View pricing, availability, product information and place orders online 24/7 at singlesource.mingledorffs.com

HVAC and Plumbing Distributor
Steven Simmons

Steven Simmons

Technical Services Manager
Southwest District

38AUZ/D with Twin Furnaces

One popular combination of Carrier equipment is twinning 2 residential furnaces together with a 6-10 ton commercial outdoor A/C unit. When this is done, it can get complicated very quickly because residential and commercial controls are different and yet they will have to be merged together for proper operation to occur.

What's different between Carrier residential controls VS Carrier commercial controls?

For the purpose of this article, the main difference is that on residential systems the transformer used to operate the system is located in the indoor unit whereas on commercial systems the transformer used for system operations is in the outdoor unit. With that being said, this combination has two furnaces each with their own transformer as well as the commercial outdoor unit with its own transformer. This gives us a total of at least 3 transformers.

On commercial buildings, it is not as simple as combining all the R (24v) wires because these buildings typically have 3 phase power. With three different incoming power combinations for the three transformers, there is a good chance that the transformers will not be “in phase”. Yes, there is phasing on low voltage 24 too (not to be discussed in this article) …. If the transformers are out of phase then R to R on two different transformers will read roughly 45vac when it should be 0vac.

So, what is the solution to this dilemma?

1st, Plan ahead- determine how the system will be controlled for furnace operation. This will be critical to the thermostat setup. The options are A) single stage furnaces acting as two stages, B) single stage furnaces acting as one single stage unit, C) two stage furnaces setup for “adaptive” heating or setup to operate traditionally with W1 & W2 inputs.

2nd, Install twinning kit- use an approved twinning kit to tie the furnaces together. When the twinning kit is installed there will need to be a “main” and a “secondary” furnace setup. These kits can be installed differently Depending on the selection from the first step. (please follow the installation directions for the appropriate twinning kit)

3rd, Install relays- there are wiring diagrams with the twinning kits for wiring residential outdoor units. Since the Carrier commercial units have their own transformers, the wiring will be slightly different. The best way (recommended way) is to utilize isolation relays to ensure that the two different low voltages (furnace vs outdoor unit) never cross paths.

Wiring the outdoor unit

The twinning kit should be used to determine how to wire the furnaces so I did not show wires for the heat side. I want to show clarity on the outdoor unit alone. Please take a look at the way the relays are used to separate the two different control voltages.

1 Stage Outdoor Wiring
2 Stage Outdoor Wiring

More to explore

Capacitor Testing

Testing Capacitors

First, let us start by saying there are a variety of capacitors in various shapes, sizes and colors. For the purpose of

The Infamous E1: Part 2

E1 fault code on Ductless equipment is communication fault between the indoor and outdoor units.When the E1 code shows up on the

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

 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
 Greewww.greeusa.comPTACs
 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