit to monitor low pressure cut-out, loss of refrigerant charge, compressor protection, lubrication and oil circulation. It also aids in the control of the electronic expansion valve in the heating mode. The outdoor coil thermistor provides the outdoor coil’s liquid line temperature to the heat pump board in order to allow the unit to operate in the low ambient operation, defrost initiation, and defrost termination, with the assistance of the outdoor air thermistor. The suction thermistor is used to assist with the control of the electronic expansion valve and monitor the suction line temperature to the compressor. The discharge thermistor is used for protection against over temperature of the compressor. Now that we have a description of what the devices do and assist within the unit, we can look at ways to check and determine if the devices are defective and causing problems.

    The suction pressure transducer receives 5 volts DC to it and sends 0 to 5 volts DC back to the control board, which is interpreted into pressure. At the connector on the control board, you can confirm that the suction transducer is receiving the 5 vdc on the ground pin and the supply pin. There is a formula that you can use once you read the output voltage between the ground pin and output pin on the control board. A formula is used to calculate the DC voltage in to pressure. You can also use the graph in the service manual. Once you get the pressure from the graph or formula, compare these calculations to your refrigerant gauge readings and compare for accuracy.

Pressure DC Voltage Formula

For example, if the measured output voltage is 3.0 VDC & Pressure is 50 psig:               

                        50 X (3.0 — 0.5) — 50 X 2.5 = 125 psig.

The temperature thermistors are electronic devices which sense temperature.  You can check these thermistors with an ohm meter. As the temperature increases, the resistance of the thermistor decreases. The outdoor air, outdoor coil, and suction thermistor are all 10K ohm thermistors. There is a graph that is in the service manual to compare the resistance of the thermistor to temperature. See below.

The discharge thermistor is a 50K ohm thermistor, which also has a graph that is in the service manual to compare the resistance of the thermistor to temperature also. See below.

50K Thermistor Resistance Versus Temperature

There is one other way to check the accuracy of these devices. With a communicating wall control, you can go into the service information of the wall control and look at the system while it is in off mode. After the unit has been idle for about 20 minutes, you can compare all the thermistors readings to each other. The readings will be close if there are no failing thermistors. The discharge thermistor can read a little higher due to the heat from the compressor if it was previously running, or the sump heater was on. While the unit is still off, or if it is running, you can compare the unit pressure from your gauges to the reading on the wall control while still in the service info screen.

    The info discussed from the service manual is to get a more accurate picture of the device that may be in question, but use the communicating wall control for a quick reference if these devices are in question. With a little understanding of what a device does in a unit, it can help determine why the unit is not operating efficiently or shutting down on a safety.

 

Sources: Service Manuals For

 25VNA8 Infinity 18VS Variable Speed Heat Pump

 24VNA9 Infinity 19VS Variable Speed Air Conditioner with Puron Refrigerant

288BNV EVOLUTION V VARIABLE SPEED HEAT PUMP

189BNV EVOLUTION V VARIABLE SPEED AIR CONDITIONER

WITH PURON REFRIGERANT

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