Manuel Reset, Series Duct Heaters. These Duct Heaters May Be Used with Heat Pumps, Cooling Units, or with Any Forced Air System. They Are Suitable For Zero Clearance Installations In Vertical or Horizontal Duct Systems. Model Numbering System: The Letters 'MR' Mean Manually Resettable Thermal Switches Are Used For The Secondary Protection. The Next Group Of Numbers Designates The Minimum Duct Size with The Side To Be Cut First and The Length Of The Insertion Next. Numbers Following The First Dash Indicate The Kw. The -1 or -3 Following Denotes Single or Three Phase. 'S' or 'F' On The End Would Designate Sequencing Relays or Flanged Construction.Example: MR 816-5-1 Means A Manual Reset Type Heater To Go Into The Side Of An 8' x 16' Duct, 5Kw, 1 Phase. Heaters May Be Ganged In or der To Fill A Duct. For Example, Two 12' x 24' Heaters May Be Used To Fill A 24' x 24' Duct. They Should Not Be Used So The Outlet Air From One Becomes The Inlet Air Of The Next Unless The Inlet To The Second Is Below 110 F. Air Flow Should Be Sufficient For The Sum Of Both Heaters.