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.