I am curious about the combustion chamber compensation port on the bottom of these valves. 0820637 Since fireplaces don't have a combustion chamber, are there some brands of heaters where they are used? How does this port work, does the pressure in the combustion chamber act directly on the back side (atmospheric) of the pressure regulator diaphragm? Does this increase the gas pressure output of the valve inch for inch? When you are adusting the output pressure of the valve, are you looking at this elevated pressure? Also, when not used is it just left open to the atmosphere?