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Bubba Moore

Technical Services Manager
Southeast District

Commercial Defrost Control

The Commercial Defrost Control Board (DFB) can be confusing and misunderstood. This DFB does a lot more than just control defrost. The DFB coordinates thermostat demands for first and second stage cooling, first and second stage heating, emergency heating and HVAC defrost control. The DFB also has an indoor fan off delay.

The DFB does not need an O or B signal to determine whether the reversing is energized in heating or cooling. That is why the units that use this DFB do not use a standard heat pump thermostat. When the DFB receives a call for first stage cooling the reversing valve(s) are energized (not by second stage cooling) and stay energized even after the demand for cooling is no longer needed. On a demand for heating the reversing valve(s) are de-energized and (stay de-energized until a call for cooling) the compressor(s) are started for first stage heating. On a call for second stage heating the DFB will energize the auxiliary (electric) heat control. The DFB also controls the auxiliary heat control during emergency heating and defrost.

HVAC Defrost Process

HVAC and Plumbing Distributor
Part# HK32EA005

The DFB when controlling defrost is a time /temperature sequence. The time is adjustable with dip switches: these times are 30, 60, 90 and 120 minutes. These times are not all the same depending on the tonnage of the equipment. The temperature function is provided by defrost thermostat(s). The tonnage will determine if the system has one or two defrost thermostats. Units with one defrost switch will defrost when the timing period is completed and the defrost switch is closed. The defrost period will end when the defrost thermostat opens, or a ten-minute time has lapsed. The larger tonnage systems have two defrost thermostats and the sequence of operations is different. Defrost thermostat number one cannot initiate a defrost cycle, only defrost thermostat number two can initiate a defrost. The DFB monitors defrost switch number one and will allow it to defrost once circuit two is in defrost. Circuit one will stay in defrost until switch opens or if circuit two defrost cycle is terminated. If space temperature is satisfied before defrost is completed, the DFB will remember where the defrost cycle was interrupted and on restart of a heating call it will resume defrost operations.

The DFB also has a defrost speedup function:  this can be done by shorting across two jumper wires that are marked on the DFB. If the defrost switches are closed and you short these jumpers for one to three seconds, it reduces the defrost time period. If the jumpers are shorted for five to twenty seconds, the DFB goes into a forced defrost and will function like a normal defrost if the defrost thermostat(s) are closed. If the defrost thermostat(s) are open, the defrost cycle will run for thirty seconds and terminate.

The DFB can be set up to allow the indoor fan to run for sixty seconds after the system shuts down if the space thermostat fan switch is set to auto. The fan delay begins when a heating or cooling demand is satisfied.

As you can see, this DFB can do a lot more than just monitor an iced up coil. The function of this DFB with input signals from the thermostat is that it controls the cooling and heating sequences, defrost stages, and aides the technician in speeding up a defrost, or checking out the defrost functions. And to top it all off, you can have a delayed fan if needed.

Source: 50TCQ Service and Maintenance Instructions (p.20)

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HVAC Defrost

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

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