HVACopedia

Carrier · Furnace

25.3 Twinned furnaces not identical

Medium severity 59SC6A

What the control is telling you

In a twinned system the two furnaces have different program numbers; the main unit will not heat.

Applies to: Carrier 59SC6A

Likely causes

  • Different furnace models twinned
  • One control loaded with the wrong program number

Test and repair steps

  1. Check the program number (inF) on both furnaces.
  2. Compare each to its own rating plate.
  3. Furnaces must be identical (same program number) to be twinned.
  4. Reprogram any control that doesn't match its rating plate; if the furnaces are different models they can't be twinned.
  5. Confirm tnn is set Pri on one unit and SEC on the other, with the twinning kit installed.

Settings for these units

Save it for the next call

Star this code in the HVACopedia app and keep the whole library on your phone for jobs with no signal.

Open the app

Source: Carrier 59SC6A Installation, Start-up, Operating and Service Instructions, Fig. 60 Service Label Information, p.54; Control Board Replacement p.53. Summarized in our own words; check the original manual for your model.

Homeowner looking at this code? See what it means for you and when to call a technician.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.