I have a 1984 Rheem gas furnace model # RGDA-100E-GR with a 11-7DU blower. The blower won't engage in auto mode in the heat setting, but it will engage in the "On" mode. It will engage in both modes in the a/c setting.

I've replaced the fan relay on the assumption it had failed. No difference. All wiring is intact, nothing is crossed, the system hasn't been worked on at all in recent times. The system worked fine until the other night when this symptom popped up. Any ideas?

Look for BFC (Blower Fan Control) on these diagrams.


https://www.rheemote.net/docstore/webdocs/ServiceDocs/HistLib/PDFS/Wiring/90-21283-10.pdf

https://www.rheemote.net/docstore/webdocs/ServiceDocs/HistLib/PDFS/Wiring/90-22748-02-01.pdf