Slide-out combustion chamber/burner assembly allows easy removal for service and ensures burner alignment. Plug-in connectors for blower and burner wiring connections simplify maintenance. Four heat exchanger cleanout ports. Advanced combustion chamber material is used to increase durability. All models are designed to provide fresh air for combustion with optional fresh air kit, increased electrical and oil line access versatility, sidewall vent option, and secondary limit control for extra safety. Sound absorbing high temperature fiber provides faster heat build-up in heat exchanger for greater efficiency and quieter operation. Cabinet insulated is to reduce sound levels.