Any HVAC engineer will tell you that flex duct is a necessary evil. It’s quick to install but retards airflow. Your RV may have quite a bit of 4” flex duct running from your furnace to your heating registers. And if it’s not installed properly, you could be paying for heat you’re not enjoying!

Introduction to RV Furnace Ducting

If you don’t know about ducting, the essentials aren’t difficult to grok. Basically, ducts are like roadways for air. Your furnace blower fan pumps hot air through these ducts to your registers. These registers (think traffic controllers) direct the heated air into the living space of the RV, and you feel comfortable. 

Some furnaces are hard-ducted directly into a distribution plenum. Other times (see pictures below), the ducts are connected directly to the furnace case.