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Bubba Moore

Bubba Moore

Technical Services Manager
Southeast District

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Two Stage Compressor Operation

25HNB6 or 286BNA units

Compressors have evolved over the years from single stage to staged compressors. Staging the capacity of a compressor helps with energy cost, humidity control, and comfort. How is this accomplished in these units and how can you tell if it is switching and functioning?

The design of a basic scroll compressor has been changed with the addition of an internal unloading solenoid that opens a bypass port in the first compression section of the scroll compressor. The opening of this bypass port effectively reduces the displacement of the scroll compressor. The single speed, high efficiency motor continues to run while the scroll modulates between the two stages. Modulation is achieved by venting a portion of the gas in the first suction pocket back to the low side of the compressor, which reduces the effective displacement of the compressor. This unloading of the compressor reduces its capacity to approximately 67% which translates to approximately 75% cooling or heating capacity at the indoor coil.

To achieve 100% full capacity, the vents that were open to reduce capacity are closed. This is achieved by a DC solenoid inside the compressor. A 24-volt AC signal sent to the rectifier and converted to a 24-volt DC signal to operate the DC solenoid inside the compressor moves the bypass ports that allow the vents to be covered and uncovered. The loading and unloading of the two step scroll is done without shutting off the compressor between first and second stage operations. During startup of the compressor, even if full load is needed, it will always start unloaded, and after five seconds it will switch to the 100% full capacity.

While the unit is operating, your suction pressure will change as it switches from low to high capacity. The compressor current will also increase as it switches from low to high stage. When there is no 24-volt DC signal, the solenoid coil is de-energized, and the compressor is in low stage. When the compressor is operating in high stage, there is a 24-volt DC signal that energies the DC solenoid.

To test the operations of the loading and unloading of the compressor, there are certain procedures you can do. First, measure the compressor amperage while the unit is running. Cycle the unloader on and off from low stage to high stage, and then back to low stage in thirty second intervals. You should see at least a twenty percent increase or decrease in compressor amperage.

Second, you can remove the solenoid plug from the compressor, turn the unit on and make sure it is in high stage. Then, with a voltmeter, set it to the DC scale. The reading should be 24-volts DC coming out of the rectifier.

The third procedure is to measure the resistance of the compressor unloader coil. This resistance should be approximately 330 to 1640 ohms. If the coil is open or shorted, the compressor will have to be replaced.

This staging is not limited to these units. It is used in residential and commercial units.

Source: Two Stage Heat Pump Installation Manual

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Alabama Locations

Anniston(256) 241-3026105 E D St, Anniston, AL 36201
Auburn(334) 734-80112295 1st Ave, Opelika, AL 36801
Birmingham(205) 250-7714516 31st St N, Birmingham, AL 35203
Dothan(334) 836-1035205 Wedgewood Dr, Dothan, AL 36303
Gulf Shores(251) 968-2660820 Commerce Dr, Gulf Shores, AL 36542
Huntsville(256) 859-2691391 Nick Fitcheard Rd NW, Huntsville, AL 35806
Loxley(251) 964-270730020 Co Rd 49, Loxley, AL 36551
Mobile(251) 476-2220703 Western Dr, Mobile, AL 36607
Montgomery(334) 262-4855861 Plantation Way, Montgomery, AL 36117
Muscle Shoals(256) 383-6904107 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
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Columbus(706) 322-38706201 W Hamilton Park Rd, Columbus, GA 31909
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Forest Park(404) 361-24745158-C Kennedy Rd, Forest Park, GA 30297
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Macon(478) 474-85445527 Thomaston Rd, Macon, GA 31220
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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

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(205) 250-7714

Macon – Southwest Hub
(800) 342-9250

Loxley – Gulf Coast Hub
(800) 239-7419

Savannah – Southeast Hub
(800) 822-7617

Commercial Parts

 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
logo-bardBardwww.bardhvac.comHeat Pumps,Gas/Electric, and Wall Mounted Air Conditioners
logo-bryantBryantwww.bryant.comCommercial, Industrial, Residential Heating & A/C Products and Controls
logo-carrierCarrierwww.commercial.carrier.comCommercial and Residential Heating & A/C Products
Data Airewww.dataaire.comPrecision Air Control for Data Processing Centers
DXairwww.dxair.comPool dehumidification systems
logo-firstcoFirst Companywww.firstco.comFan Coil Units
logo-gpsGlobal Plasma Solutionswww.gpshvac.comAir Purification and Bi-Polar Ionization
logo-greeGreewww.greeusa.comPTACs
logo-haysHayes Fluid Controlswww.haysfluidcontrols.comHose Kits and Control Valves
logo-iaireiAirewww.myiaire.comCommercial Air Purification using ionization with energy savings
logo-islandaireIslandAirewww.islandaire.comPTACs, Vertical Units, Water Source Heat Pumps
logo-ivui-Vuwww.commercial.carrier.comBuilding Automation and Controls
logo-magic-aireMagic Airewww.magicaire.comFan Coil Units
logo-micrometlMicro Metlwww.micrometl.comRoof Curbs, RTU Accessories, & ERVs
logo-modineModinewww.modine.comGas-Fired Unit Heaters, Duct, Furnaces, Hot Water Cabinet Unit Heaters, Makeup Air, and Dedicated Outdoor Air Systems
logo-network-thermostatNetwork Thermostatwww.networkthermostat.comInternet Thermostats, Wifi Themostats, Ethernet Thermostats
logo-paynePaynewww.payne.comResidential Heating & A/C Products
logo-rawalRawal Deviceswww.rawal.comContinuous Capacity Modulation and Dehumidification for DX Systems
logo-renew-aireRenew Airewww.renewaire.comFixed-Plate ERVs
logo-totalineTotalinewww.totaline.comCompressors, Parts, and Supplies
logo-united-cool-airUnited CoolAirwww.unitedcoolair.comModular All-Indoor HVAC Systems
logo-warrenWarren Technologywww.warrenhvac.comElectric Heaters
logo-wiloWilo Pumpswww.wilo-usa.comPump Systems for HVAC and Water Supply and Sewage