Rheem Furnace Code 20
Warning
Twinning fault. Two furnaces installed side by side and wired to run together (twinned) are not communicating or are not set up correctly.
Manual wording: “20 TWINNING FAULT”
Documented for: (-)801S/(-)801P and other single-stage controls with the twinning feature
Likely causes, most common first
- Twinning wiring or dipswitch setup between the two furnace controls is incorrect
- The two control boards are not the same part number, or their transformers are not in phase
When to call a technician
Twinned installations are commissioned by a technician - the twin wiring, dipswitch settings and transformer phasing all have to be checked with the power on.
A Rheem furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation Instructions for Downflow Single Stage Gas Furnaces - (-)801S / (-)801P Downflow Series (92-24161-143-03) (Rheem Manufacturing Company, R801S, R801P, (-)(-)80DSS, (-)(-)80DSP 80% single-stage gas furnaces; DIAGNOSTICS AND FAULT CODES, page 34) · checked 2026-08-25