Running extra wires to a 2-stage condensing unit has long been a challenge in retro-fit situations. If the wiring situation looks challenging, contractors might not even offer the homeowner the more comfortable multi-stage option. Or worse, it is offered and sold, but then the install crew gets to work and hits some road blocks which make the labor more costly and hurts contractor profit.
Those days are pretty much behind us now. TEC’s inventory of Bryant Evolution and Carrier Infinity communicating condensing units now includes 2-stage units that only require a 2-wire communication cable instead of 4-wire. The factory accomplished this by the use of a new control circuit board and the addition of a transformer to the condensing unit so it does not need to “borrow” 24VAC from the furnace via the C and D wires. Now you only have to run A and B for communication from the furnace or fan coil out to the A/C or heat pump, making retro-fits very easy. Below is a summary of the models of units with this new feature and the serial number ranges.
- Carrier 24ANB1 and Bryant 180B are 2-wire starting with s/n 1213E and later
- Carrier 24ANB7 and Bryant 187B are 2-wire starting with s/n 0213E and later
- Carrier 25VNA and Bryant 280ANV have always been 2-wire
- Carrier 25HNB6 and Bryant 286BNA are 2-wire starting with s/n 3112E and later
- Carrier 25HNB9 is 2-wire starting with s/n 4312E and later
- Carrier 24ANB6 and Bryant 186B remain 4-wire communication with no plans to change that – however, you can run 2-wires to these single stage units and use the traditional Y and C wiring in a pinch instead of communication cabling
By the way, you will notice the terminal boards now say “A-B-C-NO USE” instead of the old “A-B-C-D.” Even if you already have 4-wires pulled to the outdoor unit, do not terminate the D wire on the NO USE terminal. That will cause you to have 2 transformers on the same circuit which will likely blow one or both of the transformers and possible damage the circuit board.