THERE ARE THREE BASIC COLLECTOR DESIGNS      #1:  FRONT PASS     #2:  FRONT/BACK PASS COMBINATION [TRANSPIRED] AND            #3:  BACK PASS

 1: IN A FRONT PASS AIR MOVES BETWEEN THE GLASS AND ABSORBER PLATE AND IS COMMON IN BLACK PAINT AND SCREEN SYSTEMS.

  2: IN FRONT/BACK PASS [TRANSPIRED] THE AIR MOVES THROUGH THE HOLES IN THE ABSORBER PLATE LETTING AIR MOVE ON BOTH SIDES OF THE PLATE. THESE ARE NORMALLY BLACK PAINT OR SEMI-SELECTIVE PAINT.

 3: BACK PASS AIR MOVES BEHIND THE ABSORBER PLATE AND HAS A DEAD AIR SPACE BETWEEN THE ABSORBER PLATE AND THE GLASS. COATINGS RUN FORM  BLACK PAINT UP-TO HIGHLY SELECTIVE COATING.  AS IN  NORTHERN COMFORT TM. COLLECTORS

GLASS IS A VERY POOR INSULATOR      #1: GLASS IS AT THE LOW END OF THE SCALE IN PERFORMANCE AND SHOULD NOT BE USED IN ANYTHING BUT MILD CLIMATES.      #2:   IF USED WITH A GLAZING GLASS HAS BETTER PERFORMANCE BECAUSE NOT ALL OF THE AIR IS EXPOSED TO THE HEAT LOSS OF THE GLAZING.      #3:   HAS THE HIGHEST PERFORMANCE WITH THE DEAD AIR SPACE  SERVING THE SAME PURPOSE AS A DOUBLE PANE WINDOW AND IS VERY EFFECTIVE IN AIR MAKEUP AND HEATING WITH VERY LITTLE HEAT LOSS THROUGH THE GLAZING.

THREE TYPES OF GLAZING.   #1: POLY-CARBONATE WILL YELLOW AND LOOSE TRANSMITTANCE OVER TIME.    # 2: PLAIN GLASS - HIGH IRON CONTENT AND DANGEROUS IF BROKEN.    #3: SOLAR GLASS -LOW IRON CONTENT [LIKE CRYSTAL] AND FULLY TEMPERED FOR SAFETY WITH THE HIGHEST TRANSMITTANCE  AVAILABLE.