View pricing, availability, product information and place orders online 24/7 at SingleSource®


M#25VNA8 Troubleshooting Tips

Picture of 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




More to explore

Veto Promo

April 2024 Veto Pro Pac Promotion Check out the video and resources below to take advantage of our April Veto bag promotion.

Black History Month

Black History Month takes place between February 1st and March 1st every year. It’s a time to celebrate and observe the accomplishments,

Alabama Locations

Anniston (256) 241-3026 105 E D St, Anniston, AL 36201
Auburn (334) 734-8011 2295 1st Ave, Opelika, AL 36801
Birmingham (205) 250-7714 516 31st St N, Birmingham, AL 35203
Dothan (334) 836-1035 205 Wedgewood Dr, Dothan, AL 36303
Gulf Shores (251) 968-2660 820 Commerce Dr, Gulf Shores, AL 36542
Huntsville (256) 859-2691 391 Nick Fitcheard Rd NW, Huntsville, AL 35806
Loxley (251) 964-2707 30020 Co Rd 49, Loxley, AL 36551
Mobile (251) 476-2220 5522 W Commerce Blvd West, Mobile, AL 36619
Montgomery (334) 262-4855 861 Plantation Way, Montgomery, AL 36117
Muscle Shoals (256) 383-6904 107 Commerce St, Muscle Shoals, AL 35661

Georgia Locations

Albany(229) 432-5254205 Baldwin Dr, Albany, GA 31707
Athens(706) 548-2563105 Whitetail Way, Bogart, GA 30622
Atlanta(404) 875-77551290 Tacoma Dr NW, Atlanta, GA 30318
Augusta(706) 651-19503618 Wrightsboro Rd, Augusta, GA 30909
Brunswick(912) 265-5193275 Rose Dr, Brunswick, GA 31520
Columbus(706) 322-38706201 W Hamilton Park Rd, Columbus, GA 31909
Conyers(770) 918-9132335 Gees Mill Business Pkwy NE, Conyers, GA 30013
Forest Park(404) 361-24745158-C Kennedy Rd, Forest Park, GA 30297
Gainesville(770) 297-93352472 Hilton Dr, Gainesville, GA 30501
Lawrenceville(678) 442-10602111 Cedars Rd SE, Lawrenceville, GA 30043
Lithia Springs(770) 948-3451320 Thornton Rd #106, Lithia Springs, GA 30122
Macon(478) 474-85445527 Thomaston Rd, Macon, GA 31220
Marietta(770) 426-05511060 Triad Ct, Marietta, GA 30062
Newnan(770) 239-210570 Enterprise Ct, Newnan, GA 30265
Peachtree Corners(770) 239-21006675 Jones Mill Ct, Peachtree Corners, GA 30092
Savannah(912) 232-1228103 Central Jct Blvd, Savannah, GA 31405
Statesboro(912) 764-8841126 W Parrish St, Statesboro, GA 30458
Valdosta(229) 245-8200363 Connell Rd, Valdosta, GA 31602
Warner Robins(478) 953-3727110 Byrd Way, Warner Robins, GA 31088

South Carolina Locations

Hilton Head(843) 689-34043715, 21 Hunter Rd, Hilton Head Island, SC 29926

Florida Locations

Fort Walton Beach(850) 244-201776 Beal Pkwy SW, Fort Walton Beach, FL 32548
Panama City(850) 913-05853012 Lisenby Ave, Panama City, FL 32405
Pensacola(850) 438-10054405 N. Palafox Street, Pensacola FL 32505

Mississippi Locations

Hattiesburg(601) 579-96701851 Lincoln Rd, Hattiesburg, MS 39402
Ocean Springs(228) 497-5090Sunplex Industrial Park, 5717 Suntec Drive, Ocean Springs, MS 39564

Commercial Sales

Norcross – North Georgia Hub
(800) 282-4911

Birmingham – North Alabama Hub
(205) 250-7714

Macon – Southwest Hub
(800) 342-9250

Loxley – Gulf Coast Hub
(800) 239-7419

Savannah – Southeast Hub
(800) 822-7617

Commercial Parts

[email protected]
(844) 99-CHILL

 CompanyWeb AddressDescription
Airzonewww.airzoneinc.comCustom and Underfloor Air Handlers
Aladdin Metal Productswww.aladdinmetalproducts.comCurb Adapters and Roof Curbs
Aprilairewww.aprilaire.comIndoor Air Quality Products
Autaniwww.autani.comBuilding Automation and Controls
Bardwww.bardhvac.comHeat Pumps,Gas/Electric, and Wall Mounted Air Conditioners
Bryantwww.bryant.comCommercial, Industrial, Residential Heating & A/C Products and Controls
Carrierwww.commercial.carrier.comCommercial and Residential Heating & A/C Products
Data Airewww.dataaire.comPrecision Air Control for Data Processing Centers
 DXairwww.dxair.comPool dehumidification systems
 First Companywww.firstco.comFan Coil Units
Global Plasma Solutionswww.gpshvac.comAir Purification and Bi-Polar Ionization
 Hayes Fluid Controlswww.haysfluidcontrols.comHose Kits and Control Valves
 iAirewww.myiaire.comCommercial Air Purification using ionization with energy savings
 IslandAirewww.islandaire.comPTACs, Vertical Units, Water Source Heat Pumps
 i-Vuwww.commercial.carrier.comBuilding Automation and Controls
 Magic Airewww.magicaire.comFan Coil Units
 Micro Metlwww.micrometl.comRoof Curbs, RTU Accessories, & ERVs
 Modinewww.modine.comGas-Fired Unit Heaters, Duct, Furnaces, Hot Water Cabinet Unit Heaters, Makeup Air, and Dedicated Outdoor Air Systems
 Network Thermostatwww.networkthermostat.comInternet Thermostats, Wifi Themostats, Ethernet Thermostats
Paynewww.payne.comResidential Heating & A/C Products
Rawal Deviceswww.rawal.comContinuous Capacity Modulation and Dehumidification for DX Systems
Renew Airewww.renewaire.comFixed-Plate ERVs
Totalinewww.totaline.comCompressors, Parts, and Supplies
United CoolAirwww.unitedcoolair.comModular All-Indoor HVAC Systems
 Warren Technologywww.warrenhvac.comElectric Heaters
 Wilo Pumpswww.wilo-usa.comPump Systems for HVAC and Water Supply and Sewage