Why is my new propane high efficiency hot air furnace only puts out max 20C at the air registers in each room. Can not heat the house higher than 18 C. Flue pipe is also very cool , about 25 C

My supplier says it is OK, But the firnace can never warm thre house or go off . Uses a lot of propane

Unless the furnace is to small for the area being heated there is something wrong. Possible the burner orifices are to small or the unit needs adjustments. If it is new the company that sold you the furnace should come back and fix it.

If this is a high efficiency unit (90% AFUE on up) exhaust temp will be intentionally cool and tell you nothing. Regardless I will call the folks that sold and installed this unit and tell them to get it working. If its not reaching design temp for your house then they would initially responsible.