Hi/lo mixing

icy78

As I know almost nothing about the title subject, I watched a Caleffi webinar on it last night.
Caleffi puts a PRV after  the HI outlet but before the Lo outlet .
  I get how the PRV functions to limit the HI MV, and allow the LO MV to control the flow and temperature. 
 Then when the outlet pressure drops below the PRV setting the PRV will crack open and allow some flow from the HI MV.
  Now, say the flow is 7gpm (thru the LO MV) and the pressure drops to 50 psi and the PRV cracks open.
At this point the HI MV is now flowing also. 
  This is where the confusion starts for me.
IF the HI MV has a minimum flow of say 6 gpm (to control properly) well then, how is it functioning properly now that the PRV is only letting  possibly just trickle of flow. What is the difference between too-low flow at 60psi fixture pressure or at 50 psi fixture pressure?
Seems that either senario is not enough flow for the HI MV to control.

Jamie Hall

The short answer is -- it probably won't. For a pressure reducing valve to function properly, the flow -- and pressure drop -- must be within the range for which it is designed, which is usually given somewhere in the detailed specifications. Just what form of evil the malfunction will take depends on the circumstances and the valve design. Like most things, very careful engineering can give excellent downstream pressure control over very wide ranges of flow -- and like most such, can cost much money.