Hi,
I did an inspection and I found a big rust area in furnace cabinet! Actually, I did not find any evidence of leaking and furnace worked fine. So how do you write it up and suggest to the clients? And any idea what causing it?
thanks.
You should be joining NACHI.
Write up what you see and recommend service.
Nick allows these people to ask questions here on the message board. Why?
Because most are ASHI members, and are not educated, or just plain stup**.
Most likely a blocked a-coil drain hose, or blocked condensate drain line. If you need me to explain it further to all ASHI members, just let me know.
Ya attract more bees with sugar than with vinegar 
How do you know he’s an ASHI member?
Not important you know where rust is coming from … but likely the inducer fan, or flue leaking OR 2nd choice condensate. Unimportant. Whats important is:
The area inside the furnace case below the inducer fan AND outside the burners and heat exchanger area had excess rust and water stains. [Clean and service the unit]. Have a competent and licensed HVAC contractor service the unit; correct the cause of moisture / rust stainings; check it for proper operation and verify the full integrity of the heat exchanger. Cleaning and servicing sometimes reveals hidden defects if present.
Rust in the bottom of furnace, the fan compartment unless it is counterflow is usually from water in basement some time in the past. Check slab for efflorescence or stains. If it is the original furnace it may have been installed before the slab was poured (very common here) wet cement is very corrosive.