Did an experiment. I got my kitchen stem thermometer and stuck it in the vent as my oil furnace is running just now. It quickly registered almost 120 right in the vent. Held eight inches above the vent , in the stream of hot air, it was still holding 110. The variable left though would be does it run long enough to significantly heat a plant pot ( pot size affecting that) and /or excessively heat and dry the foliage
the furnace ran for maybe 10 minutes?
